Relationship between thrust-fault ramps and normal faults: Results from reflection seismic imaging and finite-element analysis

The Great Salt Lake basin in northern Utah is one of the largest, and perhaps the most continuously active, Tertiary basin in the eastern Basin and Range Province. Reflection seismic data from the basin show that the major basin-bounding faults are listric in nature. Normal-fault reactivation of Sevier-age(?) thrust-fault ramps evident on the seismic data suggests that the maximum depth of penetration and soling of these listric faults may have been strongly controlled by these older structures. Viscoelastic finite-element modeling of reactivation of an older thrust fault demonstrates that a secondary synthetic normal-fault configuration is favored in viscoelastic-brittle rocks subjected to pure extension. Slip on the thrust-fault surface causes brittle failure of rocks in a region where synthetic secondary faulting can take advantage of the preexisting weakness. Slip along the newly formed synthetic fault leads to failure in a different region, where antithetic faulting will occur, resulting in the formation of a graben. Results from the interpretation of Great Salt Lake reflection seismic data show similar relationships.