QUATERNARY FOLD DEFORMATION ASSOCIATED WITH BLIND THRUST FAULTING, LOS-ANGELES BASIN, CALIFORNIA

Detailed mapping of Quaternary deposits and geomorphic surfaces and quantitative morphometric analyses of landscape elements within the Montebello Hills and Monterey Park Hills are used to characterize the style and patterns of Quaternary deformation associated with the blind thrust fault considered responsible for the 1987 Whittier Narrows earthquake (M(L) 5.9) in the northeastern Los Angeles Basin, California. Results of these studies show that the eastern end of the Elysian Park anticlinorium above the thrust fault consists of two right en echelon asymmetric Quaternary anticlines and that there is a progressive north-to-south migration of the fold front; incipient Holocene folds are developing along the southern margin of the Monterey Park and Montebello Hills. Southward migration of the fold front reflects probable southward vergence and propagation of the underlying thrust fault. Pleistocene and Holocene fluvial terraces and geomorphic surfaces are folded across the crest of both anticlines indicating late Quaternary uplift of at least 150 m and tilting of 10-degrees-30-degrees. Based on available age data for the terraces and geomorphic surfaces, these amounts of deformation provide estimated rates of Quaternary uplift of 0.1-0.2 mm/yr and tilting of 0.2-0.5 murad/yr, roughly an order of magnitude greater rate of deformation than that reported by earlier workers in the region. Based on differences in rate of fold deformation, changes in fold axis orientation, and clear spatial separation of the folds, we infer that the underlying thrust fault consists of two distinct 6- to 8-km-long segments with independent deformation histories. In addition to Quaternary folding, we identify two surface faults, one transverse to and one parallel to the fold axes. Smaller, discontinuous transverse faults also occur in the en echelon stepover region between the two folds and may reflect tear faulting between segments of the underlying thrust fault. Deformation in the Whittier Narrows area occurs in a transitional zone between right-lateral strike-slip deformation on the Whittier-Elsinore fault system and left-oblique contractional deformation on the Santa Monica-Raymond fault system. Crustal shortening in the Whittier Narrows area represents either (1) an area of local compression within a series of progressively smaller left en echelon restraining stepovers in a regional right-lateral strike-slip system or (2) progressive onset of convergence as lateral slip along the Whittier-Elsinore fault system dies out northward and is accommodated by more westerly trending reverse and left-oblique slip faults along the southern margin of the Transverse Ranges.