Dr. Stuart N. Thomson
Research Scientist
Department of Geosciences
University of Arizona

NSF Tectonics (EAR #0635923)
Collaborative Research: Lithospheric weakening, deep crustal flow and the initiation of orogenesis at a noncollisional convergent margin in the Andes.

Together with: Keith Klepeis (University of Vermont); Geoff Clarke (University of Sydney)

Latest Publications

Thomson, S.N. Klepeis, K.A., Hervé, F. and Calderón, M. (2011). Late Oligocene erosion of Cordillera Darwin (southernmost South America) associated with rift margin uplift and opening of the west Scotia Sea. 11th International Symposium on Antarctic Earth Sciences, Edinburgh, UK.

McAtamney, J., Klepeis, K., Mehrtens, C., Thomson, S.N., Betka, P., Rojas, L. and Snyder, S. (2011). Along-strike variability of back arc basin collapse and the initiation of sedimentation in the Magallanes foreland basin, southernmost Andes (53 - 54.5°S), Tectonics, v. 30, p. TC5001, doi:10.1029/2010TC002826

Maloney, K.T., Clarke, G.L., Klepeis, K.A., Fanning, C.M. & Wang, W. (2011). Crustal growth during back-arc closure: Cretaceous exhumation history of Cordillera Darwin, southern Patagonia, Journal of Metamorphic Geology, v. 29, p. 649-672

Klepeis, K.A., McAtamney, J., Mehrtens, C.J., Thomson, S.N., Betka, P.M. and Mosher, S. 2011. Along-strike variability of coupled hinterland-foreland processes during formation of the Magallanes foreland basin, Patagonian Andes. GSA Abstracts with Programs, Vol. 43, No. 5, p. 441.

Klepeis, K.A., Betka, P.M., Clarke, G., Fanning, C.M., Hervé, F., Rojas, L., Mpodozis, C., & Thomson, S.N. (2010). Ophiolite obduction and continental underthrusting during Cretaceous closure of the Rocas Verdes basin, Cordillera Darwin, Patagonian Andes. Tectonics, v.29, TC3014, doi:10.1029/2009TC002610.

Hervé, F., Fanning, C.M., Pankhurst, R.J., Mpodozis, C., Klepeis, K.A., Calderón, M. & Thomson, S.N. (2010). Detrital zircon SHRIMP U-Pb age study of the Cordillera Darwin Metamorphic Complex: sedimentary sources and implications for the evolution of the Pacific margin of Gondwana. Journal of the Geological Society, London, 167, p. 555-568, doi: 10.1144/0016-76492009-124.

McAtamney, J., Klepeis, K.A., Mehrtens, C.J. & Thomson, S.N. (2009). The transition from extensional rift basin to compressional retro-arc foreland basin in the southernmost Andes (54.5°S): New provenance data from Bahía Brookes and Seno Otway. GSA Abstracts with Programs, Vol. 41, No. 7.

Klepeis, K.A., Betka, P.M., Alvarez, J., Poblete, F., Thomson, S.N., Gehrels, G.E. & Clarke, G. (2008). Tectonic Evolution of a Doubly-Vergent, Thick-Skinned Fold and Thrust Belt in the Patagonian Andes, Southernmost South America. GSA Abstracts with Programs, Vol. 40, No. 6, p. 548.

Intellectual Merit: A fundamental problem in continental tectonics centers on resolving how orogenesis initiates and develops at noncollisional convergent margins. Field studies and thermomechanical models demonstrate that the strength and rheology of the pre-orogenic overriding plate is a crucial factor that controls structural variability in these settings. In the Andes, the archetypal noncollisional orogen, a leading cause of pre-orogenic weakening involved the formation and destruction of large extensional basins during the Mesozoic. However, in most areas, the effects of Neogene orogenesis has obscured the geologic record of how these transitions controlled the initial rise of the Cordillera from below sea level. Even fewer areas preserve a record of how the deep crust evolved through these changing tectonic regimes. The aim of this project is to determine through direct measurement how the high temperatures, thin crust, and rheological stratifications that result from intraplate extension affected the thermal and structural evolution of the deep crust during subsequent shortening. This observational approach is possible because the Darwin Complex in southernmost Chile preserves a nearly complete record of the thermal and structural evolution of the middle crust during the formation and destruction of a large extensional basin. The proposed research involves using these unique exposures to test the contrasting ways in which pre-orogenic conditions arising from intraplate extension can influence orogenic growth during subsequent contraction. We have designed an international project that combines expertise in the fields of structural geology, metamorphic petrology, U-Pb geochronology (zircon, titanite, monazite), and thermochronology (40Ar-39Ar, K-Ar, fission track and (U-Th)/He) to determine the ages of deformation and the tectonic/thermal evolution of Darwin Complex.

Preliminary results imply that basin closure in the southernmost Andes led to the largescale underthrusting and/or partial subduction of continental lithosphere in a noncollisional, intraplate setting. If confirmed the occurrence of this process raises fundamental questions about how intraplate extension weakens the lithosphere and controls its behavior in noncollisional settings. Determining how pre-orogenic processes affect the initiation and growth of orogens is essential for understanding the long-term (≥10 m.y.) strength and rheology of continental lithosphere.

Broader Impacts: This project will enhance collaborations among U.S., Chilean and Australian institutions. We have arranged a unique collaborative arrangement among scientists with a broad range of technical expertise and extensive field experience in the southernmost Andes. This includes the first close collaboration between U.S. scientists and the Chilean National Petroleum Company in Tierra del Fuego. This and other partnerships with overseas scientists will provide access to unpublished data and archives of important geochronologic and petrologic samples. The involvement of students in these collaborations will help advance discovery"and strengthen the international partnerships. Students will benefit from cultural and scientific exchanges with overseas students and scientists. Student participation in all aspects of the project also will help promote teaching, training and learning.

Last Modified: December 2nd, 2011