Abstract: Tectonics of the Walker Lake Rift Basin, West-Central Nevada

Terence C. Blair, John G. McPherson

The Walker Lake basin, located in the Basin and Range Province of western Nevada, is a modern half graben bounded on its tectonically active west

side by the lofty Wassuk Range, and on the passive east side by the lower-relief Gillis Range. Alluvial fans and fan-deltas line the margins of the basin, whereas the axial Walker River and the perennial Walker Lake occupy the basin floor. The Wassuk range-front is the surficial manifestation of the active Walker Lake fault system, along which the basin has developed through predominantly dip-slip motion. The Walker Lake fault is characterized by a right-stepping, en echelon pattern containing at least 17 fault segments. Segment boundaries occur either: (1) coincident with lithologic contacts between Cretaceous plutons and Triassic-Jurassic volcanic rocks in the Wassuk footwall, (2) where subsidiary faults or inherited structural discontinuities intersect the range-front, or (3) where segments intersect at a high angle to produce a kink in the range-front pattern. The fault segments are packaged into two sectors, each displaying an arcuate planview pattern. Fault offset during the last 14,000 years has been greatest (+/-5 to 10 mm/yr) in the crest of the arc of each sector, diminishing to the lowest zones of offset (0.1 to 0.5 mm/yr) at the sector terminations. Local perturbations in the overall sector offset patterns occur due to the presence of geological discontinuities such as subsidiary faults, or to lithologic contacts in the footwall. Those parts of the footwall composed of volcanic rocks have undergone notably greater uplift than adjoining zones consisting of plutonic rocks, where total fault offset is dominated by hangingwall subsidence rather than footwall u lift.

The overall fault-sector offset pattern directly controls facies distributions in the basin, with deep perennial-lake or axial-river environments occupying the central fault-sector zone, where throw is maximum. In contrast, relatively large alluvial fans, fan-deltas, river deltas, and prograding recumbent spit shoreline environments are established and preserved along the fault-sector termination zones, where hangingwall subsidence is less. The direct relationship between fault segments or sectors and the positioning of sedimentary environments in extensional basins, as demonstrated by the Walker Lake basin study, should greatly enhance the predrill prediction of the sedimentary facies distribution in ancient basins of this type.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994