GEOMETRY OF HOLOCENE DISPLACEMENT TRANSFER BETWEEN NORMAL FAULTS AND STRIKE-SLIP FAULTS OF THE CENTRAL GREAT BASIN AND WALKER LANE, FOUR MILE FLAT, WEST-CENTRAL NEVADA

Displacement is transferred between transcurrent faults of the central Walker Lane and normal faults of the Central Nevada Seismic Belt across an abrupt northwest-trending physiographic boundary separating prominent NNE trending ranges of the central Great Basin from the equant mountain ranges of the Walker Lane. Along the boundary zone, the N55°W regional extension direction is not aligned with the transcurrent faults of the Walker Lane and is oblique to the normal faults of the Great Basin. Progressive displacement on the regional fault systems produces an unstable boundary zone geometry that is accommodated by the formation of a complex array of structures linking transcurrent and normal faults, many which are seismically active and show evidence of Holocene displacement. The geometry of active accommodation structures is well developed at Four Mile Flat, which forms a depression between the southern Stillwater Range, the northern Sand Springs Range, and the Cocoon Mountains on the north, east, and south, respectively. In this area, a NW-trending transcurrent fault bifurcates into subparallel strike-slip and dip-slip faults as the boundary zone is approached from the southeast and is linked to NNE trending normal faults through a series of splays that bound Four Mile Flat. Fault slip inversion on fault segments with bedrock footwall exposures indicates spatial partitioning of the displacement field into discrete strike-slip and dip-slip components. The subsurface geometry of Four Mile Flat was determined by gravity measurements made with two Scintrex CG5 gravimeters along several transects crossing the basin. Survey positioning with a vertical resolution of 2.0 cm or better was provided by dual frequency Leica GNSS receivers. A residual complete Bouguer anomaly of 12 mGals was produced using a density of 2.67 g/cm² and was inverted for depth in 3D using GMSYS modeling software. The pull-apart basin underlying Four Mile Flat has a rectilinear map pattern and is asymmetric with the a maximum depth of about 1.0 km localized along a buried, steeply dipping NNE-striking fault bounding a pediment along the western flank of the Sand Springs Range. The western margin of the basin is an active NNE trending antithetic fault that merges with the active strike slip fault to the south and a buried transcurrent fault to the north.