LATE PALEOZOIC CONTRACTIONAL TECTONICS ON THE WESTERN LAURENTIAN MARGIN: A PROGRESS REPORT

The Antler orogeny, long considered the tectonic event that ended the Paleozoic passive miogeoclinal setting on the western Laurentian margin, is now known to be only the beginning of protracted contractional tectonism that affected the margin throughout the late Paleozoic. Evidence for discrete late Paleozoic deformational events includes: angular unconformities, folds and faults with overlap age control; orogenic sediments with exotic components; and evidence for changing topography, including basin formation and destruction. New data constrain the geographic extents, geometries and kinematics of several of these deformational events.

Mid-Mississippian deformation is expressed as east-vergent folds and thrust faults in northeastern Nevada, an angular unconformity (C2) throughout central Nevada, and a shift in sediment sources and paleoflow directions in southwestern Nevada. Early Mississippian (Kinderhookian) rocks occur below the unconformity and Late Mississippian (Chesterian) rocks overlie it.

Mid-Pennsylvanian deformation is expressed as northwest- to west-southwest-vergent folds and local thrust faults in northern Nevada. There is some evidence that the intensity of this deformation increased westward. The mid-Pennsylvanian structures occur in Early Pennsylvanian (Morrowan and Atokan) rocks, and are unconformably overlain by Missourian rocks (C5) to Virgillian rocks (C6). In some places, erosion on the C6 unconformity cuts out the underlying C5 unconformity, obscuring evidence for the extent and relative significance of each.

Earliest Permian tectonism is expressed as east-vergent folds and thrust faults. These structures are found from southeastern California to northeastern Nevada, and are trimmed by a widespread unconformity (P1). Earliest Permian (Asselian) rocks are folded, and Sakmarian rocks overlie the unconformity.

The reversals in geometry from mid-Mississippian east-vergent deformation, to mid-Pennsylvanian west-vergent deformation, and back to earliest Permian east-vergent deformation appear to signal significant changes in continental margin geometry or kinematics.