VERTICAL-DISPLACEMENT HISTORY OF AN ACTIVE BASIN AND RANGE FAULT BASED ON INTEGRATION OF GEOMORPHOLOGIC, STRATIGRAPHIC, AND STRUCTURAL DATA

Knowledge of the last reactivation age of faults in the western Basin and Range Province is key to understanding the evolution of the Pacific-North America plate boundary in the last 30 Ma. It would be especially important to determine how deformation may have been propagating from south to north in the past 5 Ma, which requires knowledge of the slip history of range-bounding faults. The Dixie Valley fault system (DVFS) is an important and still-active normal fault in this region, but its slip history is not well known. Our goals are determining the age of faulting initiation, and the history of vertical displacement at the Ma time scale.

We used well data and seismic profiles from the Dixie Valley geothermal field and geologic maps to calculate offsets of geologic units. Likely tectonic events were determined from knickpoint analysis in the Stillwater Range. We then matched basin unconformities and their ages with knickpoints to obtain a vertical displacement history.

The modern DVFS started between 13 and 7 Ma. The total vertical displacement of the geologic units is between 3 and 3.5 km, and most of it occurred in the last 3 to 4 Ma. Assuming an erosion rate between 0.03 and 0.07 m/ka (i.e. compatible with the age of the basalts capping the Stillwater Range), the maximum vertical displacement rate (~ 0.8 to 1.5 mm/a) occurred between 2 and 0.5 Ma, and resulted in about 1 km of vertical displacement. Finally, the age vs. distance profile of the Stillwater Range crest is tapered at both ends, as expected for a propagating range-bounding normal fault, but it also has two maxima matching the deepest parts of the Dixie Valley basin. This indicates that the present-day DVFS may have started out as two separate strands that connected within 2 - 3 million years of inception. Our work shows that the combination of stratigraphic and structural data from the basin with geomorphological data from the adjacent mountain range is suitable for placing constraints on the displacement history at the Ma-scale of an active range-bounding normal fault system.