CROSS CUTTING RELATIONSHIPS BETWEEN LOW-ANGLE NORMAL FAULTS IN THE SLATE RANGE, EASTERN CALIFORNIA

Previous work suggests that the western Slate Range (Searles Valley, eastern California) is bound by a complex system of low-to-moderate angle faults. New results from mapping, structural, and thermochronological studies show the Searles Valley faults consist of two fault systems: an active low angle normal fault zone which cuts an older normal fault that also moved along a low-angle surface.

The Searles Valley fault zone (SVFZ) can be traced 45 km from just north of the Garlock fault over the northern crest of the range (where it is called the Manly Pass fault) and into Panamint Valley. An array of faults and fractures define a fault zone striking N-S and dipping 20-40° to the west. The fault zone offsets a complex of Mesozoic plutonic rocks (containing pendants of Paleozoic and Mesozoic volcanic and sedimentary rocks) overlain nonconformably by Miocene volcanic and sedimentary strata. Fault slip data suggest that net motion is toward the west, with little evidence for significant oblique components. Holocene faults cutting alluvial deposits closely mimic the trend of this fault zone. In one local, these faults connect downward into the main low angle fault surface.

The southeast striking older fault is exhumed in the footwall of the SVFZ in the central Slate Range and offset in the hanging wall in the northern Slate Range. This fault places Miocene (but not Quaternary) rocks onto the metamorphic basement, and currently has dips of horizontal to a few degrees westward. The fault is well documented in the Layton Pass area in the central part of the range. Here Miocene rocks, dipping 45° eastward, end downward into a subhorizontal gouge zone that grades into underlying Mesozoic plutonic rocks. The offset Miocene strata suggest a minimum net slip magnitude of ~5km.

In the northern part of the area, (U-Th)/He thermochronologic ages from apatite are consistent with these cross cutting relationships. Footwall rocks below the older fault record Eocene to Oligocene ages (30-50 Ma) indicating at most moderate fault-related cooling as well as initiation and movement along a low angle fault. Ages drop to Pliocene eastward across the SVFZ then increase again to Oligocene (~35 Ma) at the eastern extent of the exposed footwall. This records important recent cooling, indicating there has been considerable recent motion on the SVFZ.