Special San Fernando Earthquake Edition

THE GEOLOGIC SETTING

By Gordon B. Oakeshott

Although data are incomplete and "conclusions" are certain to be modified later, the big picture of what happened, geologically, is emerging. It appears likely that a block of the western San Gabriel Mountains, including a surface area of at least a hundred square miles, moved relatively upward and southwestward several feet-perhaps about 4 feet vertically and a couple of feet horizontally. Movement originated at a focal depth of about 12 km (7 to 8 miles), with the epicenter about l0 km (6 miles) north-northeast of Sylmar. Displacement probably took place along a highiy irregular fault surface dipping about 40 - 60° northward from discontinuous surface fault ruptures appearing as several traces of very much disrupted soil and sedimentary rock striking about N 80° W in the northern part of the San Fernando Valley. Thus, this earthquake originated in displacement along a reverse or thrust fault--like Arvin-Tehachapi in 1952--in contrast to all other known California earthquakes which have resulted from movement on strike-slip or normal faults.

How does this behavior fit into what we know of the geology of the area? What has been the history of development of this great mountain range which lies in the central part of the Transverse Ranges of southern California? Particularly, what has been the late history of faulting, folding, and mountain-building in this region?

The rocks

Rock formations in the San Gabriel Mountains include most major rock types in great variety, ranging from Precambrian igneous and metamorphic rocks to Holocene (Recent) alluvium. The Precambrian crystalline rocks consist principally of anorthosite and related types, radiometrically dated as 1.2 billion years old, which have intruded the 1.4 billion-year-old Mendenhall Gneiss. The Precambrian rocks are found mainly north of the San Gabriel fault. Mesozoic granitic rocks, at least some of which have been dated at about 70 million years old, crop out both north and south of the San Gabriel fault. South of the fault they carry some inclusions of Paleozoic(?) igneous and metamorphosed sedimentary rocks.

The crystalline rocks which form the central core in the highland part of the San Gabriel range are flanked on the north, west, and south by overlying younger Tertiary sedimentary and volcanic rock formations. Small bodies and fragments of Paleocene (60-70 million years old) marine sandstone and conglomerate have been sliced into the San Gabriel fault zone. Paleocene to middle Eocene marine sandstone and conglomerate occur at the extreme western end of the range.

The later Tertiary and Quaternary sedimentary rocks around the western San Gabriel Mountains were deposited in two principal basins, both representing arms of the eastern end of the great Ventura Basin: the Soledad basin northeast of the San Gabriel fault, and the San Fernando basin a few miles southwest of that fault. In the Soledad basin, late Eocene to late Miocene time is represented by about 14,000 feet (4200 m) of non-marine and volcanic rocks of the Vasquez, Tick Canyon, and Mint Canyon Formations. Remnants of the continental and nearshore middle Miocene Topanga Formation are found in the San Fernando basin. The marine middle to upper Miocene Modelo Formation is thickest in the Little Tujunga syncline in the San Fernando basin but is also present overlying the Mint Canyon Formation in the Soledad basin. Some of the fault traces of the February 9th earthquake appear as bedding plane faults in sandstone and shale of the Modelo Formation at the northern margin of the San Fernando basin. Lower Pliocene marine beds and upper Pliocene shallow-water marine to brackish-water beds are found in both basins. Continental gravel and coarse sandstone of the lower Pleistocene Saugus Formation are very widely distributed but the formation reaches its maximum thickness of over 6,000 feet on the south flank of the range in upper Lopez Canyon. Coarse reddish-brown breccia of the middle Pleistocene Pacoima Formation underlies the northern margin of the San Fernando Valley in the area from the Olive View Sanatorium to the Veterans Hospital. A succession of nearly flat-lying Pleistocene river terrace gravel s flanks the mountains. Quaternary sediments (Alluvium, Terrace deposits, and Pacoima and Saugus Formations) reach extraordinary depths along the northern margin of the San Fernando basin: an exploratory petroleum well drilled about half a mile west of the Olive View Sanatorium revealed that Quaternary(?) gravels reach a depth of at least 12,000 feet (4000m)!

The structure

Structural features of the western San Gabriel Mountains are dominated by the near-vertical, right-lateral, strike-slip San Gabriel fault, which extends obliquely across the mountains on a strike of about N65 W°. The pattern of faulting comprises (1) a principal series of steeply north-dipping fault planes along the San Gabriel fault; (2) north of the San Gabriel fault, a series of complementary left-lateral strike-slip faults trending approximately N65° E; and (3) in the block south of the San Gabriel fault, the series of discontinuous north-dipping reverse faults of the Santa Susana and Sierra Madre fault zones. The third series includes the fault ruptures of the February 9th earthquake.

The most prominent fold structure of the Soledad basin is the northeast striking, west-plunging Soledad basin syncline. In the San Fernando basin, the dominant fold is the Little Tujunga syncline whose axis lies close to the northern margin of the Quaternary sedimentary rocks. The axis of this fold closely parallels the general trace of the Sierra Madre fault zone, following it with a west-northwest trend from Tujunga to the Veterans Hospital at Loop Canyon where it seems to have been overridden by crystal-line rocks along the Hospital fault. Continuing westward, the synclinal axis turns southwest, paralleling the northeastern end of the Santa Susana fault zone.

The Sierra Madre rverse-fault zone

The Sierra Madre fault zone comprises the series of discontinuous reverse faults extending about 12 miles from the northeast end of the Santa Susana fault on the west to the Rowley fault across Big Tujunga Canyon on the east. These are arcuate, convex-southward reverse faults which separate the pre-Tertiary crystalline rocks on the north from the Tertiary and Quaternary sedimentary formations on the south. The faults are discontinuous, with dips ranging from 150 to vertical; all dip northward with the crystalline rocks thrust upward toward the south over sediments as young as the mid-Pleistocene Pacoima Formation. Displacement has been essentially of the dip-slip type and has been calculated to range from 200 to 4,000(60 - 1200m) feet on individual faults in the zone. Although the Pacoima Formation has clearly been both folded and faulted, no positive viled evidence ws found to indicat that latest Pleistocene terrace dposits were offset.

Mid-Pleistocene to present mountain building

The mid-Pleistocene orogeny was the major event in the building of the modern San Gabriel Mountains. At that time the lower Pleistocene Saugus Formation and all older formations were intensely folded by north-south directed compressional forces. Large movements-probably both vertical and horizontal-took place in the San Gabriel fault zone; the Sierra Madre zone of reverse faulting was developed; initial movements on some faults, and renewed movements on others, took place along a system of northeasttrending faults in the north block of the San Gabriel fault zone; and the central crystalline rock block was elevated several thousands of feet.

Continuation of vertical uplift is indicated by the development of very steep-sided canyons like Big Tujunga and Pacoima, as much as 3,000 feet deep, which have cut head-ward into the pre-Saugus surface of erosion but have not yet destroyed it. Essentially the same pattern of uplift appears to be continuing today, as evidenced by the February 9th fault movements and earthquake!

REFERENCES

Hill, Mason L. 1930. Structure of the San Gabriel Mountains north of Los Angeles, California: University of California Department of Geological Sciences Bulletin, vol. 19, pp. 137-170.

Jennings, C. W. and Strand, R. G. 1969. Los Angeles sheet: Geologic Atlas of California: California Division of Mines and Geology.

Oakeshott, G. B. 1958. Geology and mineral deposits of San Fernando quadrangle, Los Angeles County, California: California Division of Mines Bulletin 172, 147 pp.