HR: 08:30h
AN: T51E-03 INVITED     [Abstracts]
TI: What is the link between slab retreat and synconvergent extension in the Apennines, Italy?
AU: * Brandon, M T
EM: mark.brandon@yale.edu
AF: Yale University, Department of Geology and Geophysics, New Haven, CT 06520-8109 United States
AU: Willett, S D
EM: swillett@u.washington.edu
AF: University of Washington, Department of Earth and Space Sciences, Seattle, WA 98195-1310 United States
AB: The Apennines Mountains in Italy represent a classic case where plate convergence is accompanied by large-scale orogen-normal extension. Intuition suggests that the observed extension must somehow be related to slab retreat, but geodynamic models have generally failed to show the linkage between these two phenomena, especially in cases, such as the Apennines, where convergence is associated with accretion. The key issue in the Apennines is how to account for the closely-spaced pattern of surface deformation, with horizontal contraction on the Adriatic-Po side of the range and horizontal extension on the Tyrrhenian side of the range. We approach this problem by asking what velocity field is needed in the upper mantle to account for the paired contractional and extensional belts observed at the surface of the range. In this way, we consider the observed surface velocity field to be a highly filtered image of the velocity field in the upper mantle, with the crustal deformation in the Apennine wedge serving as the filter. We use a standard finite-element model with both frictional and thermally-activated viscous rheologies and coupled heat transport to represent the crustal deformation in the Apennines wedge. We have investigated a range of deformation scenarios, as represented by different basal velocity fields applied to an upper plate stretched by slab retreat. In order to produce surface extension that is localized and large, as observed in the Apennines, we find that horizontal deformation in the upper mantle must be localized. Distributed stretching at depth is not sufficient to induce lithospheric failure. To produce extension closely juxtaposed with contraction, the extension must be localized at a distance of about one crustal thickness from the S point, where the slab subducts beneath the upper plate Moho. We speculate that this deep localization of extension is controlled by two opposing corner flows, which is the circulation predicted for asthenospheric mantle above a retreating slab. The divergence in the flow at the top of these two corner flows may be responsible for localizing extensional failure in the upper mantle.
DE: 8102 Continental contractional orogenic belts
DE: 8109 Continental tectonics--extensional (0905)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting