REANALYSIS OF THE COCORP UTAH LINE 1 DEEP SEISMIC REFLECTION PROFILE: TOWARDS AN IMPROVED UNDERSTANDING OF THE SEVIER DESERT DETACHMENT CONTROVERSY

The Sevier Desert reflection (SDR), which underlies the Sevier Desert basin, and along which crustal extension has purportedly been accommodated in the eastern margin of the USA Basin and Range, is commonly cited as a “type example” of a low-angle normal fault (LANF) in continental crust. However, the detachment interpretation for the SDR has been much disputed. An alternative interpretation is that the SDR represents a shallow crustal unconformity aligned with a deeper Mesozoic thrust fault. The key data set for interpreting the SDR is the COCORP (Consortium for Continental Reflection Profiling) Utah Line 1 deep seismic reflection profile, which extends over the Sevier Desert basin. Our reanalysis of the SDR on the COCORP profile implies that the irregularities and discontinuities along the SDR are likely to be mostly caused by overlying lateral velocity variations. The most important processing step was to derive source and receiver domain static corrections based on the depth to the first long-wavelength rigid boundary. Similar to the classic refraction statics approach, deeper and longer-wavelength lateral velocity variations are targeted rather than the usual shallow “weathered zone” velocity variations. In this way, the velocity distortions originating above the SDR are suppressed when the seismic data are stacked in the CDP domain. Furthermore, detailed seismic attribute analysis of the reflection character suggests that the physical origin of the surface is highly variable. The reprocessed section reveals a smoother and more continuous SDR, lacking most of the offsets and apparent structural variations on the currently available version of the profile. However, two distinct slope and seismic attribute domains for the reflector appear (steeper dip deeper in the crust to the west; lesser dip shallower in the crust to the east), which are separated by an apparent “hinge” on the eastern flank of the basin. These results appear to be consistent with previous studies that challenge the LANF interpretation of the SDR, in which two reflector domains are expected. On the other hand, the smoother and more continuous nature of the SDR, after reprocessing, may point to a uniform origin for the reflector and thus could alternatively support a LANF interpretation.