T31A-0267
3D tomographic structure of the north andean subduction zone at the Colombia-Ecuador border
At the latitude of Ecuador - southern Colombia, the Nazca plate converges toward the South American plate along an ~E-W direction at a rate of about 6 cm/yr. Several large subduction earthquakes affected this area during the last century. Near the Ecuador-Colombia border the 500 km long rupture zone of the 1906 event (M = 8.8) was partially reactivated, from south to north, by a sequence of 3 thrust events in 1942 (Mw = 7.8), 1958 (Mw = 7.7) and 1979 (Mw = 8.2). From 1998 to 2005, this zone was the target of five marine geophysical campaigns in order to determine the shallow and deep structure of the margin, its deformation and the possible relation with the rupture zone of the major earthquakes. Bathymetric data, passive and active seismic data were collected off South-Colombia and Ecuador. The data suggest that the interplate earthquakes and the extension of their rupture zone are at least partly controlled by structures on the downgoing and upper plates. To the south the subduction of the buoyant Carnegie Ridge, with a up to 19 km thick crust, is inferred to partially lock the plate interface along central Ecuador. This is illustrated by the rupture zones during the 1942 and 1906 earthquakes that terminated against the subducted northern flank of the ridge. The margin wedge is segmented by transverse crustal faults that correlate with the limits of the earthquake coseismic slip zones at the limit between the 1942 and 1958 rupture zones as well as at the limit between the 1958 and 1979 rupture zones. Furthermore, seaward of the 1958 rupture zone, a 2D profile from the SALIERI experiment (2001) highlights that the margin seems to overthrust a low velocity outer basement high along a splay fault that would decouple the bulk of the margin basement from its frontal part during great earthquake rupture. During, the 3D Esmeraldas experiment, conducted from February to June 2005, 34 3-components portable stations were installed on land and 26 3-components Ocean Bottom Seismometers were deployed offshore on top of the 1958 earthquake rupture zone. This network of seismometers recorded over 19,000 shots, using a 8x16 liters air-gun seismic source, along 25 crossing profiles for a total of ~3,000 km as well as seismic activity during a period of ~3 months. We first constrained a 1-D velocity model that best fits the bathymetry-corrected travel times of Pg and Pn arrivals from OBS located on the margin by forward modeling. This velocity model is used as an initial model for tomographic inversion using the FAST code. Preliminary results of the tomographic inversion provide some new insights of the structure of the margin and of the interplate contact.
T31A-0268
The Southern Chilean Subduction Zone: Local Earthquake Tomography and State of Stress
While the northern and central part of the South American Subduction zone has been intensively studied the southern part has attracted less attention, which might be caused by its difficult accessibility and lower seismic activity. However, the southern part exhibits strong seismic and tsunamogenic potential with the prominent example of the Mw=9.5~May~22, 1960 Valdivia earthquake. Here we present data from an amphibious seismic array (Project TIPTEQ) located between 41.5--43.5°S reaching from the trench to the active magmatic arc incorporating the Island of Chiloé and the magmatic arc with the N-S trending Liquiñe-Ofqui fault zone~(LOFZ). 364 local events were observed in a 11-month period from November 2004 until October 2005 with magnitudes between 0.5 to 5.1~Ml. The observed seismicity allows us to constrain the current state of stress of the subducting plate and magmatic arc as well as the local seismic velocity structure. The downgoing Wadati-Benioff zone is readily identifiable as an eastward dipping plane with an inclination of about 29-33°. Besides events in in the Benioff Zone, 75~shallow crustal events with depths shallower than 25~km were observed mainly occurring in different clusters along the magmatic arc. These crustal clusters of seismicity are related to the LOFZ, to the volcanoes Chaitén, Michinmahuida and Corcovado, and to active faulting on secondary faults. Further activity along the LOFZ is indicated by individual events located in direct vicinity of the LOFZ. Focal mechanisms were calculated using moment tensor inversion of amplitude spectra for body waves which mostly yield strike slip mechanisms with SE-NW oriented direction of~σ1~for the LOFZ at this latitude. In contrast to the strike-slip mechanism of the events along the LOFZ, the focal mechanisms of the stronger events in the Benioff Zone yield mainly thrust mechanism down to 55~km depth. Focal Mechanism Stress Inversion (FMSI) was carried out for the crustal events and the events in the Benioff zone indicating a strike slip regime along the arc and thrust regime in the Benioff zone, respectively. We suggest that the observed deformation along the LOFZ combined with teleseismic observations is a confirmation for the proposed northward movement of the forearc sliver acting as a detached continental micro-plate as outlined by Forsythe~& Nelson (1985) and Beck et al.~(1993). A high quality subset of events was inverted for a 2-D velocity model using the SIMULPS inversion code. First results suggest a 10~km high mantle bulge (vp>7.8~km/s) below the longitudinal valley and low velocity structure below the coastal cordillera.
T31A-0269
Crustal and Upper Mantle Structure beneath Central South America Deduced from ScS Reverberation Waveforms and Receiver Functions
We investigated the crust and upper mantle structure beneath the central South America, approximately at 20 degree south, by modeling ScS reverberation waveforms and receiver functions at GSN stations. ScS reverberation waveforms are sensitive to the path-averaged S-wave velocities, attenuation, and depths and reflection coefficients of discontinuities in the crust and mantle. Our procedure to determine these parameters follows that of Kato et al. (2001) utilizing ray-based theoretical waveform modeling and grid search optimization. Finiteness of source time duration is additionally taken into consideration in synthesizing waveforms of a great earthquake. Depths of mantle discontinuities were separately estimated from arrival times of PS conversion phases on receiver functions. We also modeled receiver functions from early P coda to determine crustal structure. In this study, significant differences were found between the western coast and eastern continent. Beneath BDFB in the eastern continent, the Conrad and Moho were estimated at depths of 22 and 40 km, respectively. Beneath NNA in the western coast, four discontinuities were detected in a range of depth from 25 to 70 km. Observations on the receiver functions imply an inclination of the deep discontinuities, which can be the manifestations of the top of the subducting Nazca plate. A high value of average Q was obtained for the upper mantle beneath NNA, but not beneath BDFB. Since the value is much higher than predicted from the expected mantle transition zone temperature, the high Q region might exist in the uppermost mantle, being attributed to the subducting Nazca plate. In the transition zone, the 410-km discontinuity is shallow beneath the western coast. The mantle transition zone is thinner in the western coast than in the eastern continent. On the receiver functions in the western coast, we were able to identify a phase possibly from the 520-km discontinuity. Using SS reflection coefficients from the ScS reverberations and PS conversion coefficients from the receiver functions, we attempted to obtain the upper bounds of density and Vs contrasts at mantle discontinuities.
T31A-0270
Seismic Structure of the Crust and Uppermost Mantle of South America and Surrounding Ocean Basins
We present a new set of contour maps showing the seismic structure of South America and the surrounding ocean basins. These maps include crustal thickness, whole-crustal average P-wave and S-wave velocity, and the seismic velocity of the uppermost mantle, i.e., Pn and Sn. These maps include much new seismic reflection/refraction data from the continent. By summarizing all available data in this way, we are able to compare the crust of South America to other continents and the global crust in general. Our conclusions show: (1) The average weighted thickness of the crust under South America is 37.4 km (standard deviation, s.d. +/- 9.3 km), which is 1.8 km thinner than the global average of 39.2 km (s.d. +/- 8.5 km) for continental crust. (2) Histograms of whole-crustal P- and S-wave velocities for the South American crust are bi-modal, with the lower peak occurring for crust that appears to be missing a high-velocity (6.9-7.3 km/s) lower crustal layer. (3) A region across northern Chile and east into Argentina has an anomalously low P- and S-wave crustal velocity structure. This likely corresponds to the shallowly-subducted portion of the Nazca plate ("flat slab" of Isacks et al., 1968) which is also a region of extension. Thus, it may be tectonically analogous to the Basin and Range province of the southwestern United States. (4) The average Pn velocity beneath South America is 8.06 km/s (s.d. +/- 0.16 km/s). (5) The well-known thick crust of the Brazilian craton appears to extend into Venezuela and Colombia. (6) The crust may be thinned by extension in the Amazon basin and along the western edge of the Brazilian craton. The western edge of the continental crust correlates with the Chile-Peru trench, while the eastern edge generally correlates with the Atlantic passive margin. (7) The average Pcc velocity (P-wave velocity of the crystalline crust) is 6.43 km/s (s.d. +/- 0.25 km/s). This is only slightly lower than the global average of 6.45 km/s. (8) The average crustal P-wave velocity under the eastern Pacific seafloor is higher than under the western Atlantic seafloor due to the thicker sediment layer on the older Atlantic seafloor, as noted in previous work (Chulick and Mooney, 2002).
T31A-0271
Crustal Seismicity and 3-D Velocity Structure in the Principal Cordillera of Central Chile (33- 34.5 S, 69.5-71 W): Implications on Andean Geodynamic and Seismic Hazard
Based on data from a dense local temporary seismological network, crustal seismicity is characterized, and a 3- D body wave velocity structure is obtained by tomographic inversion down to the subducted slab. In the framework of Fondecyt 1050758, GeoAzur-IRD and ACT-18 projects, 35 broadband and short period instruments, were deployed in the studied zone for 135 days recording in continuous mode. At this zone the Andean active volcanism reappears after a gap of volcanic activity since late Miocene occurring north of 33 S due to the Central Chile flat slab subduction zone. Crustal seismicity in the depth range 0-30 km is well correlated with known geological faults that become now important in the assessment of the regional seismic hazard. This seismicity also clusters around the giant porphyry cooper deposits in the region (Rio Blanco, El Teniente), and are neither related to mine-blasts nor induced by mining activity. Moreover, the local 3-D velocity structure shows that the zone surrounding each deposit is characterized by high Vp/Vs greater than 1.8, which may indicate fluid phases located in the weakest and more fractured zone of the crust. The body wave velocity pattern shown at depth by the local tomography indicates channels of high Vp/Vs connecting the subducted slab with the surface at places where active volcanism is present, suggesting upward migration of hydrous or melted rocks. This pattern agrees with the one observed with a previous regional tomography that includes this zone, while this Vp/Vs pattern tends to be horizontal at the flat slab zone. At depths of 20-25 km, a layer of high Vp/Vs is observed beneath the Andes Cordillera that could be associated to changes in the rheological properties between the upper and lower crust, or to accumulation of magma. The average stress tensor, derived from focal mechanisms, indicate that the Andean zone is under compression in the plate convergence direction.
T31A-0272
Double-Difference Relocation of an Earthquake Nest at Bucaramanga, Colombia: Interaction Between Two Slabs?
For an earthquake nest at Bucaramanga, Colombia, that has dimensions comparable to the uncertainty of global earthquake bulletins (> 10 km), teleseismic relocation with high precision is achieved by applying a double- difference (DD) algorithm to the pick arrival times from the EHB bulletin of Engdahl et al. (1998) and waveform cross-correlation (WCC) measurements of correlated earthquakes. DD relocations using phase picks alone have relative location uncertainty less than 4 km. They concentrate on a small fault-like volume that has approximate dimensions of 20km x 10km x 10km. A map view of the relocated seismicity shows an elongated structure striking at ~ 135 degree. An on-fault view indicates, on a vertical plane, an approximately 10 km thick band of seismicity, tilting ~ 40 degree to the SE and sub-parallel to the subducting Nazca plate in this area. Further location improvement for 33 correlated events obtained by including WCC measurements in the relocation procedure images a complex near-vertical seismic zone with a width of ~ 5 km. Global CMT solutions for 23 earthquakes within this small volume show highly variable focal mechanisms, independently indicating nonuniform slip within the zone. In general, however, the P axes align with the WSW-orientation of the relative movement between the Nazca and the Caribbean plate. We propose a slab-slab interaction model, in which the nest may represent a contact zone where the two subducting slabs collide and slide past each other.
T31A-0273
Assessment of Double Benioff Zone Layer Separations Using a Modified JHD Method
We apply a modified joint hypocenter determination (JHD) method to relocate intermediate-depth events in order to revisit a global assessment of Double Benioff Zone (DBZ) layer separations. We analyze events from the EHB catalog for the slab segments in Brudzinski et al. (2007), including Nazca, Alaska, Aleutians, Central America, Kurile-Kamchatka, etc. The modified JHD algorithm computes arrival time residuals for each source-receiver pair for a given phase, which could be first arrivals, depth phases or converted phases, and solves for station corrections for each station. The inclusion of more phases, especially depth phases, during relocation significantly stabilizes the location process and improves the relative location accuracy. The relocated events yield improved estimates for the separation between the two DBZ layers. Brudzinski et al. (2007) found the separation to be highly correlated with subducting slab age, and generally consistent with antigorite dehydration as the mechanism responsible for the intermediate-depth seismicity. Our initial results suggest that the layer separation may be underestimated for some of the younger slabs, implying that chlorite dehydration may also play a role in intermediate-depth earthquake generation in younger slabs.
T31A-0274
Mantle flow and lithospheric detachment beneath the Puna plateau: Insights from seismic anisotropy.
Collision of the Nazca and South American plates in the central Andes has generated the Altiplano-Puna plateau, the second largest continental plateau on Earth. A range of mechanisms has been proposed to explain this feature, including magmatic addition, tectonic shortening and lithospheric thinning. Recent deployments of temporary seismic networks in the region are providing seismic images that help constrain the mechanism. Beneath the Puna plateau and between 23°S and 24°S, travel-time and attenuation tomography have illuminated a near-vertical high-velocity and high-Qp structure, which has been interpreted as detached continental lithosphere. Here we use observations of shear-wave splitting in both local and teleseismic phases to constrain the style of anisotropy beneath the region and help interpret deformation in the upper-mantle wedge. Splitting measurements are made at nearly 70 stations using 7 teleseismic (SKS) events and 16 local events, which range in depth from 90~km to 250~km. The magnitude of the splitting is highly variable throughout the region, ranging from 0.1secs to 1.4secs in the local data and ranging from 0.4 to 2.2 secs in the SKS data. In general the magnitude of the splitting for the local events is much larger than that seen in other subduction regions and can explain most of the SKS splitting. The polarization of the fast shear-wave is generally N-S beneath the Eastern Cordillera and to some extent also beneath the Western Cordillera. Beneath the central volcanic region the polarizations are more variable. Between 23°S and 24°S there is significant deviation in the splitting to a more E-W polarization in the fast shear-wave. We interpret this is a being due to a perturbation in the mantle flow around the region of detached lithosphere. A lack of obvious depth dependence in the splitting suggests that deformation is most pronounced in the region between the detached lithosphere and crust. Anisotropy in the thickened lower crust may also contribute to the observed splitting. It would seem that detachment has disturbed the convective regime of the upper-mantle wedge and the signature of this is revealed in the pattern of shear- wave splitting.
T31A-0275
Upper Mantle Anisotropy in South America With SKS Splitting: Predominance of Asthenospheric Flow From Absolute Plate Motion and Test of the Frequency-Dependent Polarization Filter in the SKS Analysis
Upper mantle anisotropy, due to lattice preferred orientation of minerals, like olivine, can be caused by shear deformation from the last major orogeny (usually preserved in the lithosphere) or from the lithosphere/asthenoshere strain related to the present absolute plate motion. Anisotropy causes shear wave splitting, with the fast polarization direction usually sub-parallel to the flow direction of the upper mantle rocks. Upper mantle anisotropy in Brazil has been studied with SKS and SKKS shear wave splitting, where the delay time and the fast polarization directions were measured with the method of Silver and Chan (1991). Measurements of splitting parameters have been made at 10 stations of the BLSP02 project, mainly in northern and NE Brazil, complementing the previous study of Assumpcao et al.(2006) in SE Brazil. Detectable anisotropy was found at all stations with delay times ranging from 0.5s to 1.6s (average about 1s). Our data, together with published data for the Andean region, were compared with the absolute plate motion as well as with upper mantle flow direction. Fast polarization directions tends to be roughly EW oriented, on average, consistent with the absolute plate motion direction given by the HS3-NUVEL1A model. The fact that most fast directions in Brazil tend to be roughly parallel to the absolute plate motion implies that the contribution to the shear-wave splitting from the frozen lithospheric anisotropy is relatively small, compared to other continental plates. Comparison with the upper mantle flow model developed by Conrad et al. (2007) showed some good results. While this flow model is roughly consistent with the splitting observations in SE and NE Brazil, the deviations in the Amazon craton and in the Andean region are very large. In the Amazon, the upper mantle flow model has low resolution (the San Francisco and the Amazon cratons are not separate units). In the Andes region the shear-wave splitting may be due to frozen lithospheric anisotropy, or the upper mantle flow model is not accurate. A frequency-dependent polarization filter was tested to improve estimates of SKS splitting. With synthetic data good results were obtained for larger windows and higher powers. Tests with real data will be presented.
T31A-0276
Andean earthquakes felt at long distances in Brazil! What will we do?
In the past several decades, more than 45 Andean earthquakes have been felt in Brazilian cities, thousands of kilometers away from the epicenter, especially by oscillations of high-rise buildings. Because of the increase of skyscrapers in big cities, such long-distance effects are becoming more frequent. In Sao Paulo, for example, such macroseismic effects have been observed every 3-5 years on average. We present a study on the characteristics of the earthquakes and cities more susceptible to cause high-rise buildings to oscillate. Most earthquakes occur at intermediate or large depths and those with magnitudes larger or equal than 5.0 mb, such as in the Jujuy and Santiago del Estero regions of northern Argentina, have caused long distance macroseismic effects. Most affected cities lie in sedimentary basins, such as Sao Paulo and Manaus, which can amplify the ground motions through basin resonance. In some cases, buildings with the longest axis oriented perpendicular to the largest S amplitudes (SH component) seem to be more susceptible to oscillations. As response levels are small, we used a on broad-band stations on each floor of a residential building (9-storey) in Brasilia City, in order to identify the characteristics of the building for preliminary analysis. This result is being compared with analysis of the ground motion amplitudes, recorded by nearby broadband stations.
T31A-0277
Geometrical analysis of the structure in the western flank of the Chilean Andes at 34°15'S-34°30'S
We present a structural analysis of strongly deformed Cenozoic volcanic rocks of the western Principal Cordillera between 34°15'S and 34°30'S. Deformation has been related to the inversion of the Abanico Basin occurred between 22 and 16 Ma. The structural array is characterized by a syncline with a very steep western limb and a rather flat slightly folded axial zone. On both sides of this structure, layers dip in directions opposite to the syncline flanks, suggesting disruption of anticlines by faulting. The reconstructed geometry performed on 6 cross- sections indicates that: 1. Deformation occurred by "fault-propagation folding" with considerable fault displacement, and 2. The anticlines were transported by the faults cutting their crests along the axial plane, similar to the anticlinal breakthrough deformation modes. Based on this mechanism and the relations between the axial and cut off angles, we developed a kinematic model using geometric methods and the Trishear program. Results suggests that folding was controlled by two E-vergent faults (western, WF, and eastern, EF). The most important (WF) would have a cutoff angle 40°-60°, which is compatible with inverted normal faults. Indeed, the regional nature of shortening in this region has been interpreted as the result of basin inversion. The EF has a cutoff angle of about 20 degrees, which is compatible with a neoformed thrust and allowing its interpretation as a short cut. These structures have accommodated ca. 30% (2 to 3 km) of shortening in this region. Although our proposition at depth for the main fault geometry is based on the surface analysis, it is quite consistent with the structural array interpreted by seismicity and MT images. Moreover, this structure seems to be the upward prolongation of a W-dipping ramp structure, connecting the subduction zone with the tectonic front of the mountain belt, in a zone where the ramp flattens and forms a detachment at 15-20 km depth beneath the western edge of the Principal Cordillera.
T31A-0278
The seismogenic structure of the Antofagasta subduction zone and future perspectives for the Iquique Local Network (ILN) in northern Chile
Material inhomogeneities or geometrical irregularities are suggested to act as seismogenic structures which can influence rupture nucleation, propagation, and termination of large earthquakes. To evidence such a behaviour, a direct link between tectonic and/or geological conditions and the areas of focused co-seismic moment release or slip on a rupture plane has to be established. In case of the Antofagasta region, we were able to identify two prominent asperity structures in the post-seismic aftershock sequence of the Mw8.0, 1995, earthquake and related them to geologic features located in the overriding crust. We looked at co-seismic moment release, aftershock radiated seismic energy, isostatic residual anomalies of the local gravity field, vp velocities and vp/vs ratios, and studied aftershock focal mechanism solutions in the vicinity of the asperity structures. All these parameters were able to support our presented model. With the installation of an International Plate Boundary Observatory in Northern Chile (IPOC) we want to apply our experiences gained in the Antofagasta region to the northern adjacent subduction zone segment. Our presentation will give an overview on the aims of the Iquique Local Network (ILN) as one component of the IPOC, whose major tasks are to monitor the seismic activity along the subduction interface and at the fault systems in the overriding crust, and the assumed transient seismic signals along proposed material boundaries. The data base should help to constrain changes in the local stress field and in material properties providing information on the seismic stage of the observed subduction zone segment.
T31A-0279
Reflection Seismic Imaging of the Seismogenic Coupling Zone in Southern Central Chile
The Chilean continental margin is one of the most seismically active subduction systems and serves as a natural laboratory to study mega-thrust earthquakes. We present results of the active seismic reflection survey of project TIPTEQ (from The Incoming Plate to mega-Thrust EarthQuake processes), which covers the subduction zone in southern central Chile in the area of the 1960 Valdivia earthquake hypocentre. The application of Kirchhoff prestack depth migration as well as two advanced imaging techniques (FVM, RIS) reveal a clear image of the subducted oceanic Nazca plate from the coast down to a depth of about 50 km below the Central Valley. The overriding continental crust is strongly structured showing horizontal, dipping and arching reflectors that indicate basal accretion. The reflectivity varies across the section and appears to be weak around the area of the 1960 earthquake hypocentre. Other specific features can be observed, like for instance a possible subduction channel at the top of the oceanic plate near the coast, a major crustal fault zone (LFZ) as well as a strong west dipping reflector perpendicular to the plate interface. In this paper we combine the high resolution seismic image with results from other geophysical disciplines to give a detailed picture of the Chilean subduction zone at 38° S.
T31A-0280
How Does Trench Coupling Lead to Mountain Building in the Sub-Andes?
The Andes are commonly believed to have resulted from subduction-related crustal shortening. Recent GPS data show ~30-40 mm/yr crustal shortening uniformly distributed across the central Andes, contrasting to geological observations of slower shortening (~10-15 mm/yr) that is concentrated in the sub-Andean fold and thrust belt. Previous studies have interpreted GPS-mesured crustal shortening to be mainly transient that will be recovered during future earthquakes, consistent with frequent trench earthquakes and the associated coseismic and postseismic crustal rebound. So how does the cyclic crustal shortening and rebound lead to long- term mountain building in the sub-Andes? To address this question, we have developed a 2D viscoelastic-plastic finite element model to investigate the linkage between short- and long-term strain partitioning across the central Andes. We simulate strain evolution during subduction earthquake cycles, and explore the effects of major model parameters and some of the inferred geological processes. Our results show that subduction alone is inadequate for building the Andes, because much of the interseismic crustal shortening in the upper plate may be recovered by coseismic slip and postseismic relaxation. We find that plastic deformation in the sub-Andes, in the form of sliding on the detachment faults or plastic failure of the sedimentary cover, is necessary to lead to the observed crustal shortening. Furthermore, our model suggest that numerous factors including topographic load of the Altiplano plateau, stronger trench coupling, faster western drift of the overriding South American plate, and the inferred delamination of mantle lithosphere beneath the eastern Cordillera may have contributed to the localized and apparently accelerated mountain building in the sub-Andes during the past ~10 Ma.
T31A-0281
Subduction and Denudation Dynamics Along the Andean Margin of Chile, Assessed by Low- Temperature Thermochronology.
The aim of this study is to reconstruct the evolution of the western Andean forearc between latitudes 20° and 40°S, during the early/middle Cenozoic. Little information is available for this time period, yet it is essential to reconstruct the thermal history of this period to have a full understanding of the early stages of the Andean orogeny. The dating techniques utilized are apatite (U-Th)/He, apatite fission-track (AFT) and biotite K-Ar. These techniques allow quantification of rock cooling and exhumation patterns in the forearc related to arc magmatism, changes in plate tectonics and/or erosion (burial). (U-Th)/He thermochronology is the most widely used here, since its closure temperature, between 40°and 70°C, is much lower than any used previously in regional Andean studies. Samples dated come from Mesozoic magmatic rocks outcropping in the Coastal Cordillera and Precordillera. In the Coastal Cordillera of northern Chile, biotite K-Ar ages (152¡À1 Ma), AFT ages (118-145 Ma) and (U-Th)/He (63-76 Ma) show a regional post-emplacement cooling and rapid exhumation pattern during the inland migration of the arc from late Jurassic to early Cretaceous times. A ¡Ý1-km vertical (U-Th)/He age vs. elevation trend, in the coastal cliff, reveals for the first time a regional younger phase of significant cooling at 45¡À5Ma Ma. This cooling phase is coeval with important changes in plate convergence rates, slab dip and plate coupling between the Nazca (Farallon) and South America plates. Based on our results, in this study we investigate how these changes might have effected the overriding plate during the onset of the first phase of significant uplift (Incaic phase) in Eocene times. In Central and Southern Chile, (U-Th)/He results for the Coastal Cordillera show larger differences in cooling ages. In Central Chile, ages range from 10 to 123 Ma. This geographical age distribution suggests a very different rock cooling and exhumation patterns along an area influenced by the present day flat slab and semi-arid climate (25°-35°S). Inland, results yield cooling ages in the Precordillera between 14 and 37 Ma. These ages reveal river incision rock cooling of between 50 and 70 m/myr during uplift. In southern Chile, with a humid temperate climate, we found differences in ages across the Coastal Cordillera. One west-east subvertical profile, at 37°S, yields synorogenic ages (30-47 Ma) close to the coastline, while much older ages (88-107 Ma) a few tenths of kilometers inland. These results suggest much larger amounts of exhumation (2-3 km) in the coastal regions since the onset of uplift.
T31A-0282
Turbidite sequences in marine sediments off southern Chile ? records of Quaternary climate - tectonics interactions
The active continental margin of South America is characterized by frequent, devastating and large-magnitude subduction earthquakes. In this environment the assessment of the history of major past earthquakes is hampered by the short historical and instrumental records. Here, we focus on marine turbidite records from offshore southern Chile that reflect the regional paleoseismic history of the forearc region since late Pleistocene time. The investigated records were retrieved from cores obtained from the Ocean Drilling Program (ODP), site 1232, and SONNE 50SL. The coring locations are at ~ 40ç and ~ 38çS lat, within the Peru-Chile trench, and are characterized by frequent interbedded strata of turbiditic and hemipelagic origin. Importantly, dated sections of the obtained cores indicate a recurrence time between 100 and 200 years for individual turbidite events. On the basis of the sedimentological characteristics and the nature of the active margin of southern Chile, we infer that the turbidites are seismogenic in origin, and represent paleo-earthquakes. However, the long-term changes in turbidite recurrence intervals also appear to be strongly influenced by climate and associated sea-level changes. For example, during the rising sea-level in the Holocene (ACHTUNG: es gab keine h?heren Meeresspiegel, sondern der Meeresspiegel ist seit 14 ka angestiegen) and Marine Isotope Stage (MIS) 5, recurrence of turbiditic layers is substantially higher, primarily reflecting a climate-induced reduction of sediment availability and enhanced slope stability (Dieser Satz ist inhaltlich nicht ganz klar). In addition, changes in the segmentation of tectonic uplift in the forearc and related drainage captures likely influenced the availability of sediment entering the marine realm, possibly determining the postglacial decrease in turbidite frequency. On the other hand, glacial turbidite recurrence times (including MIS 2, MIS 3, cold substages of MIS 5, and MIS 6), are within the same order of magnitude as earthquake recurrence intervals, derived from historical data and other terrestrial paleoseismic archives of the region. Only during these cold stages sediment availability and slope instability were pronounced enough to provide conditions conducive to a full record of the sequence of large earthquakes. Our data thus suggest that earthquake recurrence intervals on the order of 100 to 200 years are characteristic for southern Chile and were sustained during the last 140 kyrs.
T31A-0283
Quaternary Geologic History of the Rio Tambo, Southern Peru: Repeated Mass-Wasting Events in Western Cordillera Drainages
The amount and timing of river incision along the western margin of the Altiplano can provide insight into the climatic, volcanic, and tectonic history of this region. While many studies have focused on the Tertiary geologic history of the western margin of the Altiplano, the Quaternary geologic history of this margin remains largely unstudied. The Pacific draining Rio Tambo river valley in the forearc of southern Peru is one of the large drainages of the western margin. While much of this river is presently cutting bedrock, many locations within the main and tributary channels contain lacustrian, landslide, and volcanic ash deposits of Quaternary age (?). While geomorphologic features such as active and abandoned alluvial fans, fluvial terraces (strath and fill), and pediment surfaces exist along multiple segments of this drainage current geologic maps of the area lack the spatial resolution to effectively use these deposits for constructing the geomorphic evolution of the area. Thus, we have mapped the Quaternary (?) features through field measurements using a theodolite, as well as from remotely sensed data including SRTM DEMs, stereo-pair aerial photos, and ASTER images. Using cosmogenic 10Be to date the surfaces, we are able to place bounds on the age of terrace, lacustrian, and landslide deposits. Although fluvial incision is an important erosive process, which has resulted in this high-relief landscape, we find extensive evidence that mass-wasting is here extremely effective at facilitating the rapid movement of large amounts of material. At numerous times throughout the Quaternary, huge debris flows have dammed the steep fluvial valleys producing vast lakes and thick (up to ~100m ) lacustrian deposits upstream of the landslide deposits. At present, the landslide and lacustrian deposits have been re-incised, in many cases, to bedrock, and only stranded lake and landslide deposits remain high on valley walls. Thus, a large amount of material has been transported out of this drainage basin through first mass-wasting followed by river incision. As the Rio Tambo is located in the hyperarid climatic zone and thus receives very little annual precipitation, landsliding might result from over steepened valley walls associated with nick-point migration or possibly as a result of groundwater sapping. Further, we observe this type of rapid landscape modification in many of the bedrock-rivers in both northern and southern Peru and indeed throughout the western margin of the Peruvian Andes. While previous authors have described the late Cenozoic-present as a period of canyon cutting throughout the western margin, we suggest that mass-wasting and re-incision is the dominant process facilitating the degradation of the high topography of this portion of the the Andes.
T31A-0284
Recently active reverse faulting in the Atacama Basin area, northern Chile: Implications for the distribution of convergence across the western South America plate boundary
The western South American margin is one of the most active continental plate boundaries in the world. The ongoing convergence between the Nazca plate, or formerly the Farallon plate, and the South American plate produced the wide deformation belt of the Andes. In order to obtain more information about the active deformations in the central Andean belt to better understand the current distribution of convergence across the orogen, we attempted to map major structures that appear to be active recently by their topographic expressions using SRTM DEM and Landsat satellite images, followed by field observations. Results of our mapping show that there are many reverse faults that may be recently active in the area surrounding the Atacama Basin, in the Preandean Depression in northern Chile. These include a series of active reverse faults and related folds at the southeastern corner of the Atacama Basin, a major fold system that may be produced by an underlying fault just east of the basin, and a series of folds that forms the Cordillera de la Sal in the northern and western part of the basin. At the southeastern corner of the Atacama Basin, several geomorphic features indicate that at least some of the structures there have been active quite recently, including small drainages that cut through the folds and form active alluvial fans. Similar features of active river incision across folds are also present in the northern part of the basin. The fold system east of the basin may be one of the most important structures in the area. Deformed lava flows and deflected drainages indicate that this structure has been active recently, and growth strata near the fold suggest that it has been active for several myr. If so, the structure may be a major reverse fault system that defines the eastern boundary of the Atacama Basin, and may thus be an important onland structure that is responsible for absorbing part of the plate convergence.
T31A-0285
Influence of plate boundary geometry on the stress regime in an arc/back-arc setting
The influence of convergent plate boundary curvature on the stress distribution in an overriding plate is explored using a combination of analogue and numerical modeling techniques. Horizontal components of normal and shear stresses generated by subduction are imposed on the side of the numerical model representing a curved in-plane subduction zone. Normal stress corresponding to compressive oceanic subduction produces tensile trench-parallel stress in a lobe situated at the intersection of the plate boundary and symmetry axis of the curvature. This situation favors the development of strike-slip faults allowing horizontal extrusion of the overriding lithosphere. The horizontal component of the shear stress comprises a trench-normal component whose action resembles that of the normal stress, and a trench-parallel component that acts in the opposite direction. The trench-parallel stress exerted by subduction along both sides of a convex plate boundary produces along-strike compression near the intersection of the curvature symmetry axis and interplate zone. This lateral compression suggests that the least compressive stress is vertical, which promotes vertical thickening of the overriding plate. The role of the obliquity angle in generating lateral drag is further investigated using analogue modeling technique, while the balance between normal and shear stress is explored with the numerical technique. For compressive oceanic subduction and continental subduction, the normal stress is expected to be far greater than the shear stress and a convex geometry of the plate boundary promotes horizontal extrusion of the overriding plate. However, in specific situations, such as when the trench is starved of lubricating sediments, the shear stress magnitude may be raised thus enhancing the thickening of the arc/back-arc domain. This corresponds, for example, to the situation in the Central Andes where faster and greater crustal thickening of the Altiplano-Puna plateau is situated near the symmetry axis of the plate boundary.
T31A-0286
Effective Elastic Thickness Variations Along the Andean Margin and Their Relationship to Subduction Geometry
We present a new map of spatial variations in effective elastic thickness, Te, along the Andes, estimated using Bouguer coherence. The Te variations reflect interactions between subducting slab and pre-existing terrane structure. In the forearc, conductive cooling of the continent by the subducting slab exerts primary control on rigidity, resulting in Te that is highest (~ 40 km) where the oceanic lithosphere is oldest and coldest (~ 20° S). In the central Andes, Te is relatively low (~ 20 km) along the volcanic chain, the Altiplano and Puna plateaus. We interpret this weakening to reflect a high geothermal gradient maintained by advective magmatic processes, a shallow and hot asthenosphere, and a very weak lower crust throughout this region. East of the plateaus, high Te delineates underthrusting of the Brazilian shield. North and south of the plateaus, areas experiencing flat subduction are characterized by high Te, high shear wave velocity, thick thermal boundary layer and low heat flow, indicating that continental lithosphere there is thicker, colder and stronger. Based on these relationships we suggest that variations in slab dip along the margin relate to variations in structure of the continental lithosphere. In particular, we propose that upper plate structure influences the width and viscosity of the asthenospheric wedge, which control the suction moment responsible for the subduction angle at depths ~ 70--100 km. When oceanic lithosphere subducts beneath thin continental lithosphere, the low viscosity asthenosphere allows the slab to detach from the continent and sink into the mantle at normal angles. However, when oceanic lithosphere subducts near or beneath thick and strong continental lithosphere, the asthenospheric wedge narrows and corner flow drags high viscosity mantle from the base of the thick (> 150 km), cold continent into the wedge. Suction forces increase both with narrowing of the wedge and with increasing viscosity. We estimate the asthenospheric viscosity underneath thick, cold continents to be > 1020 Pa s, sufficient to induce flat subduction. Later, after prolonged hydration and weakening of the continent's base, asthenospheric flow into the wedge may resume, allowing the slab to sink again into the mantle at normal angles.
T31A-0287
The Andes: Geoid, Topography, and Flexure of the South American Plate
The topographic load exerted by the Andean orogenic system on the South American plate provides an opportunity to evaluate the relationship between topography, geoid, lithospheric flexure and orogenic curvature. The large aerial extent of topography in excess of 3 km between 5º S and 30º S latitude in the Andes loads the margin of an otherwise topographically "simple" continental plate, allowing us to interpret the geoid anomaly and flexure associated with loading. The Andean orogen is a curved continental/oceanic collision system for which the flexural forebulge is difficult to delineate due to the vegetative cover related to the Amazonian forest. We evaluate evidence for the presence and geometry of an Andean forebulge by integrating the global geoid anomaly with topography and flexural modeling. The GRACE global geopotential data up to degree and order 360 provides a geoid anomaly field with a minimum spatial resolution of about 100 km. The geoid signal from deep sub- lithospheric sources was removed using a long wavelength filter which passes information at shorter wavelengths with a cosine taper between degree and order 7 to 11 and ending at the maximum resolution of order/degree 360. This filter allows the evaluation of contributions to the geoid from mass distributions from depths less than 800 km. A number of features in the filtered geoid field are remarkable, including a close correlation between a geoid anomaly of 25 m over high topography, and a consistent geoid bulge of about 5 m present east of the Andes mountain chain that has no topographic expression. We successfully model this geoid signal with a topographic load on an elastic plate with an effective elastic thickness of 50 km. A good match for the geoid anomaly bulge east of the Andes is obtained between 5º and 35º S and provides a calibration of a single uniform elastic plate model. The 500 km wavelength of the bulge restricts its source to lithospheric depths. Calculations of the three- dimensional distribution of forebulge amplitudes and positions determined from our analysis suggest that two- dimensional flexural models do not correctly estimate the load and flexural parameter for curved orogens.