HR: 16:30h
AN: T32F-03    [PDF]
TI: Using Quantified Analogue Models to Investigate Rheology's Role in Causing Extension in Foldbelts During Convergence
AU: * Haq, S S
EM: saad.haq@sunysb.edu
AF: Stony Brook University, Dept. Geosciences, Stony Brook, NY 11794
AU: Davis, D M
EM: daniel.davis@sunysb.edu
AF: Stony Brook University, Dept. Geosciences, Stony Brook, NY 11794
AB: Contemporaneous margin normal compression and extension is observed in convergent margin settings. Workers have investigated this phenomenon using a single rheology and simple boundary conditions with both numerical and analogue techniques. In these experiments where a viscous rheology was used margin normal extension at the top of the wedge was observed concurrently with compression at the deformation front during active convergence. The magnitude of extension in viscous models has been directly correlated to high Argand numbers or lower viscosities. However, for similar models with viscous rheologies having low Argand numbers or with a pure frictional rheology workers have not observed margin normal extension. It has been proposed that dual rheological models may produce the geologically observed style of extensional deformation. For instance, Willet, 1999 has shown that a hybrid plastic-viscous rheology will produce extension during convergence if the viscous layer is shallow enough. Multiple rheologies have been widely used in analogue modeling but the extension observed during active convergence has not been quantified in these models. This is, in part, due to a lack of capability to accurately measure the surface strains occurring during convergence. We have developed a technique, used here, to quantify surface strains at multiple stages of the experiment, to test frictional, viscous, and frictional over viscous rheologies. In both the pure friction and viscous models our results are similar to other studies, no true extension during active convergence for friction, but significant observed rates of extension in models with pure viscous rheology. The addition of a low viscosity, or weak, basal layer results in a wedge with a small critical taper, which is predicted analytically. As dual-rheology models actively accommodate convergent strain they produce more complex deformation than either simple friction or viscous models. Hybrid models typically produce surface extension by under-stuffing. Additionally, after the succession of active convergence the viscous and hybrid models extend in areas of high topography producing additional shortening near the deformation front. In models with a sufficiently thick viscous layer a limited amount of coeval compression and extension is observed.
DE: 8020 Mechanics
DE: 8102 Continental contractional orogenic belts
DE: 8105 Continental margins and sedimentary basins
SC: Tectonophysics [T]
MN: 2003 Fall Meeting