SPATIAL AND TEMPORAL DISTRIBUTION OF LATE PALEOZOIC STRUCTURES IN NEVADA: PROGRESSIVE TECTONIC EVOLUTION OF THE WESTERN LAURENTIAN MARGIN

Overprinted late Paleozoic structures in Nevada document repeated deformation events in individual parts of the orogen. For example, at Carlin Canyon in the northern Piñon Range, a mid-Mississippian angular unconformity (C2) represents the oldest late Paleozoic structure. Subsequent mid-Pennsylvanian folds trend NE, verge NW, and are associated with NW-directed imbricate thrust faults. The unconformably overlying late Pennsylvanian rocks record open, NE-trending, early Permian folds. Farther west at Edna Mountain, mid-Pennsylvanian W-vergent folds were refolded around E-trending axes, and overlain by Middle Permian deposits. In southern Nevada near Yucca Flat, E-vergent folds and thrust faults pre-date W-vergent folds and steep reverse faults. Both sets of structures are found in Early Permian rocks, and based on regional relationships are most likely Permian in age.

Approximately synchronous unconformities throughout Nevada bracket the timing of specific tectonic events. The C2 angular unconformity crops out in the Diamond Mountains, Piñon Range, and Adobe Range. This unconformity and its laterally equivalent surface have also been identified in drillholes throughout much of central Nevada. A mid-Pennsylvanian unconformity (C5 and/or C6) truncates northwest- to west-vergent structures from the Pequop Mountains in the east to Edna Mountain in the west (~114.5° to ~117.3° W longitude). An Early Permian unconformity (P1) is exposed at Edna Mountain, Beaver Peak, and the Piñon Range.

Late Paleozoic structures formed later in southern Nevada than they did farther north, indicating that the initiation of shortening across the western margin of Laurentia propagated southward with time. At Bare Mountain, the Eleana Range and the Hot Creek Mountains, deposition was continuous across C2 time, but an abrupt change in clast composition records changes in the source area to the north. Similarly, there is no unconformity at C5 and/or C6 time in the Arrow Canyon Range, Syncline Ridge, or Mountain Springs Pass, but all record a change to shallower or more restricted conditions at this time. Subsequent, superimposed folds and faults occur in most of these localities; they are thought to be Permian. This southward progression of tectonism is consistent with the northward motion of Pangea in late Paleozoic time.