POST 3-MA INCEPTION OF THE NORTHERN WALKER LANE, NEVADA AND CALIFORNIA, BY REACTIVATION OF NORMAL FAULTS AND NORTHWEST PROPAGATION OF EXTENSION IN THE GREAT BASIN

The northern Walker Lane (NWL) is dominated by three, NW-striking, right-lateral, left-stepping, en-echelon faults. Each NWL fault cuts major basins but is outboard of, parallel to, and may have reactivated range-bounding normal faults. These normal faults were active during late Miocene-Pliocene, E-W extension that affected the Great Basin-Sierra Nevada transition. The Honey Lake fault, the westernmost NWL fault, parallels but is 3 to 5 km outboard of normal faults that separate the Diamond Mts on the southwest from Honey Lake basin. The normal faults have at least 1 km cumulative displacement, probably considerably more. Lake beds as young as 2.9 Ma are folded and thrust-faulted along the Honey Lake fault. A major segment of the middle, Warm Springs fault parallels but is ~1.5 km outboard of the Dogskin Mt frontal fault, which has at least 2 km normal displacement. Much of that displacement probably occurred after 3.6 Ma, because relatively fine-grained sediments of that age suggest that Dogskin Mt was only a minor topographic high then. These sediments are also highly deformed by displacement along the Warm Springs fault. The Pyramid Lake fault parallels but is ~2.5 km west of the frontal fault along the Truckee Range to the NE. This frontal fault has at least 1 km normal displacement, which must have occurred after 9.5 Ma, the age of basalt lavas in the range. In each case, strike-slip motion alone along the NWL faults can not have generated the deep basins.

Post-3 Ma inception of NWL faulting would seem to contradict data from the eastern California shear zone (ECSZ) and from Pacific-North American plate motions that show that the Sierra Nevada began moving northwestward as a rigid block relative to stable North America between 10 and 8 Ma. The data can be reconciled if part of the Great Basin remained connected to the Sierran block and underwent only E-W extension until 3 Ma. Northwestward motion of the connected block would have been accommodated by strike-slip along the central and southern Walker Lane and NW extension within the central and southern Great Basin. We propose that Walker Lane faulting propagated northwestward as NW extension propagated northwestward through the Great Basin. Total displacement in the NWL should be less than that in the southern Walker Lane or ECSZ by the amount of NW extension in the Great Basin.