HR: 11:50h
AN: T12D-07    [Abstracts]
TI: Changing Styles of Deformation Along the Western Margin of the Andean Plateau, Southern Peru
AU: * Schildgen, T F
EM: tfs@mit.edu
AF: Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences, Cambridge, MA 02139, United States
AU: Hodges, K V
EM: kvhodges@asu.edu
AF: Arizona State University, School of Earth and Space Exploration, Tempe, AZ 85287, United States
AU: Whipple, K X
EM: kxw@asu.edu
AF: Arizona State University, School of Earth and Space Exploration, Tempe, AZ 85287, United States
AU: van Soest, M
EM: Matthijs.vanSoest@asu.edu
AF: Arizona State University, School of Earth and Space Exploration, Tempe, AZ 85287, United States
AB: The Cordillera Occidental in southern Peru provides an important record of the deformational signature of the Andean Plateau formation. Early deformation along the western margin of the developing plateau was primarily accommodated along a series of high-angle, west-vergent reverse faults. This fault system crops out in a 100-km wide region from the Cordillera Occidental to the Peruvian coast. Although several research teams have attributed much of the construction of the western plateau margin to these features, a wide variety of high-angle faults and broad monoclines also occur along this margin. Understanding the relative significance of these features to plateau margin development has been hampered by limited of constraints on the timing and magnitude of fault movement, as well as uncertainty about the history of surface uplift. Structural mapping and thermochronologic studies in southern Peru provide new insights into the timing and magnitude of late Cenozoic surface uplift as well as the style of structural accommodation. The west-vergent reverse fault system deforms Oligocene sediments and older bedrock, but is covered by 14-16 Ma volcanic units. More recent faulting close to the range front is extensional in nature and has been an important contributing factor to the formation of the modern range-front escarpment. These structures deform the 14-16 Ma volcanic units, but most show only limited offsets. Valley-bottom transects of thermochronologic data from Cotahuasi valley show no major discontinuities in age, indicating that offset on any of the structures cannot exceed several hundred meters. Farther to the east, normal fault movement has not been as tightly constrained, but empirical relationships between fault length and maximum throw suggest at most 1 km of movement, or 500 to 800 m of vertical offset considering a reasonable range of fault dips. We combine these observations with new apatite (U- Th)/He data from vertical and valley-bottom transects of samples that provide robust constraints on the timing of surface uplift. These data suggest that the major phase of 2 to 2.5 km of plateau uplift that started at c. 10-12 Ma cannot have been solely accommodated by surface-breaking fault movement. Instead, regional long-wavelength warping and possibly a component of block uplift dominated the late Cenozoic deformation history of the western margin.
DE: 1140 Thermochronology
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
DE: 8104 Continental margins: convergent
DE: 8175 Tectonics and landscape evolution
SC: Tectonophysics [T]
MN: 2007 Fall Meeting