foreland shortening and crustal balancing in the andes at 30 s latitude excellent surface exposures known benioff zone geometry a dynamic morphology and the availability of industry seismic reflection data all make the andes at 30 s an excellent transect for investigating crustal scale balanced sections 150 170 km of horizontal shortening has occurred in three major belts located between the trench and the foreland the thin skinned east verging precordillera of western argentina accounts for 60 75 of the total shortening and formed mostly since major volcanism ceased at 10 ma industry seismic reflection data show that the decollement of the precordillera belt is located anomalously deep at 15 km the belt is dominated by fault propagation folds and contains several prominent out of sequence thrust faults seismic stratigraphie analysis shows that miocene strata in the iglesia valley located between the precordillera and the crest of the andes accumulated in a piggy back basin onlap relations on the western side indicate that the high cordillera was uplifted as a major fault bend fold over a buried ramp thrusting in the two western belts both in the high cordillera of chile formed during the waning stages of arc volcanism 11 16 ma and account for 25 40 of the shortening the observed shortening is probably greater than can be accounted for with reasonable crustal thicknesses indicating the possibility of continental truncation or erosion along the plate margin or an anomalously thick root held down by the nearly flat subducted nazca plate our preferred crustal geometry puts the ramp between upper and lower crustal deformation west of the high topography requiring crustal scale tectonic wedging to thicken the crust beneath the crest of the andes this non unique model provides a simple explanation of the first order morphology of the andes at this latitude

models of crustal thickness for south america from seismic refraction receiver functions and surface wave tomography abstract an extensive compilation of crustal thicknesses is used to develop crustal models in continental south america we consider point crustal thicknesses from seismic refraction experiments receiver function analyses and surface wave dispersion estimates of crustal thickness derived from gravity anomalies were only included along the continental shelf and in some areas of the andes to fill large gaps in seismic coverage two crustal models were developed a by simple interpolation of the point estimates and b our preferred model based on the same point estimates interpolated with surface wave tomography despite gaps in continental coverage both models reveal interesting crustal thickness variations in the andean range the crust reaches 75 km in southern peru and the bolivian altiplano while crustal thicknesses seem to be close to the global continental average 40 km in ecuador and southern colombia despite high elevations and along the southern andes of chile argentina elevation lower than 2000 m in the stable continental platform the average thickness is 38 5 km 1 st deviation and no systematic differences are observed among archean paleoproterozoic cratons neoproterozoic fold belts and low altitude intracratonic sedimentary basins an exception is the borborema province ne brazil with crust 30 35 km thick narrow belts surrounding the cratons are suggested in central brazil parallel to the eastern and southern border of the amazon craton and possibly along the transbrasiliano lineament continuing into the chaco basin where crust thinner than 35 km is observed in the sub andean region between the mid plate cratons and the andean cordillera the crust tends to be thinner 35 km than the average crust in the stable platform a feature possibly inherited from the old pre cambrian history of the continent we expect that these crustal models will be useful for studies of isostasy dynamic topography and crustal evolution of the continent

deep structure of the southern ural mountains as derived from wide angle seismic data p and s wave modelling of the data obtained during the seismic refraction wide angle reflection experiment of the urseis95 project demonstrate the presence of a 15 18 km thick crustal root beneath the magnitogorsk tagil zone in the central part of the urals orogen however the centre of this crustal root is displaced by 50 80 km to the east of the present day maximum topography also beneath the magnitogorsk tagil island arc zone an upper crustal body with a high p wave velocity of 6 3 km s 1 at 4 9 km depth can be interpreted as consisting of mafic and or ultramafic rocks this in turn would help to explain the positive bouguer gravity anomaly and the surface heat flow minimum associated with the zone and would also be consistent with the known surface geology of the zone another major feature of the seismic model is the presence of high p and s wave velocities 7 5 and 4 2 km s 1 respectively at the base of the crustal root if the deeper parts of the thickened crust also have high densities small density contrast of about 0 1 g cm 3 with respect to the uppermost mantle then this helps to explain the absence of a pronounced gravity minimum associated with the root these high velocities and densities can be most easily explained by mafic rocks or a mix of mafic and ultramafic rocks within the structural framework of berzin et al 1996 these rocks would belong to the lower russian plate which was being subducted beneath the siberian plate during the uralian orogeny it is possible that the crustal root is formed from the remnants of oceanic crust or a mix of oceanic crust and mantle attached to the russian plate this would mean that little or no continental crust has been subducted or that subduction and hence the uralian orogeny stopped when there was no more oceanic crust or when an attempt was made to subduct lighter continental crust

the accretionary history of southern south america from the latest proterozoic to the late palaeozoic some palaeomagnetic constraints

seismic investigation of the continental margin off and onshore valparaiso chile abstract at the latitude of valparaiso chile a fundamental change in the configuration of the benioff zone volcanic activity and the structure of the continental margin occurs opposite the subducting juan fernandez ridge three legs of the german r v sonne cruises so101 so103 and so104 surveyed the continental margin and oceanic plate offshore valparaiso aiming at studying the crustal structure and investigating possible causes for the change in slab configuration sonne cruise so101 investigated the tectonic setting with swath mapping bathymetry magnetics and high resolution seismics following these investigations cruise so103 collected land sea wide angle seismic data and coincident deep seismic reflection data were acquired during cruise so104 coincident near vertical and wide angle seismic measurements were made along two profiles profile 1 located at the south of the study area away from the influence of the subducting ridge crosses the margin where thick trench sediment and an accretionary wedge near the trench is observed profile 2 located in the north runs from the juan fernandez ridge to the chilean coast the crustal velocity models obtained for the two profiles show that the continental crust extends to the middle lower slope boundary which is also reflected in morphology in addition they show that the crustal structure of the oceanic plate is rather similar but the plate seems to be slightly more inclined along the northern profile 13 versus 10 in the south the two profiles are only about 70 km apart but their structures differ significantly no straightforward correlation exists between the two profiles that can be attributed to ridge collision the data support that the 1985 central chile earthquake ruptured the plate boundary in the area that includes the segment boundary and mainly where continental crust forms the upper plate

lithospheric density structure of the southern central andes constrained by 3d data integrative gravity modelling the southern central andes sca between 27 s and 40 s is bordered to the west by the convergent margin between the continental south american plate and the oceanic nazca plate the subduction angle along this margin is variable as is the deformation of the upper plate between 33 s and 35 s the subduction angle of the nazca plate increases from sub horizontal 5 in the north to relatively steep 30 in the south the sca contain inherited lithological and structural heterogeneities within the crust that have been reactivated and overprinted since the onset of subduction and associated cenozoic deformation within the andean orogen the distribution of the deformation within the sca has often been attributed to the variations in the subduction angle and the reactivation of these inherited heterogeneities however the possible influence that the thickness and composition of the continental crust have had on both short term and long term deformation of the sca is yet to be thoroughly investigated for our investigations we have derived density distributions and thicknesses for various layers that make up the lithosphere and evaluated their relationships with tectonic events that occurred over the history of the andean orogeny and in particular investigated the short and long term nature of the present day deformation processes we established a 3d model of lithosphere beneath the orogen and its foreland 29 s 39 s that is consistent with currently available geological and geophysical data including the gravity data the modelled crustal configuration and density distribution reveal spatial relationships with different tectonic domains the crystalline crust in the orogen the magmatic arc and the main orogenic wedge is thicker 55 km and less dense 2900 kg m3 than in the forearc 35 km 2975 kg m3 and foreland 30 km 3000 kg m3 crustal thickening in the orogen probably occurred as a result of stacking of low density domains while density and thickness variations beneath the forearc and foreland most likely reflect differences in the tectonic evolution of each area following crustal accretion no clear spatial relationship exists between the density distribution within the lithosphere and previously proposed boundaries of crustal terranes accreted during the early paleozoic areas with ongoing deformation show a spatial correlation with those areas that have the highest topographic gradients and where there are abrupt changes in the average crustal density contrast this suggests that the short term deformation within the interior of the andean orogen and its foreland is fundamentally influenced by the crustal composition and the relative thickness of different crustal layers a thicker denser and potentially stronger lithosphere beneath the northern part of the sca foreland is interpreted to have favoured a strong coupling between the nazca and south american plates facilitating the development of a sub horizontal slab

upper lithospheric structure of the subduction zone offshore of southern arauco peninsula chile at 38 s velocities 8 4 km s 1 in the trench outer rise area the top of incoming oceanic plate is pervasively fractured and likely hydrated as shown by extensional faults horst and graben structures and a reduction of both crustal and mantle velocities these slow velocities are interpreted in terms of extensional bending related faulting leading to fracturing and hydration in the upper part of the oceanic lithosphere the incoming mocha fz coincides with an area of even slower velocities and thinning of the oceanic crust 10 15 thinning suggesting that the incoming fracture zone may enhance the flux of chemically bound water into the subduction zone slow mantle velocities occur down to a maximum depth of 6 8 km into the upper mantle where mantle temperatures are estimated to be 400 430c in the overriding plate the tomographic model reveals two prominent velocity transition zones characterized by steep lateral velocity gradients resulting in a seismic segmentation of the marine fore arc the margin is composed of three main domains 1 a 20 km wide frontal prism below the continental slope with vp 3 5 km s 1 2 a 50 km area with vp 4 5 5 5 km s 1 interpreted as a paleoaccretionary complex and 3 the seaward edge of the paleozoic continental framework with vp 6 0 km s 1 frontal prism velocities are noticeably lower than those found in the northern erosional chile margin confirming recent accretionary processes in south central chile

seismicity and shape of the subducted nazca plate an updated compilation of earthquake locations and focal mechanism solutions from the international seismological centre and preliminary determination of earthquakes is the basis of a comprehensive study of the geometry of the wadati benioff zone beneath western south america the new data support previous mapping of a sharp flexure rather than a tear in the subducted nazca plate beneath southern peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with 30 dip at latitude 33 s in contrast the transition from 30 slab dip beneath bolivia to a nearly horizontal dip in the region between 28 s 32 s is more gradual occurring over several hundred kilometers of along strike distance between 20 s and 32 s this southward flattening corresponds to a broadening of a horizontal benchlike part of the subducted plate formed between 100 and 125 km depth the transition in continental tectonic style near 27 s 28 s from a wide volcanically active plateau to a narrow nonvolcanic cordillera appears not to be associated with the main slab flattening which begins to the north of these latitudes but with a more abrupt change in curvature of the subducted slab from convex upward to concave upward immediately below the plate boundary interface the concept of gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the south american plate we hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths

late miocene high and rapid surface uplift and its erosional response in the andes of central chile 33 35 s 1 we address the question of the late cenozoic geomorphological evolution of the central chile andes 33 35 s using uplift markers river incision previous and new ages of volcanic bodies and new fission track ages the uplift markers consist of relicts of high elevated peneplains that evidence 2 km of regional surface uplift lasting 2 ma with variable amount along an e w transect the eastern coastal cordillera was uplifted 1 5 2 1 km at 33 34 s and 2 5 at 33 45 s and 1 5 km at 34 30 s erosional response to uplift was characterized by the retreat of a sharp knickpoint with celerities between 10 and 40 mm a 1 extrapolation using a stream power law shows that uplift began shortly before 4 ma or at 10 5 4 6 ma 7 6 ma central age depending on the morphostructural units involved the first alternative implies simultaneous uplift of the continental margin the second model the most reliable one implies that the uplift affected together the eastern coastal cordillera and the principal cordillera while the rest of the western fore arc subsided this regional uplift can be mostly balanced by crustal thickening resulting from coeval shortening related to the out of sequence thrusting event in the principal cordillera and the uplift of the frontal cordillera simultaneously emplacement of the southern edge of the flat slab subduction zone might have partially contributed to this uplift event

crustal thickness estimation beneath the southern central andes at 30 s and 36 s from s wave receiver function analysis summary we have used the s wave receiver function srf technique to investigate the crustal thickness beneath two seismic profiles from the charge project in the southern central andes a previous study employing the p wave receiver function method has observed the moho interface beneath much of the profiles they found however that the amplitude of the p to s conversion was diminished in the western part of the profiles and have attributed it to a reduction of the impedance contrast at the moho due to lower crustal ecologitization with srf we have successfully detected s to p converted waves from the moho as well as possible conversions from other lithospheric boundaries the continental south american crust reaches its maximum thickness of 70 km along 30 s between 70 w and 68 5 w beneath the principal cordillera and the famatina system and becomes thinner towards the sierras pampeanas with a thickness of 40 km negative phases possibly related to the base of the continental and oceanic lithosphere can be recognized in the summation traces at different depths by comparing our results with data obtained from previous investigations we are able to further constrain the thickness of the crust and lithosphere beneath the central andes

temporal and spatial relationships of thick and thin skinned deformation a case study from the malargue fold and thrust belt southern central andes abstract in this paper we analyse two end member models of temporal and spatial interactions between thick and thin skinned structures in a thrust front with pre existing rift structures in the most commonly accepted model a hinterland to foreland sequence of inversion of pre existing normal faults is proposed as a result the emplacement of shallow thrust sheets in the sedimentary cover occurs before the basement inversion in the foreland in the other model basin inversion occurs early in the deformation history of the external part of a fold and thrust belt as the result of a foreland to hinterland sequence of inversion the malargue fold and thrust belt 34 36 s formed in response to compression of the mesozoic neuquen basin during neogene to pleistocene times integrating detailed structural data from the northern part of this belt with new ar ar dating we propose a revised kinematic model of thick and thin skinned interaction and define the temporal spatial evolution of the belt comparison of the timing of deformation in the thick and thin skinned areas strongly supports the hypothesis that the reactivation of normal faults was coeval with the insertion of shallow detachments and low angle thrusting along the migrating front of the thrust belt and occurred from the foreland to the hinterland detachments occur at several stratigraphic horizons including a deep basement decollement related to the basement involved thrusting and shallow detachments located within the jurassic and cretaceous beds these shallow and deep detachments were coeval producing simultaneous development of thrusts during the complex deformation of the thrust front between 15 and 8 ma

crustal deformation in the south central andes backarc terranes as viewed from regional broad band seismic waveform modelling summary the convergence between the nazca and south america tectonic plates generates a seismically active backarc region near 31 s earthquake locations define the subhorizontal subducted oceanic nazca plate at depths of 90 120 km another seismic region is located within the continental upper plate with events at depths 1 80 th 45 55 km than those of the eastern sierras pampeanas vp 6 0 6 2 km s 1 vp vs 1 70 th 27 35 km in addition we observed an apparent distribution of reverse crustal earthquakes along the suture that connects those terranes finally we estimated average p and t axes over the charge period the entire region showed p and t axis orientations of 275 and 90 plunging 6 and 84 respectively

revealing the deep structure and rupture plane of the 2010 maule chile earthquake mw 8 8 using wide angle seismic data the 27 february 2010 maule earthquake mw 8 8 ruptured 400 km of the nazca south america plate boundary and caused hundreds of fatalities and billions of dollars in material losses here we present constraints on the fore arc structure and subduction zone of the rupture area derived from seismic refraction and wide angle data the results show a wedge shaped body 40 km wide with typical sedimentary velocities interpreted as a frontal accretionary prism fap landward of the imaged fap the velocity model shows an abrupt velocity contrast suggesting a lithological change which is interpreted as the contact between the fap and the paleo accretionary prism backstop the backstop location is coincident with the seaward limit of the aftershocks defining the updip limit of the co seismic rupture and seismogenic zone furthermore the seaward limit of the aftershocks coincides with the location of the shelf break in the entire earthquake rupture area 33 s 38 5 s which is interpreted as the location of the backstop along the margin published seismic profiles at the northern and southern limit of the rupture area also show the presence of a strong horizontal velocity gradient seismic backstop at a distance of 30 km from the deformation front the seismic wide angle reflections from the top of the subducting oceanic crust constrain the location of the plate boundary offshore dipping at 10 the projection of the epicenter of the maule earthquake onto our derived interplate boundary yielded a hypocenter around 20 km depth this implies that this earthquake nucleated somewhere in the middle of the seismogenic zone neither at its updip nor at its downdip limit

sedimentary fill of the chile trench 32 46 s volumetric distribution and causal factors the chile trench of the convergent continental margin of central chile is a sediment filled basin that stretches over 1500 km in a north south direction the sediment fill reflects latitudinal variations in climate as well as in the morphology and geology of chile but also of sediment transport processes to the trench and within the trench we try to untangle these signals by calculating the total volume and the latitudinal volume distribution of trench sediments and by relating this distribution to a number of factors that affect this pattern the volume calculation is based on a model geometry of the top of the subducting oceanic plate that is buried beneath trench sediments and the sea floor as measured by swath bathymetry we obtain the model geometry of the subducting plate by interpolating between depth converted seismic reflection profiles that cross the trench the total volume of the trench fill between 32 and 46 s is calculated to be 46000 500 km 3 the resulting latitudinal volume distribution is best explained by a sedimentation model that alternates between 1 glacial phases of high sediment flux from southern chile combined with active latitudinal sediment transport within the trench and 2 interglacial phases over which sediment input is dominated by local factors supplementary material top of the oceanic basement tob grid is available as ascii raw data files xyz at www geolsoc org uk sup18664

crustal structure of the south central andes cordillera and backarc region from regional waveform modelling summary we investigate the crustal structure in the andes cordillera and its backarc region using regional broadband waveforms from crustal earthquakes we consider seismic waveforms recorded at regional distances by the chile argentina geophysical experiment charge during 2000 2002 and utilize previous seismic moment tensor inversion results for each single station earthquake pair we fixed the source parameters and performed forward waveform modelling using ray paths that sample the crust of the highest elevation cordillera and the accreted terranes in the backarc region our investigation indicates that synthetic seismograms for our earthquake station geometry are most sensitive to crustal parameters and less sensitive to mantle parameters we performed a grid search around crustal thickness p wave seismic velocity vp and p t os wave seismic velocity ratio vp vs fixing mantle parameters we evaluated this waveform analysis by estimating an average correlation coefficient between observed and synthetic data over the three broadband components we identified all acceptable crustal models that correspond to high correlation coefficients that provide best overall seismogram fits for the data and synthetic waveforms filtered mainly between 10 and 80 s our results indicate along strike variations in the crustal structure for the north south high cordillera with higher p wave velocity and thickness in the northern segment north of 33 s and persistently high vp vs ratio 1 85 in both segments this is consistent with a colder mafic composition for the northern segment and a region of crustal thickening above the flat slab region in contrast the results for the current volcanic arc south of 33 s agree with a warmer crust consistent with partial melt related to quaternary volcanism presumably of an intermediate to mafic composition a distinctive feature in the backarc region is the marked contrast between the seismic properties of the cuyania and pampia terranes that correlates with their heterogeneous crustal composition the cuyania terrane composed of mafic ultramafic rocks exhibits high vp high vp vs and a thicker layered crust versus the thinner more quartzrich crust of the eastern sierras pampeanas associated with low vp and low vp vs these differences may have some effect on the mechanism that unevenly generates crustal seismicity in the upper 30 km in this active compressional region in particular the seismic properties of the cuyania terrane which shows evidence for a high vp and high vp vs crust may be reflecting the complex tectonic evolution history of this terrane including accretion rifting and re accretion processes since the palaeozoic that promote a high level of crustal seismicity in the upper 30 km enhanced by the flat slab subduction in the segment between 31 s and 32 s another possible mechanism could be directly related to the presence of a strong lower crust above the flat slab that efficiently transfer stresses from the slab to the upper crust generating higher seismicity in the cuyania terrane between 30 s and 34 s

a laurentian grenville age oceanic arc back arc terrane in the sierra de pie de palo western sierras pampeanas argentina

crustal thickness in the central andes from teleseismically recorded depth phase precursors summary although crustal thickness is a critical constraint for geodynamic models of andean orogenesis relatively few measurements exist from local seismic studies in this study we use reflections from the underside of the moho as illuminated by intermediate depth earthquakes to provide new estimates of crustal thickness for the central andes 16 34 s these reflected signals pmp are identified as precursors to the depth phase pp when recorded at teleseismic distances 35 85 although relatively small in amplitude the pmp phase is often clear even on single seismograms less obvious pmp phases were enhanced by stacking traces from arrays this method was most effective for events of m w 6 and depth 100 km crustal thickness determined in this study ranges from 59 to 70 km in southern peru from 49 to 80 km across the puna altiplano and from 50 to 60 km above the pampean flat slab the lack of such phases for certain events may be evidence of heterogeneity at the moho and some precursors appear to correspond to intracrustal magma and lithospheric mantle discontinuities the results obtained using the pmp technique combined with those from other studies show the crustal thicknesses under the central andes are more variable than expected from purely isostatic considerations we infer that one or a combination of geodynamic processes involving lower crustal flow lower crustal delamination and mantle crustal lithospheric coupling can explain the highly variable topography on the moho beneath the central andes

tectonic inversion in the santa barbara system of the central andean foreland thrust belt northwestern argentina 1 the santa barbara system sbs of northern argentina is a 400 km long segment of the subandean foreland thrust belt it is characterized by predominantly west verging relatively high angle thrust faults many of these faults are reactivated normal faults from one branch of a complex cretaceous to paleogene rift system heteroaxial folding in parts of the sbs is probably due to a slight component of range parallel dextral strike slip motion acting on preexisting faults striking north and ne during inversion regional balanced cross sections along two transects across the northern sbs indicate that the major faults flatten into a detachment in the basement at about 10 km depth neogene e w contraction in the sbs is of the order of 21 26 km rift extension is not very well constrained but was probably less than 10 km the structural style of the sbs differs from the thin skinned subandean thrust belt to the north and from the large wavelength sierras pampeanas basement uplifts to the south the changes between the different styles are sharp and coincide with the northern and southern boundaries of the rift in the foreland suggesting that crustal or lithospheric heterogeneities exert an overriding control on foreland structural style

navigational infrastructure at the east pacific rise 9 50 n area following the 2005 2006 eruption seafloor benchmarks and near bottom multibeam surveys 1 four seafloor benchmarks were deployed with rov jason2 at frequently visited areas along the northern east pacific rise nepr ridge crest near 9 50 n within the ridge2000 epr integrated study site iss bull s eye when used in concert with established deep ocean acoustic positioning techniques these benchmarks provide navigational infrastructure to facilitate the integration of near bottom data at this site by allowing efficient and quantitative coregistration of data and observations collected on multiple dives and over multiple cruises high resolution near bottom multibeam bathymetric surveys also were conducted along and across the ridge crest to provide a morphological and geological context for the benchmark areas we describe the navigation and data processing techniques used to constrain the benchmark positions and outline operational details to effectively use benchmarks at this and other deep ocean sites where multidisciplinary time series studies are conducted the well constrained positions of the benchmarks provide a consistent geospatial framework that can be used to limit navigational uncertainties during seafloor sampling and mapping programs and enable accurate spatial coregistration and integration of observations these data are important to test a range of multidisciplinary hypotheses that seek to link geological chemical and biological processes associated with crustal accretion and energy transfer from the mantle to the hydrosphere at mid ocean ridges

waveform tomography reveals channeled flow at the base of the oceanic asthenosphere understanding the relationship between different scales of convection that drive plate motions and hotspot volcanism still eludes geophysicists using full waveform seismic tomography we imaged a pattern of horizontally elongated bands of low shear velocity most prominent between 200 and 350 kilometers depth which extends below the well developed low velocity zone these quasi periodic fingerlike structures of wavelength 2000 kilometers align parallel to the direction of absolute plate motion for thousands of kilometers below 400 kilometers depth velocity structure is organized into fewer undulating but vertically coherent low velocity plumelike features which appear rooted in the lower mantle this suggests the presence of a dynamic interplay between plate driven flow in the low velocity zone and active influx of low rigidity material from deep mantle sources deflected horizontally beneath the moving top boundary layer

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

structure and tectonics of the central chilean margin 31 33 s implications for subduction erosion and shallow crustal seismicity the pre and current collision of the juan fernandez ridge with the central chilean margin at 31 33 s is characterized by large scale crustal thinning and long term subsidence of the submarine forearc caused by subduction erosion processes here we study the structure of the central chilean margin in the ridge trench collision zone by using wide angle and multichannel seismic data the transition from the upper to middle continental slope is defined by a trenchward dipping normal scarp with variable offsets of 500 2000 m height beneath the scarp the 2 d velocity depth models show a prominent lateral velocity contrast of 1 s 1 that propagates deep into the continental crust defining a major lateral seismic discontinuity the discontinuity is interpreted as the lithological contact between the subsided collapsed outermost forearc composed of eroded and highly fractured volcanic rocks and the seaward part of the uplifted coastal cordillera made of less fractured metamorphic igneous rocks extensional faults are abundant in the collapsed outermost forearc however landward of the continental slope scarp both extensional and compressional structures are observed along the uplifted continental shelf that forms part of the coastal cordillera particularly at the landward flank of the valparaiso forearc basin 32 33 5 s shallow crustal seismicity has been recorded in 2008 2009 forming a dense cluster of thrust events of mw 4 5 the estimated hypocentres spatially correlate with the location of the fault scarp and they highlight the upper part of the seismic crustal discontinuity

gravitational potential as a source of earthquake energy abstract some degree of tectonic stress within the earth originates from gravity acting upon density structures the work performed by this gravitational tectonics stress must have formerly existed as gravitational potential energy contained in the stress causing density structure according to the elastic rebound theory reid 1910 the energy of earthquakes comes from an elastic strain field built up by fairly continuous elastic deformation in the period between events for earthquakes resulting from gravitational tectonic stress the elastic rebound theory requires the transfer of energy from the gravitational potential of the density structures into an elastic strain field prior to the event an alternate theory involves partial gravitational collapse of the stress causing density structures the earthquake energy comes directly from a net decrease in gravitational potential energy the gravitational potential energy released at the time of the earthquake is split between the energy released by the earthquake including work done in the fault zone and an increase in stored elastic strain energy the stress associated with this elastic strain field should oppose further fault slip

the early paleozoic evolution of the argentine precordillera as a laurentian rifted drifted and collided terrane a geodynamic model paleontologic and stratigraphic evidence points to the early paleozoic precordilleran terrane of western argentina as being the conjugate rift pair of the appalachians stratigraphic similarities of the cambrian and early arenig carbonate series and very strong affinities among trilobite conodont and brachiopod faunas show their close relationship the most probable provenance areas are the appalachian ouachita rifted margin and the newfoundland appalachians although the former fits better with geometric and drifting paths suggested by faunal affinities increasing celtic and baltic brachiopod genera and divergent stratigraphy since the arenig indicate the drifting of the precordilleran terrane collisional foredeeps developed on collapsed former platform carbonates as flexural subsidence progressed the collision of the precordillera with western gondwana occurred during the mid llanvirn to llandeilo a magmatic arc related to eastward subduction present coordinates was active in the famatina range east of the precordillera this region of celtic affinity shows faunal exchange with the precordilleran terrane since the late arenig and may represent accreted intra iapetus volcanic island arc complexes the rifting and drifting stages are consistent with paleoclimatic and paleomagnetic data that show the migration of the precordilleran terrane from periequatorial to peripolar latitudes between the cambrian and latest ordovician the deep ocean to the west of the precordillera started to close by the late ordovician with the eastward drift of the chilenian terrane absence of volcanic or pyroclastic arc derived rocks in the precordillera indicate west dipping subduction as chilenia approached the continental margin a new forebulge was established on the former collided precordilleran terrane developing an erosional unconformity in central precordillera talacasto tambolar arch a gondwanic signature was fully developed by the middle silurian when the malvinokaffric clarkeia fauna flourished before then the late ordovician glacial record and associated hirnantia fauna were the first clear tie to gondwana during the silurian the marginal basin behaved as a foreland with lithosphere rheology and eustasy governing the sequence stratigraphy wrench faulting along its eastern boundary displaced the precordillera toward the south continued shortening during closure with the chilenian terrane in the mid devonian produced thrust loading of the basin and generated a thick graywacke succession final accretion of chilenia late devonian generated a regional angular unconformity between the lower and upper paleozoic new eastward subduction was initiated west of the accreted chilenian terrane during the late permian triassic as indicated by the choiyoi volcanic complex which presently outcrops in the frontal cordillera

seismic structure of the north central chilean convergent margin subduction erosion of a paleomagmatic arc we study the erosive convergent margin of north central chile at similar to 31 degrees s by using high resolution bathymetric wide angle refraction and multichannel seismic reflection data to derive a detailed tomographic 2 d velocity depth model in the overriding plate our velocity model shows that the lowermost crustal velocities beneath the upper continental slope are 6 0 6 5km s which are interpreted as the continental basement composed by characteristic metamorphic and igneous rocks of the coastal cordillera beneath the lower and middle continental slope however the presence of a zone of reduced velocities 3 5 5 0km s is interpreted as the outermost fore arc composed of volcanic rocks hydrofractured as a result of frontal and basal erosion at the landward edge of the outermost fore arc the bathymetric and seismic data provide evidence for the presence of a prominent trenchward dipping normal scarp similar to 1km offset which overlies a strong lateral velocity contrast from similar to 5 0 to similar to 6 0km s this pronounced velocity contrast propagates deep into the continental crust and it resembles a major normal listric fault we interpret this seismic discontinuity as the volcanic continental basement contact of the submerged coastal cordillera characterized by a gravitational collapse of the outermost fore arc subduction erosion has most likely caused large scale crustal thinning and long term subsidence of the outermost fore arc

geodynamic evolution and tectonostratigraphic terranes of northwestern argentina and northern chile in ordovician time gondwana in the area of northwestern argentina and northern chile had a west facing active margin the evolution of this margin culminated in the ocloyic orogeny at the end of ordovician time this orogeny was caused by the collision of the allochthonous arequipa antofalla terrane with this margin the early paleozoic evolution of northwestern argentina and northern chile contrasts markedly with the accretionary history of central argentina and central chile where the precordillera and chilenia terranes docked in the late ordovician and late devonian periods respectively an inspection of the available stratigraphic and geochronological data on sedimentary volcanic and plutonic units of the southern central andes of northern chile and northwestern argentina reveals a lull in magmatic and metamorphic activity lasting for 100 m y from early silurian to early late carboniferous time this is interpreted as corresponding to a tectonic scenario in which the present andean margin was a passive margin of gondwana this passive margin developed in response to the rifting off of a part of the arequipa antofalla terrane the present location of this block is unknown late carboniferous time marks the renewed onset of subduction initiating the andean plate tectonic setting still prevalent today recently proposed models explain the late ordovician orogeny by the collision of laurentia with western south america during laurentia s clockwise motion around south america and away from its position in the neoproterozoic supercontinent in its present form this hypothesis is difficult to reconcile with the paleozoic tectonostratigraphic evolution of the southern central andean region

3d density model of the central andes abstract we developed a 3d density model of the continental crust the subducted plate and the upper mantle of the central andes between 20 29 s and 74 61 w through the forward modelling of bouguer anomaly the goal of this contribution is to gain insight on the lithospheric structure integrating the available information geophysical geologic petrologic and geochemical in a single model the geometry of our model is defined and constrained by hypocentre location reflection and refraction on and offshore seismic lines travel time and attenuation tomography receiver function analysis magnetotelluric studies thermal models and balanced structural cross sections the densities allocated to the different bodies are calculated considering petrologic and geochemical data and pressure and temperature conditions the model consists of 31 parallel e w vertical planes where the continental crust comprises distinct bodies which represent the different morphotectonic units of the central andes we include a partial melting zone at midcrustal depths under the altiplano puna low velocity zone and consider the presence of a rheologically strong block beneath the salar de atacama basin according to recent seismic studies contour maps of the depth of the continental moho the thickness of the lower crust and the depth to the bottom of the lithosphere below south america are produced the possible percentage of partial melt in the central andes low velocity zone is estimated the residual anomaly is calculated by subtracting from the bouguer anomaly the gravimetric effect of the modelled subducted slab and of the modelled moho isostatic anomalies are calculated from regional and local isostatic mohos calculated with and without internal loads derived from our gravity model which are then compared to the modelled continental moho this study contributes to a more detailed knowledge of the lithospheric structure of this region of the andes and provides an integrated 3d density model which may be used by the geoscientific community as a tool that can help to understand and interpret the geodynamic features and processes acting along the central andes

tectonic shortening and crustal thickness in the central andes how good is the correlation exceptional thickening of continental crust beneath the central andes is believed to be mainly a result of tectonic shortening of the south american plate in neogene time this shortening has been estimated to have contributed as much as 70 80 of the present crustal volume a compilation of published shortening values and our own estimates based on balanced cross sections from the central andes between lat 3 s and 40 s suggest that 70 80 is a maximum rather than an average value for this part of the andes tectonic shortening and the crustal cross section area are only loosely correlated variations in tectonic shortening are more abrupt than those of crustal areas particularly near the northern and southern ends of the altiplano puna high plateau where thick crust is associated with relatively small amounts of shortening shortening there may account for no more than about 30 of the present crustal cross section area the processes that created the remaining crustal area are not clear but are likely to involve poorly constrained pre neogene tectonic shortening moderate magmatic additions to the crust tectonic underplating of material derived from the forearc and possible flow of ductile lower crust along strike

lithospheric and upper mantle structure of central chile and argentina summary the tectonics of central chile and argentina have been greatly affected by the shallow dips of the subducting nazca plate which controlled patterns of magmatism and deformation nearly 1000 km away from the plate boundary we calculate receiver functions from data recorded by the charge array which transected the andes and sierras pampeanas in central chile and argentina to better constrain the crustal structure of this region beneath the northern transect of the charge array where the nazca slab flattens near 100 km we find the crust is over 60 km thick beneath the andes and thin to the east the thick crust however extends 200 km to the east of the high elevations estimates of vp vs obtained from receiver functions vary along ancient terrane boundaries exhibiting higher values to the west interestingly we observe that the amplitude of the phase corresponding to the moho on receiver functions diminishes to the west complicating our images of crustal structure we proposed that the observations presented here of thickened crust within a region of low elevations diminished receiver function arrivals and reports of high shear wave speeds atop of the mantle wedge overlying the shallowly subducted nazca slab can be explained by partial eclogitization of the lower crust the moho appears simpler across the southern transect where it can be identified near 50 km depth at its deepest point beneath the andes and shallows eastwards volcanism remains active near the latitudes of our southern transect and we observe multiple crustal lowvelocity zones indicative of regions of partial melt near the centres of volcanism signals related to the nazca slab remain more elusive suggestive of a small impedance contrast between the slab and overlying mantle

provenance variations in the late paleozoic accretionary complex of central chile as indicated by detrital zircons abstract we present detrital zircon u pb shrimp age patterns for the central segment 34 42 s of an extensive accretionary complex along coastal chile together with ages for some relevant igneous rocks the complex consists of a basally accreted high pressure low temperature western series outboard of a frontally accreted eastern series that was overprinted by high temperature low pressure metamorphism eleven new shrimp detrital zircon age patterns have been obtained for meta turbidites from the central 34 42 s segment of the accretionary complex four from previously undated metamorphic complexes and associated intrusive rocks from the main andean cordillera and three from igneous rocks in argentina that were considered as possible sediment source areas there are no mesozoic detrital zircons in the accretionary rocks early paleozoic zircons are an essential component of the provenance and grenville age zircons and isolated grains as old as 3 ga occur in most rocks although much less commonly in the western series of the southern sector in the northernmost sector 34 38 30 s proterozoic zircon grains constitute more than 50 of the detrital spectra in contrast with less than 10 in the southern sector 39 42 s the youngest igneous detrital zircons in both the northern western 307 ma and eastern series 345 ma are considered to closely date sedimentation of the protoliths both oxygen and lu hf isotopic analyses of a selection of permian to neoproterozoic detrital zircon grains indicate that the respective igneous source rocks had significant crustal contributions the results suggest that early paleozoic orogenic belts pampean and famatinian containing material recycled from cratonic areas of south america supplied detritus to this part of the paleo pacific coast in contrast in the southern exposures of the western series studied here permian detrital zircons 253 295 ma dominate indicating much younger deposition the northern sector has scarce early to middle devonian detrital zircons prominent south of 39 s the sedimentary protolith of the northern sector was probably deposited in a passive margin setting starved of devonian achalian detritus by a topographic barrier formed by the precordillera and possibly chilenia terranes devonian subduction related metamorphic and plutonic rocks developed south of 39 s beyond the possible southern limit of chilenia where sedimentation of accretionary rocks continued until permian times

thick skinned and thin skinned thrusting in the la ramada fold and thrust belt crustal evolution of the high andes of san juan argentina 32 sl abstract the cenozoic structural evolution of the la ramada fold and thrust belt in the high andes of argentina between 32 00 s and 32 30 s shows in time and space the interaction of thin skinned and thick skinned deformations based on geometric and kinematic constraints three different deformation stages have been recognized the first began with thin skinned nnw trending structures detached in upper jurassic gypsum in the second stage the basement was involved by tectonic inversion of triassic normal faults producing a thick skinned fold and thrust belt this inversion combined with the ductile rheology of the jurassic gypsum resulted in refolding of older structures the uplift of the basement by high angle reverse faults at the ramada espinacito massif produced a sticking point in the foreland propagation of the thrust belt this was responsible for the third stage of deformation characterized by thin skinned nnw out of sequence thrusts developed in the westernmost sector the total andean shortening calculated in the area was 21 24 km near 30 the structure of the la ramada fold and thrust belt was integrated into a crustal scale cross section at 32 s latitude based on gravimetric models and surface geology a crustal model is proposed for the andes of chile and argentina a shortening of 180 km was calculated between the chile trench and the sierra de pie de palo this implies a mean deformation rate of 2 8 mm year 1 during cenozoic times an initial crustal thickness of ca 33 km was computed a two layer model with a middle detachment level is proposed to explain the spatial decoupling between maximum shortening at the surface and maximum crustal thickening the upper layer was balanced based on surface structural information the lower layer is interpreted to have been constructed by an imbricate thrust system controlled by the mercedario rift master shear and the precordillera chilenia suture

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

the late jurassic to present evolution of the andean margin drivers and the geological record 1 uncommonly long lived subduction and variable plate geometry along the south american andean plate margin resulted in diverse relationships between magmatic flux and extensional and contractional deformation as recorded by the overriding continental plate convergence velocities absolute overriding plate velocities and subducting slab ages were resolved along the trench from 170 ma to the present using a recently developed kinematic global plate model to identify any relationship between subduction conditions deformation style and magmatic features in the overriding plate key correlations reflect the dependence of macroscopic crustal strain style on subduction mechanism and relative plate vectors extensional back arc basins involving mafic oceanic crust developed only when the overriding plate velocity of south america was directed away from the trench and the modeled age of the subducting slab was older than 50 myr the development of fold and thrust belts and uplift of major plateaus was accompanied by trench normal convergence velocities in excess of 4 cm yr parameters investigated in this study revealed no correlation with the timing of major magmatic events nor was any correlation observed with the structural style of fold and thrust belts

goce satellite derived gravity and gravity gradient corrected for topographic effect in the south central andes region summary global gravity field models derived from satellite measurements integrated with terrestrial observations provide a model of the earth s gravity field with high spatial resolution and accuracy the earth gravity model egm08 a spherical harmonic expansion of the geopotential up to degree and order 2159 has been used to calculate two functionals of the geopotential the gravity anomaly and the vertical gravity gradient applied to the south central andes area the satellite only field of the highest resolution has been developed with the observations of satellite goce up to degree and order 250 the topographic effect a fundamental quantity for the downward continuation and validation of satellite gravity gradiometry data was calculated from a digital elevation model which was converted into a set of tesseroids this data is used to calculate the anomalous potential and vertical gravity gradient in the southern central andes region the geological structures are very complex but not well resolved the processing and interpreting of the gravity anomaly and vertical gradients allow the comparison with geological maps and known tectonic structures using this as a basis a few features can be clearly depicted as the contact between pacific oceanic crust and the andean fold and thrust belt the seamount chains over the oceanic nazca plate and the famatinian and pampean ranges moreover the contact between the rio de la plata craton and the pampia terrain is of great interest since it represents a boundary that has not been clearly defined until now another great lineament the valle fertil desaguadero mega lineament an expression of the contact between cuyania and pampia terranes can also be clearly depicted the authors attempt to demonstrate that the new gravity fields can be used for identifying geological features and therefore serve as useful innovative tools in geophysical exploration

crustal structure of the eastern sierras pampeanas of argentina using high frequency local receiver functions abstract the eastern sierras pampeanas are basement cored outcrops uplifted in the andean foreland where the easternmost segment of the pampean flat slab segment starts dipping more steeply into the deeper mantle these ranges of central argentina known as the sierras de cordoba have an enriched quartz composition and are bounded by a series of reverse faults different models have been suggested to represent the style of the thick skinned deformation in this area however the overall structure linking the exposed faults and terrane boundaries with their probable continuation at depth is unknown in this paper we present images of the crustal structure beneath the sierras de cordoba using the common conversion point stacking method of high frequency local receiver functions recorded by the esp broadband seismic array the work consists of two transects located around 31 s and 32 s across the sierras de cordoba the results show a consistent sharp moho signal associated with a high contrast in seismic velocities in good agreement with the granitic character of the crust lying above the mafic upper mantle the moho morphology varies exhibiting thicknesses of 38 km in the west to 35 km in the east with a vertical shifting under the sierra chica we relate this variable character to the presence of the boundary between the pampia terrane and the rio de la plata craton our results for the intra crustal structure indicate the presence of three discontinuities in the northern transect and at least two discontinuities in the southern transect these discontinuities appear vertically displaced beneath the surface traces of the major range bounding faults providing evidence for the continuation to mid crustal depths of the exposed reverse faults thus the reverse faults seem to have displaced several horizontal intra crustal interfaces finally in the lower crust we found a region which seems to be aseismic

anatomy of the andean subduction zone three dimensional density model upgraded and compared against global scale models summary we present an upgraded version of a previously published 3 d density model of the andean subduction zone between 18 s and 45 s this model consists of 3 d bodies of constant density which geometry is constrained by independent seismic data and is triangulated from vertical cross sections these bodies define the first order morphology and internal structure of the subducted nazca slab and south american plate the new version of the density model results after forward modelling the bouguer anomaly as computed from the most recent version of the earth gravitational model egm2008 the 3 d density model incorporates new seismic information to better constrain the geometry of the subducted slab and continental moho cmh and has a trench parallel resolution doubling the resolution of the previous model as an example of the potential utility of our model we compare the geometry of the subducted slab and cmh against the corresponding global models slab1 0 and crust2 0 respectively this exercise demonstrates that although global models provide a good first order representation of the slab and upper plate crustal geometries they show large discrepancies up to 40 km with our upgraded model for some well constrained areas the geometries of the slab lithosphere asthenosphere boundary below the continent cmh and intracrustal density discontinuity that we present here as supporting information can be used to study andean geodynamic processes from a wide range of quantitative approaches

a lithospheric velocity model for the flat slab region of argentina from joint inversion of rayleigh wave phase velocity dispersion and teleseismic receiver functions fil ammirati jean baptiste ministerio de ciencia tecnologia e innovacion productiva agencia nacional de promocion cientifica y tecnologica argentina universidad nacional de san juan facultad de ciencias exactas fisicas y naturales departamento de geofisica y astronomia argentina

late paleozoic to jurassic silicic magmatism at the gondwana margin analogy to the middle proterozoic in north america a vast region of upper paleozoic to middle jurassic 300 150 ma silicic magmatic rocks that erupted inboard of the gondwana margin is a possible phanerozoic analogue to the extensive middle proterozoic 1500 1350 ma silicic magmatic province that underlies much of the southern mid continent of north america like the north american rocks the gondwana silicic magmas appear to be melts of crust that formed about 200 300 m y earlier in the north american case this older crust formed and was accreted to the continent during a major period of crustal formation 1700 1900 ma whereas in the gondwana case the crust that melted consisted mainly of magmatic are terranes accreted to the continental margin during the paleozoic in both cases basic to intermediate magmatic rocks are extremely rare and magmatism is less abundant in regions that contain older and previously melted crust the similarities between the north american and gondwana silicic rocks suggest that both suites formed in extensional settings where basaltic magmas ponded at the base of the preheated crust caused extensive crustal melting that inhibited upward passage of the basalts in both cases silicic volcanism occurred after major assembly of a supercontinent by subduction and accretion processes and before breakup of the supercontinent by analogy with the polar wander curves for gondwana the granite rhyolite provinces may have formed during a period of very slow motion of the supercontinents relative to the poles

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

three dimensional density model of the nazca plate and the andean continental margin 1 we forward modeled the bouguer anomaly in a region encompassing the pacific ocean 85 w and the andean margin 60 w between northern peru 5 s and patagonia 45 s the three dimensional density model that reproduces the gravity field is a continental scale representation of density structure to 410 km depth that characterizes the mantle and crust of the oceanic nazca plate subducted slab and continental margin with a minimum number of bodies we predefined the density of each body after studying the dependency of density on composition of crustal and mantle materials and pressure temperature conditions appropriate for the andean setting a database of independent geophysical information constrains the geometry of the top of the subducted slab locally the moho of the oceanic and continental crusts and indirectly the lithosphere asthenosphere boundary underneath the continental plate other boundaries notably the intracrustal density discontinuity separating upper from lower crust below the continent were not constrained and their geometry is the result of fitting the observed and calculated bouguer anomaly during forward modeling this contribution presents the model to the andean geoscientific community and contains some tools like a sensitivity analysis that helps potential users of the model to interpret its results we describe and discuss some of these results in order to illustrate the application of the model to the study of a wide range of phenomena e g modification of oceanic plate structure by hot spots shape of the subducted slab thermal structure of the continental lithosphere compensation mechanism and formation of orogenic relieve causes of andean segmentation

the morning glory wave of southern california a pulse like disturbance traveling across the los angeles basin was observed on 12 october 2001 with seismographs of the trinet network this wave had a period of about 1000 s and a propagation speed of about 10 m s much slower than seismic waves the seismograph data were compared with barograph data and a good correlation was found so the wave was determined to be atmospheric in origin it had amplitude of about 1 mbar but it was not known what process could produce such a wave since the initial finding we have inspected all the trinet barograph and seismograph data for a period of two and a half years from january 2000 to august 2002 and found four more similar events each of the events has amplitude between 0 8 and 1 3 mbar a period between 700 and 1400 s and a propagation speed between 5 and 25 m s we conclude that these waves are internal gravity waves trapped in a stable layer formed by a temperature inversion some of these waves have large amplitudes and develop into solitary waves nonlinear internal gravity waves similar to the spectacular morning glory wave observed in australia we call these waves the la morning glory waves the la morning glory wave is probably excited by either stormy weather winds such as the santa ana winds or large teleseismic events the morning glory wave could contribute to the recently reported excitation of the background free oscillations of the earth additionally because of its large amplitude it could have important implications for aviation safety as was suggested earlier for the morning glory waves in australia

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

coupled tectonic evolution of andean orogeny and global climate abstract the largest tectonic relief breaking the earth s surface 13 km vertically is at the subduction margin of the andes which generates routinely megathrust earthquakes mw 8 5 and drives the paradigmatic andean orogen here we present key geologic evidence to reassess first order features of geomorphology and tectonics across the central andes where the orogen includes the altiplano plateau and attains its maximum integrated height and width the andean subduction margin has a stepped morphology dominated by the low relief atacama bench which is similar to a giant uplifted terrace slopes gently over a width of 60 100 km from the andes to the pacific and runs over more than 1000 km of coastal length we find that the genesis of stepped morphology at the andean seaboard is due to concomitant development of large west vergent thrusts parallel to the subduction interface and increasing aridity in the atacama desert which keeps an unprecedented large scale record of interplaying tectonics and cenozoic climate change incorporating our results with published geological knowledge demonstrates that andean orogeny is characterized by trench perpendicular bivergent and trench parallel bilateral growth over the past 50 myr associated with positive trench velocity toward the continent trench advance and subduction of a wide slab under south america we hypothesize that a global plate tectonic reorganization involving long lasting viscous mantle flow has probably forced both andean orogeny and global climate cooling since 50 ma in contrast two important stepwise pulses of increasing aridity and trench perpendicular andean growth appear to be results of changes in erosion rates due to global late eocene and middle miocene cooling events

tectonic control of the subducting juan fernandez ridge on the andean margin near valparaiso chile near the latitude of valparaiso chile a fundamental change in configuration of the benioff zone volcanic arc activity and the structure of the continental margin occurs opposite the subducting juan fernandez ridge upper plate tectonics related to subduction of the ridge were studied by an international group of geoscientists in the two degree segment offshore valparaiso extending from the shelf edge seaward across the eastern end of juan fernandez ridge near the o higgins group of seamounts the juan fernandez ridge strikes northeast rather than continuing its east west trend across the pacific basin the ridge uplifts the upper plate and sediments of the valparaiso basin are deformed against its southern flank this deformation is consistent with the southward migration required by an oblique trending ridge and the nearly trench normal vector of plate convergence in the trench axis the ridge forms a basement barrier behind which sediments 2 5 km deep have ponded the lower slope over the ridge appears eroded whereas the margin not yet affected by ridge subduction is fronted by an accretionary prism about 25 km wide nazca plate relief clearly influences tectonism of the margin where it subducts beneath thin continental crust its relation to deeper processes segmenting the andean orogen appears to involve prior tectonic events

automatic boundary extraction from magnetic field data using triangular meshes boundary extraction is a collective term that we use for the process of extracting the locations of faults lineaments and lateral boundaries between geologic units using geophysical observations such as measurements of the magnetic field the process typically begins with a preprocessing stage where the data are transformed to enhance the visual clarity of pertinent features and hence improve the interpretability of the data the majority of the existing methods are based on raster grid enhancement techniques and the boundaries are extracted as a series of points or line segments in contrast we set out a methodology for boundary extraction from magnetic data in which we represent the transformed data as a surface in 3d using a mesh of triangular facets after initializing the mesh we modify the node locations such that the mesh smoothly represents the transformed data and that facet edges are aligned with features in the data that approximate the horizontal locations of subsurface boundaries to illustrate our boundary extraction algorithm we first apply it to a synthetic data set we then apply it to identify boundaries in a magnetic data set from the mcfaulds lake area in ontario canada the extracted boundaries are in agreement with known boundaries and several of the regions that are completely enclosed by extracted boundaries coincide with regions of known mineralization

the control of pre existing extensional structures on the evolution of the southern sector of the aconcagua fold and thrust belt southern andes abstract the aconcagua fold and thrust belt located in the andean mountains at 32 30 to 34 s has been described as a classic model of a thin skinned thrust belt however new structural data from its southern sector have shown that it has a complex structural framework reflected in multiple mesozoic extensional phases overprinted by structural inversion as well as thin and thick skinned tectonics two major superimposed extensional structural styles have been identified for the mesozoic characterized by distinctly oriented stress fields a key role in the evolution of this part of the fold and thrust belt was played by a late triassic to early jurassic depocentre and by late jurassic block faulting shortening was accommodated by a combination of inversion of pre existing normal faults development of footwall short cuts and both thin and thick skinned thrusting synrift and postrift sedimentary rocks were uplifted by reactivation of normal faults with further shortening along newly formed thin skinned thrust faults the geometry of thin skinned fault systems is controlled by the architecture of the rift basin competent footwalls forming barriers to the lateral propagation of detachments

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

global shear speed structure of the upper mantle and transition zone s u m m a r y the rapid expansion of broad band seismic networks over the last decade has paved the way for a new generation of global tomographic models significantly improved resolution of global upper mantle and crustal structure can now be achieved provided that structural information is extracted effectively from both surface and body waves and that the effects of errors in the data are controlled and minimized here we present a new global vertically polarized shear speed model that yields considerable improvements in resolution compared to previous ones for a variety of features in the upper mantle and crust the model sl2013sv is constrained by an unprecedentedly large set of waveform fits 3 4 of a million broad band seismograms computed in seismogram dependent frequency bands up to a maximum period range of 11 450 s automated multimode inversion of surface and s wave forms was used to extract a set of linear equations with uncorrelated uncertainties from each seismogram the equations described perturbations in elastic structure within approximate sensitivity volumes between sources and receivers going beyond ray theory we calculated the phase of every mode at every frequency and its derivative with respect to sand p velocity perturbations by integration over a sensitivity area in a 3 d reference model the normally small perturbations of the 3 d model required to fit the waveforms were then linearized using these accurate derivatives the equations yielded by the waveform inversion of all the seismograms were simultaneously inverted for a 3 d model of shear and compressional speeds and azimuthal anisotropy within the crust and upper mantle elaborate outlier analysis was used to control the propagation of errors in the data source parameters timing at the stations etc the selection of only the most mutually consistent equations exploited the data redundancy provided by our data set and strongly reduced the effect of the errors increasing the resolution of the imaging our new shear speed model is parametrized on a triangular grid with a 280 km spacing in well sampled continental domains lateral resolution approaches or exceeds that of regionalscale studies the close match of known surface expressions of deep structure with the distribution of anomalies in the model provides a useful benchmark in oceanic regions spreading ridges are very well resolved with narrow anomalies in the shallow mantle closely confined near the ridge axis and those deeper down to 100 120 km showing variability in their width and location with respect to the ridge major subduction zones worldwide are well captured extending from shallow depths down to the transition zone the large size of our waveform fit data set also provides a strong statistical foundation to re examine the validity field of the jwkb approximation and surface wave ray theory our analysis shows that the approximations are likely to be valid within certain time frequency portions of most seismograms with high signal to noise ratios and these portions can be identified using a set of consistent criteria that we apply in the course of waveform fitting

shear wave velocities in the pampean flat slab region from rayleigh wave tomography implications for slab and upper mantle hydration 1 the pampean flat slab region located in central argentina and chile between 29 and 34 s is considered a modern analog for laramide flat slab subduction within western north america regionally flat slab subduction is characterized by the nazca slab descending to 100 km depth flattening out for 300 km laterally before resuming a more normal angle of subduction flat slab subduction correlates spatially with the track of the juan fernandez ridge and is associated with the inboard migration of deformation and the cessation of volcanism within the region to better understand flat slab subduction we combine ambient noise tomography and earthquake generated surface wave measurements to calculate a regional 3d shear velocity model for the region shear wave velocity variations largely relate to changes in lithology within the crust with basins and bedrock exposures clearly defined as low and high velocity regions respectively we argue that subduction related hydration plays a significant role in controlling shear wave velocities within the upper mantle in the southern part of the study area where normal angle subduction is occurring the slab is visible as a high velocity body with a low velocity mantle wedge above it extending eastward from the active arc where flat slab subduction is occurring slab velocities increase to the east while velocities in the overlying lithosphere decrease consistent with the slab dewatering and gradually hydrating the overlying mantle the hydration of the slab may be contributing to the excess buoyancy of the subducting oceanic lithosphere helping to drive flat slab subduction

overriding plate controls on subduction evolution geologic and geophysical observations indicate that the thickness density and strength of the lithosphere vary on the earth however the role of the overriding plate lithosphere properties on the evolution and morphology of subduction is not well understood this paper presents time dependent numerical models of subduction that vary the overriding plate thickness strength and density and allow for a plate interface that evolves with time via an anisotropic brittle failure rheology we examine the effect of these parameters on subduction evolution in particular the emergence of a asymmetric versus symmetric subduction b trench retreat versus advance c subduction zone geometry d slab stagnation versus penetration into the lower mantle and e flat slab subduction almost all of the models presented result in sustained asymmetric subduction from initiation trench advance occurs in models with a thick and or strong overriding plate slab dip measured at a depth below the plate boundary interface has a negative correlation with an increase in overriding plate thickness overriding plate thickness exerts a first order control over slab penetration into the lower mantle with penetration most commonly occurring in models with a thick overriding plate periods of flat slab subduction occur with thick strong overriding plates producing strong plate boundary interface coupling the results provide insight into how the overriding plate plays a role in establishing advancing and retreating subduction as well as providing an explanation for the variation of slab geometry across subduction zones on earth where similar patterns of evolution are observed

cenozoic tectonics of the high andes of west central argentina 30 36 s latitude abstract the structure of the central andes shows three distinctive segments characterized by different geometries these geometries are superimposed on the present large scale plate tectonic setting characterized by distinct subduction segments the northern la ramada segment is a thick skinned fold and thrust belt formed by tectonic inversion of a late triassic rift the central aconcagua segment consists of a thin skinned fold and thrust belt while the southern malargue segment like the first one is a thick skinned fold and thrust belt developed by tectonic inversion of a late triassic early jurassic rift system during late cenozoic times the amount of shortening gradually decreases from north to south as indicated by the crustal roots of the central andes the different geometries along the principal cordillera controlled the abrupt changes in the shortening among segments the structure of precordillera and sierras pampeanas has also been considered in order to account for the total shortening in the la ramada segment the main shortening occurred in the precordillera in the aconcagua segment in the principal cordillera while in the malargue segment the shortening is widely distributed in a broader principal cordillera because south of the flat slab subduction segment the precordillera and sierras pampeanas are missing