T34B-01
From Ecuador to Patagonia: Andean deformation from InSAR 1992-2007
We use Interferometric Synthetic Aperture Radar (InSAR) observations from 6 satellites (ERS-1, ERS-2, ENVISAT, RADARSAT, ALOS, and JERS-1) along with published GPS displacements to constrain the myriad deformational processes in the central and southern Andes between 1992-2007. In this contribution, we review the types of deformation that are occurring (volcanoes, earthquakes, post-seismic and inter-seismic deformation, glaciers, groundwater, and anthropogenic processes) and present new observations and models of these processes. In the central Andes, InSAR observations have been possible in all seasons and have documented numerous sources of deformation: at least 7 volcanic areas, more than a dozen earthquakes (5 < Mw < 8.4), both natural and human-induced groundwater movements, and some signals that are mysterious. On the other hand, it is difficult to constrain inter-seismic deformation given the various sources of noise in the InSAR data, including: changes in the water vapor content of the troposphere, perturbations in the ionosphere, and uncertainty in the precise orbital positions of the satellites. In the southern Andes, because of the more humid climate, deformation measurements with C-band radar are only successful during the austral summer. In addition, due to a lack of data acquisitions, observations are limited to only a subset of the potentially active volcanoes and a fraction of the time period for which there is satellite data. In spite of the difficulties, we observe deformation at at least two two volcanoes, from the oil production in the San Jorge basin, as well as motion of glaciers at the Northern and Southern Patagonian Icefields. Preliminary L-band InSAR data from ALOS indicates great potential for further illuminating deformational processes in the southern Andes.
T34B-02
Recent Contractile Deformation in the Forearc of Southern Peru: A Geomorphologic Analysis And 10Be Surface Exposure Ages
The style, amount, and timing of deformation along the margins of the Altiplano are important components of our working model of the formation and maintenance of this high elevation plateau. While much of the convergence- related shortening is accommodated along the eastern margin in the Subandean fold and thrust belt, a significant amount of uplift and crustal thickening has occurred in the western margin during the past ~20 Myr. In addition to ancient uplift and deformation, various styles and amounts of Recent deformation that reflect the current lithospheric state of stress have been documented within the forearc. Some of the first order variables that affect the state of stress and therefore the style of deformation within the forearc of the western margin include: 1) the variable dip of the subducting slab along the South American margin, 2) the orientation of convergence relative to the margin, and 3) the subduction of aseismic ridges (e.g., Nazca Ridge). Other potential influences on the state of stress include addition of material to the western margin through lower-mid crustal flow, subduction erosion, and magmatic additions. In southern Peru, previously documented active deformation in the forearc includes coastal normal faults trending perpendicular to the trench, and transform faults oriented parallel to the trench, including the left-lateral Incapucio fault system, of the Precordillera. Our new field mapping and geochronologic studies in the Longitudinal Basin and Precordillera of southern Peru reveal recent contractile deformation along structures trending sub-parallel to the trench. Here, a southwest propagating anticline related to a blind thrust deflects the active stream channels within the Pampa Cabeza de Vaca region. Incision along the active drainages is localized to areas near active structures and has produced strath terraces that provide datable geomorphologic markers to quantify incision rates and constrain the timing of deformation. Cosmogenic 10Be surface exposure ages from a set of three distinct abandoned terraces in the Pampa Cabeza de Vaca region yield ages ranging from ~35-550ky and incision rates of ~0.04-0.09mm/yr. Thus, the contractile deformation within this region has been active for at least the last 500ky and is plausibly presently active. The documentation of recent contractile deformation within the forearc of southern Peru stylistically contrasts with previously held view active deformation in this region is dominated by extensional topographic collapse. Moreover, active shortening within the Peruvian forearc bears on our models of how the Altiplano plateau is currently being maintained along the western margin. Indeed, by identifying and quantifying active deformation within the Peruvian forearc, we can begin to address the potential links between surface processes related to climate and active tectonics, and the dynamics of the lithosphere.
T34B-03 INVITED
Late Cenozoic Deformation of the Coastal Cordillera, Northern Chilean Forearc, 18- 25°S
Overlying the only part of the South American continental crust that is in direct contact with the subducting Nazca Plate, the Coastal Cordillera of northern Chile and southern Peru should provide the most complete geological record of the coupling between the two plates. This record of coupling is exquisitely preserved in the hyperarid Atacama Desert. This preservation is both one of the major advantages and major challenges of working in this region: On the positive side, exposure of geomorphic surfaces is complete and unencumbered by erosion or vegetation, and brittle saline soils preserve subtle deformation features that would quickly be obliterated in more humid environments. On the negative side, ancient geomorphic features are just as fresh as Recent one and the lack of organic material precludes radiocarbon dating, a traditional tool of paleoseismology. During the last several years, we have concentrated on documenting three fundamental characteristics of late Cenozoic forearc deformation: (1) NS shortening on reverse faults striking at a high angle to, and dextral-reverse faults striking oblique to, the continental margin; (2) N-striking normal faults of the forearc and their reactivation, locally, as reverse faults; and (3) extensive suites of tension cracks. Reverse faults striking at a high angle to the margin are present between 19 and 21.5°S, straddling the topographic symmetry plane that marks the axis of the Bolivian orocline. Limited dating of tuffs and surfaces shows that these structures have been active for at least the last 6 Ma. At least 5 of these structures -- Atajaña, Pisagua, Iquique north and south, and Barranco Alto -- cut the Pleistocene marine terraces of the coastal platform, producing 20 to 50 m of vertical offset. A forearc crustal earthquake just south of Pisagua in March 2007 demonstrates that margin parallel shortening continues to the present and that permanent deformation occurs during the interseismic part of the plate boundary seismic cycle. NS-striking normal faults are well known in the forearc thanks to spectacular exposure on the Mejillones Peninsula, where normal faults dip 45-70° E. Simple kinematic analysis suggests that these listric faults flatten at a few kilometers depth, well above the subduction zone, and thus do not directly facilitate subduction erosion. High-angle normal faults in the Atacama fault zone near Antofagasta and in the Salar Grande (Paposo, Salar del Carmen, Hombre Muerto) area have locally experienced minor reactivation as steep reverse faults, indicating that the fault zones are very weak and may experience movement during both interseismic and coseismic phases. Open tension cracks are widespread and well preserved due to the gypcrete crust that characterizes the Coastal Cordillera between 500 and 1200 m, but also penetrate well into bedrock. We have mapped more than 75,000 cracks, the majority due to tectonic mechanisms, on 1 m and 2.5 m resolution satellite imagery. Tectonic cracks can be subdivided into to groups: those related to bending on the crest of fault bend/propagation folds associated with the EW reverse faults, and those produced coseismically. We suggest, based on orientation and distribution, that this latter group may actually be used to define long-term segment boundaries of rupture zones of interplate earthquakes.
T34B-04 INVITED
Implications of Paleogene Foreland Basin Evolution in NW Argentina for Timing of Andean Orogenesis
The timing and paleogeography of early Andean mountain building are topics of ongoing debate. We track development of the early central Andes in Bolivia and NW Argentina by studying widespread Tertiary deposits in the Altiplano-Puna and Eastern Cordillera. These deposits accumulated on top of Late Cretaceous post-rift marginal marine facies and Precambrian-Paleozoic basement. Over a north-south distance of 1000 km, the Paleocene-Oligocene succession consists of three stratigraphic assemblages: (1) Paleocene-Eocene fluvial- lacustrine siltstone and marl, up to ~200 m thick; (2) a 10-100 m thick zone of stacked Eocene paleosols, including stage III-IV Calcisols, Vertisols, and strongly reduced Gleysols; and (3) an upward coarsening, several km-thick sequence of fluvial to alluvial fan deposits. Locally, upper Eocene-Miocene rocks consist of thick (>2 km), proximal alluvial fan deposits containing growth structures. Modal petrographic data indicate derivation from metasedimentary and plutonic source terranes, and paleocurrent data show eastward sediment transport. Detrital zircon U-Pb ages indicate derivation from Paleozoic low-grade metasedimentary and igneous source rocks. Eocene detrital AFT ages from apatite grains that yield early Paleozoic U-Pb crystallization ages require rapid exhumation (0.4 mm/yr to >1mm/yr) of western source areas in neighboring regions such as the Eastern Cordillera and indicate a constructional orogen. Combined with previous structural studies, our data are consistent with a Late Cretaceous-Eocene thrust belt in the Cordillera de Domeyko in northern Chile, flanked to the east by a several hundred km wide foreland basin system. Flexural subsidence dominated the proximal region, and back-bulge and post-rift thermal subsidence may have operated in the distal eastern part of the basin. Approximately 500 km of eastward migration of the foreland basin system produced the vertical succession preserved in the Paleogene of the Puna and Eastern Cordillera. Shortening estimates for NW Argentina combined with simple flexural modeling raise the issue of the possible whereabouts of a roughly 300 km long slab of underthrust South American lower crust and lithosphere.
T34B-05 INVITED
The Subducted Chile Ridge Imaged with Teleseismic Travel-time Inversion
We present a teleseismic travel-time inversion for upper-mantle velocity structure beneath the Chile triple junction region. Data were recorded at 46 seismic stations deployed in southern Chile from December 2004 to February 2007 (for details, see the Chile Ridge Subduction Project, http://seismology.geology.ufl.edu/chile). The area covered by the network (42-46 degrees South and 72-78 degrees West) lies above the projected position of the subducted Chile ridge, which separates the Nazca and Antarctic oceanic plates. Because the Nazca plate subducts nearly 5 cm/yr faster than the Antarctic plate, the trailing edge of the last Nazca lithosphere formed before the ridge subducts has been inferred to separate steadily from the leading edge of the Antarctic lithosphere, forming progressively larger slab windows with depth. The relative delay times are obtained via a multi-channel cross correlation of band-passed waveforms for each teleseismic event. These data are then inverted using an iterative, robust, non-linear scheme, which parameterizes the 3-D velocity variations as splines under tension constrained at over 30,000 nodes beneath the region.. We image a high-velocity slab in the upper mantle dipping steeply to the East, which we associate with the subducted Nazca oceanic lithosphere, and a distinct low-velocity anomaly at the projected location of the subducted Chile ridge. http://seismology.geology.ufl.edu/chile
T34B-06
Shear Wave Splitting and Seismic Anisotropy in the Chile Ridge Subduction Region
We present new shear wave splitting measurements of SK(K)S and PKS phases recorded at 39 broadband seismic stations in the Chile triple junction region. The network, deployed December 2004-February 2005 and operated jointly by the University of Florida and the Universidad de Chile (Santiago), spans the region where the Chile Ridge subducts beneath South America, from the Pacific coast of the Taitao Peninsula to the Argentine border, and extends 250 km north and south of the actual triple junction. Given increasing temperature with depth, it has long been hypothesized that ridge subduction should result in creation of slab windows – asthenosphere-filled gaps between continually separating edges of oceanic lithosphere formed at the Earth's surface. The Chile Ridge Subduction Project was formulated in part to test this notion. The network was demobilized during January-February 2007. In conjunction with teleseismic travel time inversions and studies of seismic attenuation in the Chile Ridge subduction region, shear wave splitting as recorded at the Project network provides an excellent snapshot of upper mantle flow in the region: Shear wave splitting in the study area, which may be caused by mineral alignment during upper mantle flow or possibly by aligned pockets of partial melt, is strong (delay times up to 3 s) and highly variable, with a marked change from trench-parallel in the northern network to trench normal in the western Taitao Peninsula, near the most recently subducted Chile ridge segment. http://seismology.geology.ufl.edu/chile
T34B-07
Structure of the upper mantle and crust beneath the Amazonian Craton and the Southern Andes
The structure of the upper mantle and crust beneath South America has been inhomogeneously resolved because of the particularly heterogeneous distribution of earthquakes and seismic stations in the continent. We recently improved resolving power for northeastern Brazil, which contains the oldest geology of South America, through the BLSP02 seismic project. We incorporated BLSP02 data in a joint inversion of about 5700 Rayleigh wave group velocity dispersion curves, 1537 regional S and Rayleigh wave trains, and 58 receiver functions. The new model shows no significant difference in crustal thickness between Archean and Proterozoic regions in South America, with the Moho roughly at a depth of 42 km depth. Imaged Moho depth ranges from 30 km in the central Chaco basin to 45-70 km beneath the orogenic Andean belt. The imaged S-velocity indicates an average lithosphere thickness of around 160 km for the Amazonian craton. Extremely low average velocities are imaged for the mantle wedge beneath the Andes and appear to be strongest in regions of normal-dip subduction. However, upper mantle structure beneath the southern portion of the continent, including for the mantle wedge beneath the southern Andes, remains poorly resolved. To investigate spatial variations in the low velocity mantle wedge along the southern Andes, we incorporate regional waveform data from the Chile Ridge Subduction Experiment, which provides path coverage as far south as the triple junction between the South American, Nazca, and Antarctic plates. Comparing the S velocities beneath the Andes above the slab window near the triple junction where the Chile Ridge meets the trench, to those beneath the Andes north of the triple junction in regions of normal- and small-dip subduction, will facilitate assessing the relative roles of wet and hot mantle in the Andean mantle wedge.
T34B-08 INVITED
Scotia Arc Kinematics From GPS Geodesy
We obtain the first Scotia and South Sandwich plate Euler vector estimates not dependent on closure using a combination of GPS crustal velocity data from the Scotia and South Sandwich plates, transform azimuths, spreading data, and an updated earthquake slip vector catalog. Neither the GPS data, which are tied to the global plate reference frame but sample only limited portions of the two plates, nor the geologic data, which are not tied to the global spreading circuit, are sufficient to individually define the Euler vectors. In addition, the Scotia plate GPS measurements do not sample the stable plate interior and it is necessary to model the plate boundary deformation field for the Euler vector estimation. Our GPS based South America-Antarctic and Scotia-South Sandwich Euler pole estimates are in good agreement with previous estimates from either GPS or geologic data alone. The new South America-Scotia Euler pole, however, is significantly different and is near the South America- Antarctica Euler vector.