Retrodeformable cross sections across the Los Angeles area interpret the Pliocene to Quaternary deformation to be a developing basement‐involved fold and thrust belt. The fold and thrust belt is seismically active as evidenced by the 1987 Whittier Narrows earthquake (ML = 5.9) and the 1971 San Fernando earthquake (MW = 6.6). The structural geology of the Los Angeles area is dominated by three major compressional uplift trends: (1) the Palos Verdes anticlinorium and western shelf, (2) the Santa Monica Mountains anticlinorium, and (3) the Verdugo Mountains‐San Rafael Hills and the San Gabriel Mountains. These trends result from major thrust ramps off a detachment(s) at 10–15 km depth. Thrusts of the Verdugo Mountains‐San Rafael Hills and the San Gabriel Mountains reach the surface; the other two uplifts are associated with blind thrusts. Compressional seismicity is concentrated along these thrust ramps. The 1987 Whittier Narrows earthquake probably occurred on the Elysian Park thrust which underlies the Santa Monica Mountains anticlinorium. The thrust interpretation accounts for the geometry of the anticlinorium, the seismological characteristics of the earthquake, and the geometry of coseismic uplift. The earthquake and aftershocks occurred within a structurally complex, narrow zone of Miocene and Pliocene northwest trending faults that cross the anticlinorium at a high angle. These northwest trending faults are interpreted to be reactivated faults now behaving as tears in the Elysian Park thrust and not the result of active right‐lateral deformation extending into the Whittier Narrows area. Our analysis suggests the Whittier Narrows earthquake sequence occurred within a structurally weakened zone along the Elysian Park thrust. We also suggest that the Whittier fault is not an important Quaternary structure and may not be seismogenic. The regional cross section is a nonunique solution, and other possible solutions are considered. Multiple solutions arise from the presence of two intersecting compressional belts in the Los Angeles area: the Transverse Ranges and the northern Peninsular Ranges. The belts may be due to one or more regional detachments and the northern Peninsular Ranges may be northeast or southwest vergent. The deformed top of the crystalline basement along the regional cross section requires a minimum of 15.0 km of north‐south convergence between the Palos Verdes Hills and the San Andreas fault regardless of the structural solution. Restoration of our cross section solution requires 21.4 km of north‐south convergence on top of the crystalline basement (including 6.4 km of slip continuing offshore to make structures of the continental borderland) and 29.7 km of convergence on the basal detachment. Geologic relationships suggest major shortening started between early and late Pliocene time (2.2–4.0 Ma) which yields a minimum convergence rate of 3.8–6.8 mm/yr between the Palos Verdes Hills and the San Andreas fault. Convergence rates for our solution range from 5.4 to 13.5 mm/yr between the edge of the...

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The Whittier Narrows earthquake sequence (local magnitude, M(L) = 5.9), which caused over $358-million damage, indicates that assessments of earthquake hazards in the Los Angeles metropolitan area may be underestimated. The sequence ruptured a previously unidentified thrust fault that may be part of a large system of thrust faults that extends across the entire east-west length of the northern margin of the Los Angeles basin. Peak horizontal accelerations from the main shock, which were measured at ground level and in structures, were as high as 0.6g (where g is the acceleration of gravity at sea level) within 50 kilometers of the epicenter. The distribution of the modified Mercalli intensity VII reflects a broad north-south elongated zone of damage that is approximately centered on the main shock epicenter.

The Santa Ana Mountains expose one of the most complete strati• graphic sections in coastal southern California, and much of the section contains distinctive rocks and diagnostic fossils. The section includes a basement complex of crystalline and semicrystalline rocks of Mesozoic age, unconformably overlain by as much as 5,200 m of Upper Cretaceous and Cenozoic clastic strata ranging in composition from organic shale to boulder conglomerate. The basement consists of the marine clastic Bedford Canyon Formation of Middle Jurassic age and is perhaps 5,500 m thick; the Upper Jurassic(?) and Lower(?) Cretaceous Santiago Peak Volcanics, 790 m thick; and extensive middle Cretaceous quartz plutonite bodies. The superjacent sedimentary deposits are mostly marine and of Late Cretaceous, Paleocene, Eocene, Miocene. Pliocene, and Pleistocene age. In addition, nonmarine units interfinger with marine, especially just above major unconformities. The northeastern, highest part of the Santa Ana Mountains is a sharply folded anticline, plunging northwest and having a downfaulted, narrow northeast limb and a similarly truncated nose. The downthrow to the north, on the Whittier fault zone, is variable, with an apparent stratigraphic separation of about 900 m. That to the northeast, on the Elsinore fault zone, may be slightly greater. The low western parts of the mountains are underlain by rocks that are only moderately folded, broken by numerous northwest-to north-trending normal faults. A complex, horstlike positive structure extends east-west along lower Santiago Creek. The broad Lorna Ridge syncline farther south underlies !lluch of the southern part of the mapped area. At the west edge of the mountains, the post-middle Miocene formations thicken slightly westward toward the subsiding Los Angeles basin, though both thickening and subsidence reverse directions in the subsurface anticlinal Anaheim nose, 5 or 6 km west of the mountains. The geologic map, structure sections, and supporting data show that the geologic record of the northern Santa Ana Mountains characterizes that of the northern Peninsular Ranges, including the San Joaquin Hills to the south and southwest, the Los Angles basin to the west, the Puente Hills to the north, and the Perris Block to the east. Thus, the mountains furnish evidence of regional significance: the boundary between basement and superjacent sedimentary rocks is of Early or middle Cretaceous age; northeastward transgression of Paleocene strata onto successively older units records an early Tertiary southwestward tilt of the mountain mass, and northward and eastward onlap of successively younger strata onto the basement surface is interptreted as additional evidence of an extensive, persistent early Tertiary peneplain; the tilting of the mountain mass and associated deformation continued to middle Miocene time, when relative depression of the Los Angeles basin began; and continuing deformation produced pronounced erosional unconformities in upper Miocene, Pliocene, and upper Pleistocene strata. TABLE ...

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Igneous rocks _ ________________________________ 316 Tuffs of the Topanga formation _ _____________ 316 El Modeno volcanics_ ______________________ 316 Basalt flow member..___________________ 316 Palagonite tuff and tuff breccia member.. _ 319 General description. _______________ 319 Palagonite tuff breccias__________ ___ 319 Bedded palagonite tuff______________ 319 Andesite flow and flow breccia member_ ___ 320 General description_________________ 320 Flow breccias _____________________ 320 Lava flows_________________________ 320 Hydrothermal alteration...__________ 321 Limy volcanic breccia_______________ 321 Undifferentiated volcanic rocks.__________ 321 Andesite dikes-__-____-____________-_-321 Associated igneous rocks.____________________ 322 Breccia-___________________________ 322 Basalt dikes__________________________ 322 Descriptive geology-Continued