Contrasting lithospheres: does one size fit all for the structure of mantle under foreland arches?
Tuesday, 16 December 2014
Foreland arches’ particular placement far inland relative to active subduction boundaries prompts this long-standing question: do these arches form as a result of lithospheric rheology inherent in the zone between more ancient mobile belts and cratons, or do they result from stress⁄strength conditions driven by more recent processes, such as low-angle subduction? The Sierras Pampeanas basement-cored uplifts in Argentina correspond spatially with present-day low angle subduction, which many consider an analogue for western U.S. Laramide basement arches uplifted contemporaneously with low-angle subduction ˜50 Ma. However, significant questions remain about the lithospheric rheology of the two regions, in particular, how much have they been modified by subduction processes? Seismic anisotropy can potentially indicate how continental lithospheres above subduction zones deform or resist deformation in response to stress, and therefore may constrain possibilities for lithospheric rheology. We contrast two comparable shear wave splitting datasets, for the Sierras Pampeanas and Bighorn Mountains in Wyoming, to examine the similarity of continental lithospheric structure in these two regions. Heterogeneous shear wave splitting patterns for Wyoming suggest the existence of Archean lithosphere preserved under the region, supported by seismic fast directions consistent with expected patterns resulting from Wyoming craton assembly orogenic events. Regional tomography shows a thick zone of fast velocity upper mantle, also consistent with ancient lithospheric mantle. In contrast, seismic anisotropy for the Sierras Pampeanas is more homogeneous over a similarly-sized region, with fast directions inconsistent with our expectations for patterns resulting from the assembly of the Andean foreland, and quite consistent with modern deformation expected to result from subduction processes. Seismic wavespeeds indicate that much of the Sierras Pampeanas is not underlain by pristine ancient continental lithosphere, having been modified significantly by the subduction process. These comparisons indicate that different histories accompany the lithospheres of deformed foreland regions, and if these lithospheres have a similar rheology, there are multiple ways to produce such a rheology.