Prograde metamorphism, strain evolution, and collapse of footwalls of thick thrust sheets: a case study from the Mesozoic Sevier hinterland U.S.A.

Analysis of prograde metamorphic fabrics in the exhumed footwall of the Mesozoic Windermere thrust, northeast Nevada, reveals fabrics and style of ductile flow developed during tectonic burial metamorphism of a sedimentary footwall of a thick thrust sheet. In response to structural burial, footwall strata underwent Barrovian metamorphism synchronous with development of bedding-parallel or near parallel S and S–L tectonites. Footwall rocks range from unmetamorphosed at high structural levels and progressively increase in metamorphic grade to upper amphibolite facies at deep levels. Attenuation of footwall stratigraphic units accompanied tectonite development whereby the amount of attenuation varies with metamorphic grade from ∼0% in lower greenschist facies to ∼30–50% in upper amphibolite facies rocks. Microstructures in metacarbonate, metapsammite, and metapelite, and analysis of crystallographic preferred orientations of quartz c-axes in quartzite, suggest a dominantly coaxial strain path with flattening strain predominating at lower metamorphic grade and plane strain more dominant at higher metamorphic grade. The microstructural data indicate that dominantly coaxial deformation during metamorphism accommodated extension of the footwall. Ductile extension most likely records footwall collapse induced by a loss of strength due to relative upward migration of isotherms during and or following burial. Extension may act as a mechanism that in part facilitates isostatic accommodation or sinking of the overlying load resulting in a reduction of topography. Attenuation of footwalls of thick thrust sheets and production of predominantly coaxial layer-parallel or near parallel fabrics may be an intrinsic process once sufficient structural overburden is developed and thermal relaxation and prograde metamorphism ensues.