Relationships between kinematics of arc-continent collision and kinematics of thrust faults, folds, shear zones, and foliations in the Nevadan orogen, northern Sierra Nevada, California
The Mesozoic Nevadan orogeny in the northern Sierra Nevada metamorphic belt, California, may be attributed to arc-continent collision, stratigraphic data and macroscopic cross cutting relations suggest successive accretion of two arcs along an active continental margin. The younger accretion event involved the Early Jurassic Slate Creek terrane, which is a 3-5 km thick pseudostratigraphic arc fragment. The Slate Creek thrust, an isoclinally folded fault with a subhorizontal median surface, carries the Slate Creek terrane at least 40 km eastward (continentward) over the pre-existing continental margin terrane amalgam. No rock units can be correlated across the Slate Creek thrust which is thus interpreted as an arc-continent suture. In addition to the Slate Creek thrust, the Nevadan orogen includes a major east-vergent imbricate thrust set east of and beneath the Slate Creek thrust, and steep west-vergent reverse shear zones, macroscopic upright folds, and steep foliations that overprint and cut the east-vergent structures. These data suggest a model for the relationships between the kinematics of arc collision and Nevadan orogenic structures. The Slate Creek arc terrane accreted by westward partial subduction of the continental margin along the east-vergent Slate Creek thrust. The continental margin was imbricated along an east-vergent thrust set. The structurally higher, inactive Slate Creek thrust-suture was deformed by steep west-vergent shear zones, folds, and foliations which may have accommodated shortening of the east-vergent thrust sheet. This deformation occurred within an active, continental margin arc that probably initiated by subduction flip after collision of the Slate Creek arc. This kinematic model is consistent with the structural geometry and chronology of the Nevadan orogen while qualitatively maintaining lithosphere-scale strain compatibility. This model has implications for problems related to emplacement of large crystalline thrust sheets, displacements beneath and at the margins of shortened crustal segments, and interaction of oppositely-verging structures. The Nevadan orogen is a slate belt, and the structural-plate tectonic model presented for the Nevadan orogeny may be testable in slate belts of other orogens.