Miocene Rifting in the Los Angeles Basin: Evidence From the Puente Hills Half-Graben, Volcanic Rocks, and P-Wave Tomography

Tom Bjorklund, Kevin Burke, Hua-wei Zhou, & Robert S. Yeats

Published May 2002, SCEC Contribution #642

Formation of the Puente Hills half-graben (PHHG) in the northeastern Los Angeles basin (the NELAB) and eruption of the Glendora and El Modeno Volcanics (16-14 Ma) help to constrain the timing of extension in the basin. At ca.14 Ma, normal faulting on the proto-Whittier fault established the PHHG in which sedimentary strata accumulated between ca.14 and 10 Ma and into which diabase sills intruded. At ca. 7 Ma, N-S contraction began to invert the PHHG, leading to formation of the Puente Hills anticline and the Whittier fault. Our high-resolution 3-D P-wave velocity model shows two anomalous higher-velocity (6.63 km/s) bodies at depths between 9 and 18 km, which we attribute to dioritic plutons named here for Whittier Narrows and El Modeno. The stock-like Whittier Narrows pluton could have been a source for the Glendora Volcanics and the diabase sills in the PHHG. The sill-shaped El Modeno pluton was a likely source for the El Modeno Volcanics. The northwesterly alignment of the plutons may mark the location of the NELAB rift boundary that is associated with the clockwise rotation of the western Transverse Ranges. Three active faults, the Elysian Park blind thrust, the Puente Hills blind thrust and the Whittier fault, converge on the Whittier Narrows pluton, which may have played a role in their location and segmentation.


Citation
Bjorklund, T., Burke, K., Zhou, H., & Yeats, R. S. (2002). Miocene Rifting in the Los Angeles Basin: Evidence From the Puente Hills Half-Graben, Volcanic Rocks, and P-Wave Tomography. Geology, 30(5), 451-454. doi: 10.1130/0091-7613(2002)030<0451:MRITLA>2.0.CO;2.