Sierra Madre thrust fault, Arcadia, California

Abstract One of the best exposures of the Sierra Madre thrust fault can be seen in the west wall of Santa Anita Canyon in Wilderness Park, city of Arcadia, 15 mi (25 km) northeast of downtown Los Angeles. Figure 1 shows how one can drive to the fault exposure by turning left into the first available parking area in Wilderness Park. The park gates are open 900 am to 5:00 pm daily. The 54-mi-long (90 km) Sierra Madre fault zone forms the southern base of the San Gabriel Mountains along which they have been thrust over the valleys to the south. The 1971 San Fernando M 6.4 earthquake, caused by this fault, created 15 mi (25 km) of surface ruptures beginning 24 mi (40 km) west of this exposure.At the west side of Arcadia’s Wilderness Park (Fig. 2), banded gneiss is thrust over old alluvium containing large boulders. The fault consists of several feet of gouge and crushed rock generated from the gneiss; it dips 35 degrees north. The fault cannot be traced into the upper part of the old alluvium and probably has been inactive since it was deposited, roughly 2,000 to 5,000 years ago (Crook and others,1978). Two of the earliest geologists to describe the San Gabriel Mountains (Davis, 1927; Miller, 1928) believed the range was bounded on the south by normal faults. Mason Hill (1980) was first to show that the western part of the range was not created by the typical basin and range-type faulting. In the late 1930s, John P. Buwalda (1940) mapped portions of the fault zone between La Canada and Monrovia. He was the first to describe the Sierra Madre fault as “a rather wide zone complexly braided as to pattern of fracture lines” rather than a single line trace. It was not until the 1960s that portions of the fault zone were mapped in detail. This mapping was done bya group of Metropolitan Water District (MWD) of Southern California geologists of which the authors were a part. The 1:12,000 scale mapping was carried out between the ArroyoSeco on the west and Sycamore Canyon on the east andestablished 11 localities where crystalline basementrocks could be seen thrust over Quaternary alluvium. The Wilderness Park site was, to our best recollection, discovered during this project by Daniel C. Kalin and isjudged by us to be the best easily accessible example. The first published map showing the fault zone and the Wilderness Park site was produced by Douglas M. Morton(1973). His map shows this fault trace to branch into the interior of the mountain range rather than follow the southern base as does the main fault zone. From 1975–78 the authors, along with C. R. Allen, B. Kamb, and C. M. Payne mapped the fault zone in greater detail, and trenched the fault, as part of a USGS grant to Caltech (Crook and others, 1978). The area mapped is a strip 1.2 to 2.4 mi (2 to 4km) wide extending 24 mi (40 km) from Big Tujanga to San Gabriel Canyons. The 14C dates obtained from the Caltech work reveal that the central part of the Sierra Madre fault has notmoved in 5,000 years. This contrasts with the 1971 earthquake to the west and the recent evidence that the Cucaonga branch to the east has a recurrence interval of about 800 years (Morton and others, 1982). Is the central part of the fault zone truly orders of magnitude less active, or has it accumulated enough strain energyto be currently worrisome?