ESTIMATING NORMAL FAULT SLIP RATES OVER A TEN MILLION-YEAR INTERVAL USING APATITE FISSION TRACK AND (U-TH)/HE THERMOCHRONOLOGY

Modern geodetic techniques allow rates of deformation to be measured very accurately over periods of years, and dating of individual slip events permits deformation and fault-slip rates to be reconstructed for intervals spanning thousands to tens of thousands of years. Major faults are often active for millions of years, however, and understanding how slip-rates measured over shorter timescales compare to slip rates over the lifetime of a fault requires better constraints on fault-slip rates over geologic (million-year) timescales. Large normal faults in the Basin and Range Province exhume, uplift, and cool footwall rocks as they move, permitting the use of low-temperature apatite fission-track (FT) and (U-Th)/He (He) thermochronology to date the timing and rate of exhumation and fault slip. These methods have been widely used to address the relative timing of faulting across the Basin and Range, and are now being applied to more detailed studies of exhumation and fault-slip rates. Here, we present apatite FT and He data from the Pine Forest and Santa Rosa Ranges in northwestern Nevada that document apparently constant exhumation rates of about 0.3–0.4 km/my for 8-–10 m.y. Apatite FT ages from deeper (> 2 km) levels of the Pine Forest footwall block decrease systematically from 12 Ma to 7 Ma, and apatite He ages decrease from ca. 12 Ma to 3 Ma, consistent with exhumation and fault slip beginning about 12 Ma and continuing to near the present time at a steady rate of about 0.3 – 0.4 km/my. Sparser FT and He data from the nearby Santa Rosa Range indicate a similar exhumation rate of 0.2 –0.3 km/my beginning ca. 11–12 Ma. In a tilting normal fault footwall block, exhumation is related to fault slip and extension rate by fault dip angle and tilt block size; exhumation rates 0.3–0.4 km/my in the Pine Forest Range correspond to a fault slip rate on the order of 0.6–1.2 km/my, and a horizontal extension rate of about 0.2–0.5 km/my. These estimates are within the range of uplift and fault slip rates determined from geomorphologic studies of active faults elsewhere in the Basin and Range, and comparable to exhumation rates determined from recent apatite FT and He studies of other large normal fault systems. The data demonstrate that FT and He methods can be used to constrain normal fault slip rates over intervals of up to 10 million years.