APATITE (U-TH)/HE AND FISSION TRACK DATA FROM THE CARSON RANGE, NEVADA: IMPLICATIONS FOR NORMAL FAULTING IN THE LAKE TAHOE AREA

Cenozoic extension in the transition zone between the Sierra Nevada and the Basin and Range province at ~39°N latitude has been primarily accommodated by the Genoa fault system along the active eastern flank of the Carson Range. Structures along the W side of the Carson Range, the eastern margin of the Tahoe-Truckee depression, are less well understood. Apatite fission track and (U-Th)/He transects across the Carson Range were devised to constrain the timing and amount of westward fault block tilting and to elucidate the nature of the eastern margin of the Tahoe-Truckee depression. Thermochronological samples from near the eastern range front, close to the Genoa fault, yield apatite (U-Th)/He ages that trend as young as ~10 Ma and apparent fission track ages that are <45 Ma with mean track lengths <12.8 µm. However, these samples do not directly date the onset of exhumation of the Carson Range, since normal displacement along the Genoa fault system has not been sufficient in magnitude to exhume completely reset ages. Fission track modeling results and extrapolation of the age-paleodepth profiles of the (U-Th)/He and fission track systems indicate that the inception of major faulting in the Carson Range occurred around 4 ± 2 Ma, which is in agreement with constraints from N of the Carson Range (Henry and Perkins, 2001). Structurally shallower samples near the crest of the range and along the western range flank of the Carson Range display mean track lengths >13.4 µm and apparent ages between ~60 Ma and 70 Ma, delineating a 60 Ma isochron surface that dips ~15° to the W. No systematic decrease in fission track or (U-Th)/He ages was detected along the western range flank that would be attributable to west-dipping normal faults. These data indicate that the Carson Range is an asymmetrically west-tilted fault block that has undergone ~15° of westward tilting since the Pliocene and strongly support the interpretation that the Tahoe-Truckee depression is a half-graben bound by E-dipping faults, such as the West Tahoe fault, along the western margin of the depression.