Testing the Extensional Detachment Paradigm: A Borehole Observatory in the Sevier Desert Basin, Utah

The Sevier Desert basin, Utah represents a world-class target for scientific drilling and for the development of an in situ borehole observatory of active faulting, with potential for establishing that normal-sense slip can occur along a brittle low-angle fault and, by determining the conditions under which that may take place, for resolving the mechanical paradox associated with such structures. The Sevier Desert detachment was defined in the mid- 1970s on the basis seismic reflection data and commercial wells as the contact between Paleozoic carbonate rocks and Cenozoic basin fill over a depth range of ~0-4 km. Today, the interpreted fault is thought by most workers to root into the crust to the west, to have large estimated offset (< 47 km), to have been active over most of its history near its present 11° dip, and to be associated with contemporary surface extension (a 30- km-long zone of prominent Holocene fault scarps immediately west of Clear Lake). Although no seismicity has been documented on the detachment, its scale is consistent with earthquake magnitudes as large as M 7.0. A published alternative interpretation of the Paleozoic-Cenozoic contact as an unconformity rather than a fault has not been generally accepted. Deformation at detachment faults is commonly spatially restricted, and may have been missed in well cuttings. Exhumation of the detachment would have made it possible to remove critical footwall evidence prior to later sedimentary onlap, particularly at updip locations. The incomplete coverage and uneven quality of seismic reflection data on which the detachment interpretation depends, and an unresolved debate about stratigraphic ties to a critical well, leave room for discussion about interpretive details, including the possibility that deformation was distributed across several closely spaced faults. An apparent mismatch between stratigraphically based ages and fission-track evidence for the timing of footwall exhumation cannot be resolved with available well data. Drilling is now needed to make in situ measurements at depth, to obtain critical core of fault rocks at a down-dip site where offset should be large, and to establish more clearly the relationship between basin development and displacement along the interpreted fault. A workshop will take place from July 15-18, 2008, in Utah, under the auspices of the International Continental Scientific Drilling Program, to flesh out objectives, strategies and operational details, and to develop a consensus on the location of a drill site.