Abstract

Recently recovered archival leveling data from southwestern Arizona and adjacent parts of California compel major revisions in the configuration and interpretation of the southern California uplift, both at its culmination and following its partial collapse. Re-examination of the older vertical-control record indicates that similar modifications may be equally appropriate in any reconsideration of the early-20th-century analogue of the southern California uplift. The impact of these revisions is limited to the southeastern part of the uplift, which is now believed to have projected well into northern Mexico. The chief changes in our earlier reconstructions appear as a sharply diminished isobase gradient south of Cottonwood Pass, dramatically reduced tectonic subsidence centering on the Salton Sea, and short-lived uplift of at least 0.3 m at the latitude of El Centro. A newly refined qualitative interpretation of the data implies contraction and decoupling at the base of the seismogenic zone, in conjunction with right-stepping movement and extension between an en echelon transform pair through the Salton Basin, combining to produce both the regional uplift and concurrently developed and relatively localized differential subsidence within the Salton Trough.

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