Footwall Deformation along the Willard Thrust, Sevier Orogenic Belt: Implications for Mechanisms, Timing and Kinematics

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Geological Society of America Bulletin






Geological Society of America

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Map-, outcrop-, and microstructural-scale examination of the footwall of the Willard thrust sheet in the Sevier fold-and-thrust belt, northern Utah, reveals deformation of footwall rocks before or during emplacement of the Willard thrust sheet. The Willard thrust sheet contains Proterozoic metasedimentary rocks in its hanging wall and has a footwall lateral ramp that cuts up section southward. In the northern part of the study area, Cambrian Maxfield and Ophir Formations were pervasively ductilely deformed at all scales. There, east and northeast-vergent recumbent isoclinal folds with well-developed axial-planar cleavage, kink folds, small shear zones, and solution cleavage are developed in a 100- to 200-m-thick zone of footwall deformation. Calcite deformed by dynamic recrystallization and diffusive mass transfer; rare twinned dolomite crystals deformed by intracrystalline glide. In the central part of the study area, the Cambrian Maxfield, Ophir, and Nounan Formations are complexly folded and faulted below the Willard thrust. Plastic deformation is largely restricted to a 5- to 10-m-thick limestone mylonite in the Maxfield Formation, with cataclastic deformation predominant below the mylonite. Axes of map-scale folds are nearly parallel with the Willard thrust, and a small thrust appears to be cut by the Willard thrust. Southward, the thrust climbs section in its footwall to Devonian and Mississippian strata, and deformation in the footwall is predominantly brittle, superimposed on locally developed ductile structures. The change from pervasive plastic to brittle deformation and accompanying change in structural style of footwall rocks occurs in the Cambrian Ophir and Maxfield Formations. The lateral changes of footwall deformation may be due to (1) lateral variations of footwall rock rheology and thus a variation in bulk strain of the rocks; (2) different temperature or confining or fluid pressure conditions along strike, perhaps due to a pre-existing footwall high; or (3) lateral variations in stress and strain conditions in the footwall. Whatever the cause, the data show that significant amounts of deformation occurred in the footwall of the Willard thrust before and(or) during motion along the thrust.