Projects

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Applications of low temperature thermochronology to plate tectonic reconstructions and orogenesis

At present, we are working on two projects, one in southern Mexico and the other in the central California coast. A brief description of the problems we address is given below:

A. Cenozoic plate tectonic history of southwestern Mexico; constraints from low temperature thermochronology

The southern coast of Mexico marks the southern margin of the North American plate, and the location of the passing of the triple point between the North American, Cocos, and Caribbean plates. The exhumation history of the Sierra Madre del Sur mountain range, whose southern reaches extend into the Xolapa terrane in southern Mexico, helps constrain the timing of the passing of the triple junction. The timing of this passing is currently unclear, and the triple point has been postulated to have been at the western end of the Xolapa terrane in the Oligocene by some and at the eastern end of the Xolapa by the Oligocene by others. The Xolapa terrane is characterized by Cretaceous to Tertiary undeformed plutons intruding as a result of the subduction of the Cocos plate. Six transects along the Xolapa terrane were sampled in order to more tightly constrain the passing of the North American-Cocos-Caribbean triple junction. We link the exhumation of the southern Sierra Madre del Sur to the change in plate geometry by dating the samples using two low-temperature thermochronometers, the apatite fission track and apatite (U-Th)/He systems. Because the plutons in the Xolapa terrane are undeformed, they exhibit little risk of thermal resetting due to deformation. Preliminary data from one transect yields relatively slow exhumation until ~10 Ma, indirectly suggesting that exhumation since ~10 Ma must have been significantly faster.

Personnel: Ducea, graduate student Sarah Shoemaker, Peter Reiners (Yale University), John Garver (Union College), Maria Campa (University of Guerrero).

Publications: S. Shoemaker, M. Ducea, P. Reiners, J. Garver, M.F. Campa, 2002, Cenozoic plate tectonic history of southwestern Mexico; constraints from low temperature thermochronology, extended abstract.

B. Determining Late Cenozoic erosion rates from bedrock and surface uplift rates in a non-glaciated transpressional environment: The Santa Lucia range, California

Apatite U-Th/He ages from a vertical transect from the Santa Lucia Mountains, Central California Coast Ranges, indicate a young, Plio-Pleistocene bedrock uplift of these mountains. The inferred exhumation rates are constant, about 0.35 mm/yr between 8 and 2 Ma. Exhumation rates were higher, about 0.8 mm/yr for the past 2 Ma. An average surface uplift rate of 0.15 mm/yr is constrained by independent geologic data in the Santa Lucia for the period 4-0 Ma. Using these data, we calculate an average erosion rate of 0.5 mm/yr for the Santa Lucia Mountains. It is proposed that mass wasting via landslides is the dominant mechanism responsible for the erosion rates in the Santa Lucia Mountains. Plio-Pleistocene shortening is known to be about 10-12% in the area suggesting that to a first order the bedrock uplift rates are tectonic, with little or no isostatic component.

Personnel: Ducea, Martha House (Caltech).
Publications:

· Ducea, M.N., House, M., Kidder, S., 2003, Late Cenozoic denudation and uplift rates in the Santa Lucia Mounains, California. Geology, February 2003, v. 31, No. 2, p. 139-142. Click Here to download article (pdf).

· Shoemaker, S., Ducea, M., Reiners, P.W., Garver, J.I., Campa, M.F, 2002, Cenozoic plate tectonic history of southwestern Mexico; constraints from low temperature thermochronology. Geotemas, vol. 4, 137-138.

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Page last updated May 22, 2003. Questions or Comments Mail to mducea@geo.arizona.edu