HR: 0830h
AN: H51E-1132    [PDF]
TI: Evolution Of The Coastal Margin Of Central Peru Through Fission-Track and (U-Th/He) Analysis
AU: * Wipf, M
EM: martin.wipf@erdw.ethz.ch
AF: Department of Earth Sciences, ETH Zuerich Sonneggstrasse 5, Zuerich, ZH CH-8092 Switzerland
AU: Harrison, D
EM: harrison@erdw.ethz.ch
AF: Department of Earth Sciences, ETH Zuerich Sonneggstrasse 5, Zuerich, ZH CH-8092 Switzerland
AU: Seward, D
EM: diane.seward@erdw.ethz.ch
AF: Department of Earth Sciences, ETH Zuerich Sonneggstrasse 5, Zuerich, ZH CH-8092 Switzerland
AU: Schlunegger, F
EM: fritz.schlunegger@geo.unibe.ch
AF: Institute of Geological Sciences, University of Bern Baltzerstrasse 1, Berne, BE CH-3012 Switzerland
AB: The Coastal Batholith of Peru comprises granitoid bodies which range in age from 100-60 Ma and 50-20 Ma. It forms a well defined linear feature along the coastal margin. Low temperature geochronological methods such as fission-track and (U-Th)/He are best suited to reconstruct the later exhumation of the coastal margin. Factors such as variability in climate and in rock type are constrained reducing the number of parameters that might have an effect on erosion rates. The Coastal Batholith has been analyzed at several locations between Lima (Lat. 12$\deg$S) and Chala (Lat. 16$\deg$S) using fission-track (zircon and apatite) and (U-Th)/He dating methods. Modeling of the preliminary fission-track data set shows an almost identical thermal history for all locations North of San Juan (Lat. 15.30$\deg$S). Rapid exhumation in Late Eocene was followed by heating and renewed cooling during Oligocene time. The heating phase is interpreted as burial by Tertiary sediments, and the cooling, erosion of this sediment cover and the granite itself. An Early Miocene age (zircon fission-track age) of an ignimbrite stratigraphically just above the granites from the Nazca area (Lat. 15$\deg$S) shows that significant erosion of Eocene and Oligocene sediments had taken place in the coastal margin of central Peru, prior to the Early Miocene. Modeling further suggests that South of San Juan no significant amounts of sediment were deposited or eroded since the early Eocene. This southern region has been undergoing only simple long-term exhumation since as far back as Late Cretaceous. To explain these changes in age, which occur south of the leading edge of the subducting Nazca Ridge the geomorphology of the coastal area of central Peru was analyzed. Topographic cross sections perpendicular to the coast suggest an effect, caused by the collision with the ridge in the immediate on-land coastal areas. Over the area of the presently subducting ridge, a coast-parallel cross section shows an increase in altitude of approximately 500-1000 meters. No affect of the subducting Nazca Ridge is observable along the Western Cordillera further inland. The new fission-track ages can be interpreted in the terms of events that have affected the coastal region of Peru. These events disclosed through the apatite fission-track modeling are in agreement with those of Sebrier (1988), i.e. 45 to 40 Ma and 28-26 Ma. The zircon fission-track ages are thought to represent cooling ages of the Upper Cretaceous to Paleocene intrusions of the Coastal Batholith. This is the first time that such quantitative data has been available for this region. Remarkably, the region to the south of the ridge has been undergoing only long slow exhumation during the Late Cretaceous -Tertiary to Recent, uninterrupted by any events that have caused a reheating such as burial by sedimentation.
DE: 1824 Geomorphology (1625)
DE: 1886 Weathering (1625)
SC: Hydrology [H]
MN: 2003 Fall Meeting