We thank Drs. Elisabetta Erba, Paul R. Bown, and Robert Whitmarsh for their thoughtful and careful critical reviews of this paper and acknowledge with pleasure our ODP Leg 173 colleagues and co-chief scientists for many helpful discussions. Dr. Paul Wallace and the ODP staff most expeditiously provided many items necessary for the study. Discussions with Drs. Gianreto Manatschal and Nikolaus Froitzheim aboard ship and on a field trip they led to view detachment faults in the Swiss Alps following the Leg 173 postcruise meeting were especially instructive and insightful. This study was supported by USSAC funds. The first author thanks the Consejo Nacional de Investigaciones Cientificas y Técnicas-CONICET (Argentina) for facilitating her study-in-residence in Tallahassee. We thank David Davenport for his skillful execution of the figures and Kristeen Roessig and Bryan Ladner for helpful assistance in manuscript preparation. Laboratory facilities were supported by NSF grant OPP-9422893.