HR: 0800h
AN: T31A-0286    [Abstracts]
TI: Effective Elastic Thickness Variations Along the Andean Margin and Their Relationship to Subduction Geometry
AU: Perez-Gussinye, M
EM: mperez@ija.csic.es
AF: Institute of Earth Sciences "Jaume Almera", CSIC, Barcelona, 08028, Spain
AU: * Lowry, A R
EM: arlowry@cc.usu.edu
AF: Dept. of Geology, Utah State University, Logan, UT 84322-4505, United States
AU: Phipps Morgan, J
EM: jp369@cornell.edu
AF: Dept. of Earth Sciences, University of Cornell, Ithaca, NY 14853-1504, United States
AU: Tassara, A
EM: andres@dgf.uchile.cl
AF: Dept. of Geophysics, Universidad de Chile, Santiago, 13518, Chile
AB: We present a new map of spatial variations in effective elastic thickness, T_e, along the Andes, estimated using Bouguer coherence. The T_e 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 T_e that is highest (~ 40 km) where the oceanic lithosphere is oldest and coldest (~ 20° S). In the central Andes, T_e 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 T_e delineates underthrusting of the Brazilian shield. North and south of the plateaus, areas experiencing flat subduction are characterized by high T_e, 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.
DE: 8104 Continental margins: convergent
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8138 Lithospheric flexure
DE: 8162 Rheology: mantle (8033)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting