Four-dimensional analysis of the inversion of a half-graben to form the Whittier fold-fault system of the Los Angeles basin

Analysis of structural and stratigraphic data from hundreds of oil wells, surface geologic maps and earthquake focal mechanisms reveals a three-phase structural evolution of the Whittier fold-fault system. That evolution progressed from an extensional phase (16 Ma), accommodated by the formation of a half-graben, through an episode of graben filling (14-8 Ma) to lithospheric shortening, which began (ca. 8 Ma) with contractional reactivation of the graben-forming fault as a blind reverse fault and accompanying anticlinal uplift. About 1 Ma, the blind reverse fault broke through onto the sea floor, initiating the activity of the present Whittier fault system. Horizontal contraction estimated from reconstructed cross-sections ranges from 1.4 to 3.2 km and has been at an average rate of 0.3 mm/year in the P direction (N-S) of the 1987 Whittier Narrows earthquake for the past ca. 8 My. That rate may have accelerated after breakthrough of the blind reverse fault to as much as 1.0 min/year. The ratio of strike-slip to dip-slip displacement on the Whittier fault over the past ca. 8 My is about 0.4. (C) 2002 Elsevier Science Ltd. All rights reserved.