Crustal Architecture and Post-2.5 Ma Shortening Rates Parallel to the LARSE II Profile, Eastern San Fernando Valley, California
A. J. Meigs1 and J. D. Yule2
1Department of Geosciences, Oregon State University, Corvallis, OR
2California State Univ, Northridge, Northridge, CA
A new crustal transect east of and parallel to the LARSE II profile extends from the San Gabriel Fault within the Transverse Ranges to the Los Angeles basin (LAB). Crustal structure is dominated by southern and northern anticlinoria (SA and NA), linked by a syncline beneath the San Fernando Valley (SFVS). The SA extends from the LAB to the SFVS axis and comprises a buried, actively growing monocline in the LAB subsurface, a broad structural high expressed by lower Pliocene-Miocene rocks (cut by the Hollywood fault), and the shallowly north-dipping south limb of the SFVS from south to north, respectively. Growth strata developed across the monocline document growth since ~5 Ma. Geomorphic and paleoseismic data suggest the anticlinorium is underlain by an active blind thrust. The NA comprises a faulted anticline-syncline pair across the Verdugo fault, the Merrick syncline (MS), and two basement-cored folds from south to north, respectively. A conformable Miocene-Pleistocene section underlies the MS whereas upper Saugus Fm. (<2.5 Ma), unconformably overlie pre-Tertiary basement in the northern folds (demonstrating growth prior to 2.5 Ma). The 1971 San Fernando earthquake likely occurred on the low-angle crustal ramp underlying the NA. Roughly 7 km of shortening are accommodated by the SA and NA. Uplift of the base of the SFVS (~2000 m), and the MS (~4400 m), with respect to the central LAB is reflected by the depth to Delmontian strata. North-south shortening estimates using either interpretation imply rates (1.4–2.8 mm/yr), that are 2–4 times lower than geodetic rates. Although the near surface consists of a number of active faults, the characterization of the overall structure as two discrete anticlinorium suggests that the deeper structure is dominated by two moderately north-dipping (~30–40o), crustal ramps. If correct, these ramps are likely splays from the shallowly northdipping master fault imaged by LARSE I at ~15–20 km depth. This structural style probably holds for much of the Transverse-Peninsular Ranges collisional zone.
AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California