Abstract

Recent debates on the development of detachment faults during continental extension have been centered on whether they were active at low angles and initiated as relatively planar faults. In addition, intense investigations have concentrated on how the detachment-fault geometry evolved in time and space and how the extension was accommodated in their upper and lower plates. The superb exposure of the mid-Tertiary Whipple Mountains detachment fault system in south-eastern California provides an unusual opportunity to address these questions. Analysis of the stratigraphy and structures of upper-plate rock units constrains the three-dimensional kinematic evolution of the mid-Tertiary Whipple-Chemehuevi detachment fault system in southeastern California. Tertiary volcanic strata in the northwestern Whipple Mountains and central Chemehuevi Valley consist of similar rock types that have undergone about 15% extensional strain. In contrast, a different succession of Tertiary volcanic and sedimentary rocks underlies the northeastern Whipple Mountains along the structural trend of central Chemehuevi Valley; these strata have been extended by 40%. Differences in magnitudes of extension and mid-Tertiary stratigraphies between these two areas argue for the presence of a synformal, gently east-dipping normal fault above the Whipple fault in the central Chemehuevi Valley. This fault accommodates 15 km of the total 40-km extension along the Whipple-Chemehuevi detachment fault system. In addition, tilting patterns of upper-plate strata and the presence of an areally extensive syn-extension landslide (>70 km2)show that (1) the Whipple detachment fault was active at low angles (<30°); (2) the Whipple detachment fault was relatively planar at the time of its initiation and became antiformally warped during its late stage of development; and (3) the antiformal warping of the Whipple detachment fault produced enough topographic relief to expose the lower-plate mylonitic rocks and to initiate a large landslide (>70 km2) from the north side of the Whipple Mountains; its emplacement occurred before the end of detachment faulting.

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