A NEW METHOD TO CONSTRAIN FAULT DIPS USING LOW-TEMPERATURE THERMOCHRONOLOGY: AN EXAMPLE FROM THE GOLD BUTTE BLOCK OF SOUTHEASTERN NEVADA

There has been much debate concerning whether slip occurs on low-angle detachment faults, especially when classical fault mechanics (Andersonian theory) are taken into account. Does slip occur on low-angle faults or are these faults initiated at high angles and then rotated into a low-angle geometry? Constraining the initial dips of detachment faults, or the evolution of dip during movement (e.g., perhaps indicative of a rolling-hinge mechanism) is an essential ingredient to understanding extensional tectonics. Low-medium temperature thermochronologic techniques such as ⁴⁰Ar/³⁹Ar, fission track and (U-Th)/He use a number of approaches to constrain fault dip. These are mainly (but not exclusively) associated with determining pre-extensional paleotemperature contours in the lower plate and a pre-extensional geothermal gradient (assumed or otherwise constrained) to constrain the initial dip of a fault. The Gold Butte block (GBB) is an easterly tilted crustal block, tectonically exhumed along the Lakeside Mines fault (LMF). The initial dip of ca. 60° on the LMF was previously constrained by restoration of easterly dipping Paleozoic strata into paleo-horizontal. A large database of apatite fission track ages from the GBB plotted vs paleo-depth reveal the classic form of an exhumed apatite PAZ and indicate the onset of rapid cooling due to tectonic exhumation at ca. 17 Ma and the vertical exhumation rate as ≈7.2 km/m.y. A plot of AFT age vs horizontal distance for rapidly cooled samples projected onto a line parallel to the extension direction yields a slip rate along the fault of ≈8.6 km/m.y. Treating the vertical exhumation rate and the slip rate as vectors, the angle (Φ) between them constrains the dip of the fault (90-Φ = fault dip). Using this approach, the dip of the LMF as it slipped was ≈57°, in agreement with the structural reconstruction. While uncertainties on the regression-constrained rates can be potentially quite large, large databases and higher precision techniques will reduce the uncertainties. Application of this approach to multiple thermochronometers will theoretically trace the movement (and evolution of the dip) of the fault through a range of temperatures, taking into account all caveats about the dynamic nature of thermal regimes during tectonic exhumation.