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 mm/year. The ratio of strike-slip to dip-slip displacement on the Whittier fault over the past ca. 8 My is about 0.4.