SEGMENTATION OF RAPID VERTICAL CRUSTAL MOTIONS IN THE SOUTHERN SAN ANDREAS FAULT SYSTEM (Invited Presentation)

The southern San Andreas fault (SAF) system provides an excellent natural laboratory for the study of rapid vertical crustal motions within an active strike-slip fault network. Here we combine information from studies of stratigraphy, geomorphology, geophysics, and low-T thermochronology to assess the geometries, rates, and underlying controls on patterns of vertical deformation. A number of areas in this region have experienced rapid (>1 mm/yr) vertical motion since a major reorganization that initiated the San Jacinto and Elsinore faults ~1.2 Ma. Rapid vertical motions in the southern SAF system occur primarily in: (1) narrow fault-bounded crustal slivers measuring 1-5 km across strike and up to 15 km along strike; and (2) larger, relatively intact tilt blocks 25-30 km across strike and 25-40 km along strike. Narrow slivers include the Yucaipa block north of San Gorgonio Pass (5-7 mm/yr uplift), the San Jacinto basin (2-4 mm/yr subsidence), and active post-760 ka uplifts in the Indio and Mecca Hills (1-2 mm/yr uplft). Rapid vertical motions of small fault slivers occur along strike-slip fault segments that are oriented both oblique and parallel to the relative plate motion, suggesting that plate-motion obliquity is not the main controlling factor. This class of rapid vertical motion probably is driven by local stresses arising from geometric complexities in zones of strong transform deformation, and can be explained with modified strike-slip duplex models. Large tilt blocks include: (i) the southern Coachella Valley and Santa Rosa Mts, where crustal-scale NE tilting is indicated by steep range-front morphology on the Santa Rosa fault, large alluvial fans on the NE flank of the Santa Rosa Mts, and asymmetric NE-ward thickening of subsurface sediments toward a steep boundary at the SAF; and (ii) Fish Creek-Vallecito basin, bounded by the Elsinore fault on the SW and the Fish Creek Mts fault on the NE. The Fish Creek Mts occupy the uplifted NE limb of a large W-plunging syncline, and expose basement that has been uplifted and exhumed ~6 km since ~1.2 Ma at ~5 mm/yr. Rapid crustal-scale tilting and folding appear to result from translation of blocks through large bends in the fault zone, and may also be influenced by vertical loading of dense mafic intrusions in the zone of lithospheric rupture beneath the southerm Salton Sea.