Subsurface Structure of the Northern Los Angeles Basin South of the Santa Monica and Hollywood Faults
R. S. Yeats
Geosciences, Oregon State Univ, Corvallis, OR
In contrast to Ventura basin, where modern deformation rates have
operated since the Miocene, northern Los Angeles (LA) is an evolving
deformation system with modern rates and deformation styles
based on structures younger than 1 Ma. The Las Cienegas blind reverse
fault
steps left and eastward to the Elysian Park blind
fault
,
which is taking over the slip rate of 1.5–2 mm/yr formerly concentrated
on the Las Cienegas
fault
. The 1987 Whittier Narrows earthquake
uplifted the anticlinal Montebello and Santa Fe Springs oil
fields and the La Habra syncline between them, consistent with the
lack of thickening of the San Pedro Formation between these eastwest
structures. The dominant Pliocene structure of the Coyote Hills
is an east-striking
fault
with 1.2 km left slip, the Stern
fault
of East
Coyote and South Flank
fault
of West Coyote and Leffingwell oil
fields. This
fault
was folded, along with Plio-Pleistocene strata, above
a north-dipping blind reverse
fault
not penetrated by wells, uplifting
the Coyote Hills. Inactive structures defined by isopachs of the underlying
Plio-Pleistocene Fernando Formation trend northwestsoutheast;
these include the LA trough, the Anaheim nose, and the
proposed Compton-Los Alamitos thrust. The Whittier
fault
is strike
slip based on late Quaternary evidence but its earlier history is mainly
dip slip, including a footwall anticline at Whittier, Sansinena, and
Brea-Olinda oil fields. The change in tectonic style may be due to
the northwest propagation of Peninsular Ranges right-lateral faults
into the LA basin. Earthquakes generated on these northern LA
structures are expected to be in the M 6–7 range, like the 1987 event,
unless they rupture in a cascade, in which case larger magnitudes are
possible.
AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California