DEFORMATION OF THE PREBLE FORMATION, NORTHWEST NEVADA: IMPLICATIONS FOR PALEOZOIC-MESOZOIC TECTONICS OF WESTERN NORTH AMERICA

Mapping and structural analysis of the Cambrian Preble Formation in the Edna Mountains and Sonoma Range, northwest Nevada, provide evidence for a significant west-directed deformational event of Paleozoic age. The Preble consists of phyllite, impure quartzite, and marble with a well-developed tectonic fabric. Primary lithologic layering (S0-1) has been transposed and tight-to-isoclinally folded about N-S trending axes. Folds are west vergent, with moderately to steeply E-dipping axial surfaces. The dominant phyllitic foliation (S2) is axial planar to these folds and is penetrative throughout the rock mass. Because the strongly deformed Preble Fm. is unconformably overlain by relatively undeformed Middle Permian rocks (Edna Mountain Formation), the deformation beneath the unconformity must be of Paleozoic age. Candidates for this deformational episode include: 1) Mid-Pennsylvanian-Early Permian event(s), as documented in upper Paleozoic rocks farther east in Nevada; 2) the Late Devonian-Early Mississippian Antler Orogeny; and/or 3) an unrecognized event of pre- late Devonian age. If the deformation is Pennsylvanian-Permian, then this event covered a much larger area than currently appreciated, and the intensity of deformation apparently increased from east to west in Nevada. If of Antler age, deformation in the Preble Fm. is interesting because of its W-directed geometry. The presence of a Paleozoic E-dipping planar anisotropy in the crust may have influenced the geometry of younger, Mesozoic structural development in this region. For example, Jurassic E-W contraction was accommodated by a belt of anomalous W-directed thrusting/folding that is spatially coincident with the structural basement containing the older (pre-Middle Permian), east-dipping fabric.