Towards Geodetic Observation of Active Mountain Growth in the Sierra Nevada/Great Basin Transition

Hammond, W.C.; Blewitt, G.; Plag, H.-P.; Kreemer, C.
Nevada Geodetic Laboratory, Nevada Bureau of Mines and Geology; whammond@unr.edu

Topographic relief across the Sierra Nevada and Great Basin of the western United States is dominated by mountain ranges and valleys that are the product of active tectonic deformation. The rates of growth of these mountains, i.e. the generation of relief through slip on range-front faults and tilting of crustal blocks are the subject of controversy. For example, estimates of the age of modern Sierra topography vary by one order of magnitude, from 3 to 30 million years. With present elevations near 3 km, the more rapid of these implied rates is within reach of GPS measurement precision (likely <0.5 mm/yr for permanent continuous stations with many years of data). We evaluate the ability of data from western U.S. high precision GPS networks, e.g. EarthScope Plate Boundary Observatory (PBO) and the University of Nevada Mobile Array of GPS for Nevada Transtension (MAGNET) to constrain 1) the rate of vertical motion and tilt of the Sierra Nevada/Great Valley microplate, and 2) normal faulting rates which indicate active uplift on fault-bounded blocks in the Sierra Nevada range front, Walker Lane and Basin and Range. To facilitate this we employ a block model that is constrained by vertical and horizontal GPS components, which uses measurements made in the interseismic time to infer long-term rates. Because we seek to infer long term uplift rates we must also correct for postseismic transients from earthquakes that can distort the GPS velocity field. The signal of viscoelastic relaxation from historic earthquakes in Central Nevada is clearly visible in the vertical velocity field. We remove this transient relaxation by subtracting the predictions from a model of the viscoelastic process. The vertical component from the rebound of postglacial Pleistocene lakes in the Great Basin (e.g. Lake Lahontan), secular and annual changes of snow load on the Sierra Nevada also need to be considered, however, the contributions that these processes have to the modern geodetic rates is less certain and needs to be explored. Preliminary results indicate that several long-running stations on the west slope of the Sierra Nevada move upward at near 1 mm/yr, in agreement with models that call for a younger Sierra Nevada. PBO stations running over 3 years corroborate this rate of motion but are less precise owing to their shorter time series.