TESTING FAULT GROWTH MODELS WITH LOW-TEMPERATURE THERMOCHRONOLOGY IN NORTHWEST NEVADA

Observed scaling between fault length and finite displacement is the basis for the most common fault growth model that describes simultaneous increases in displacement and length via repeated earthquakes. An alternative model suggests that fault length is established early, and any further growth is accomplished by increases in cumulative displacement. Unfortunately, a paucity of data documenting the lateral growth of faults from field observations has prevented full understanding of the evolution of natural fault systems and thorough testing of these contrasting fault growth models. This study explores how apatite (U-Th)/He thermochronology (AHe) can be used to constrain the spatial evolution of normal fault growth. We report 11 new cooling ages, supplemented by 17 existing cooling ages, to quantify the long-term faulting and exhumation history of the normal-fault bounded Pine Forest Range in the Nevada Basin & Range province. Using multiple vertical sample transects we calculate the spatiotemporal variability in footwall exhumation to determine the best-fit model of fault growth and calculate the rate of lateral propagation. Rapid exhumation began at ~13 Ma in the central Pine Forest Range and continued until at least 10 Ma at a rate of 0.2 km/Myr. Exhumation began at ~11.5 Ma in the northern Pine Forest Range and continued until ~5 Ma at a rate of 0.4 km/Myr, with moderate slip since the late Miocene. The disparity in fault-driven exhumation timing at different locations along strike reveals a lateral propagation rate of ~6 km/Myr. These results show that the fault reached its current fault length within 3-4 Myr and has continued to grow dominantly by displacement accumulation with negligible lengthening since ~8-10 Ma. Our results indicate that the constant-fault length model more accurately describes Basin and Range fault behavior and suggests that its modern physiography was established by the mid-late Miocene. This study is the first to use low-temperature thermochronology to test fault growth models with broader implications for basin evolution, landscape evolution, and earthquake hazard models.