TESTING THE EXTENSIONAL DETACHMENT PARADIGM: SCIENTIFIC DRILLING IN THE SEVIER DESERT BASIN, WESTERN UNITED STATES

Low-angle normal faults or detachments are widely regarded as playing an important role in crustal extension and the development of passive continental margins. However, no consensus exists on how to resolve the mechanical paradox implied by such faults or to account for the general absence of evidence for seismicity. Drilling to a depth of 2-4 km in the Sevier Desert basin of west-central Utah will test the extensional detachment paradigm through coring, downhole logging, biostratigraphic, isotopic and fission-track dating, magnetostratigraphy, and in situ measurement of pore pressure, permeability, fluid chemistry, temperature and stress orientation/magnitude at an example for which evidence of large normal-sense slip on a still-active detachment of particularly low dip (11°) is considered by many to be among the most convincing. Geological studies in the proposed borehole will evaluate evidence for both ductile and brittle deformation vs an unconformity at the Paleozoic-Cenozoic contact, and how the timing of sediment accumulation relates to exhumation of the footwall of the hypothesized detachment. In situ measurements will determine the conditions under which displacement may have taken place, as recently as the Holocene, consistent with the generally accepted interpretation of the geology. A workshop is being organized for 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 (http://www.ldeo.columbia.edu/sevier/icdp/).