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FOLDING AND UPLIFT [c4, p104-106]

The southern section of the San Andreas fault system resembles an inclined block ( Figure. 4.21), with its raised northeast edge along the San Andreas fault and its downtilted southwest half submerged beneath the Pacific Ocean. Near Los Angeles, topographic relief across this block exceeds 4,300 m: from Mount San Antonio (3,067 m) in the San Gabriel Mountains to the floor of the Catalina Basin (-1,300 m). This sloping and apparently tilted surface actually incorporates the results of strike-slip and reverse or thrust faulting, and regional uplift.

Marine Quaternary deposits of the Los Angeles Basin, Ventura Basin, and adjoining offshore areas are locally as thick as 1,200 m (Yerkes, 1972), and lower Pleistocene beds, folded along northwestward trends, exhibit structural relief of more than 1 km in some parts of the Los Angeles Basin. Upper Pleistocene strata are deformed nearly as much, and evidence from drill holes and surface geologic mapping (Durham and Yerkes, 1964; Yerkes and others, 1965; Yerkes, 1972; Shoellhamer and others, 1981) show that much of the folding is of late Quaternary age. Offshore Pliocene and Quaternary basins and folds also trend northwest. These basins, which border the coast as far south as San Diego (Howell and others, 1978), contain a thinner section of Upper Cenozoic strata than the Los Angeles Basin.

Some of the Quaternary folding, like that along the Newport-Inglewood fault zone, is probably directly related to strike slip on deeper wrench faults (Wilcox and others, 1973). Other, more westerly striking folds may reflect a compressional component of deformation similar to that shown by folds in the central and southern Coast Ranges. Fold patterns in the Los Angeles Basin are complicated by unknown amounts of differential compaction over a faulted and folded basement surface. This surface, nearly 10,000 m deep at the center of the basin (Yerkes and others, 1965, Figure. 2), underlies a nearly complete section of marine Neogene strata, of which the near-surface Quaternary deposits make up a small part. The basin fill thus records almost continuous subsidence and marine deposition from Miocene time to the present.

Northeast of these late Cenozoic coastal basins, crystalline basement rocks either crop out or lie within a few hundred meters of the surface. Neogene subsidence of the coastal marine basins contrasts sharply with sustained and evidently rapid uplift, accompanied by thrust faulting, of the mountainous region to the east and northeast. The north-dipping San Fernando fault provided an example of such range-front tectonics in February 1971. Failure on this fault caused an M=6.6 earthquake, opened a 15-km-long zone of surface thrusting, and elevated the land north of the fault by as much as 2 m. Similarly, major Quaternary upwarping and faulting have helped shape the San Gabriel and San Jacinto Mountains; the San Bernardino Mountains (just beyond the boundary of the fault system as defined here) also display evidence of Quaternary uplift and thrust faulting and have apparently deformed with the other ranges.

Geologically recent uplift and tilting of the mountain ranges is indicated by extensive summit areas above 2,000 m, steep southwesterly-flowing streams in narrow V-shaped canyons, extensive alluvial fans and fan complexes at the northeast and southwest range fronts, and incompletely dissected Quaternary surfaces at elevations near 2,000 m. The dissected upland surfaces, most evident in the San Bernardino (Dibblee, 1975) and San Gabriel Mountains, appear as accordant ridge crests and as remnants of originally extensive uplands, some of which are underlain by Quaternary fan deposits and alluvium.

Other geologic evidence also implies rapid late Pleistocene and Holocene uplift. The Blackhawk and Silver Reef debris-avalanche deposits at the north edge of the San Bernardino Mountains record high-velocity (31 m/s), voluminous (0.28 x 109 m3) debris flows (Woodford and Harriss, 1928; Shreve, 1968), which surged northward from the range front as recently as 17.4 ka (Stout, 1975, 1977), leaving relict lobes of slide debris on the desert floor 10 km from the slide source. Smaller and slower debris flows and rock slides still occur from time to time in both the San Gabriel and San Bernardino Mountains.

Rates of late Quaternary uplift can be estimated for parts of the southern and central San Gabriel Mountains: near the mouth of Little Tujunga Canyon (lat 34�17' N., long 118�22' W.) (Menges and others, 1979), on the North Fork of the San Gabriel River (lat 34�09' N.,long 117�31' W.) (McFadden and others, 1982), and across the Cucamonga fault (lat 34�10' N., long 117�31' W.) (Matti and others, 1982). Strath and fill terraces at the first two localities have been dated by physiographic position, 14C methods, and soil maturity; they range in age from middle Pleistocene to late Holocene (approx 1,000-700 ka). Terrace elevations above modern stream channels reflect both climatic and tectonic influences, but Bull and others (1979) and Menges and others (1979) separated these effects to obtain uplift rates of about 0.3 cm/yr for the Holocene and about 0.03 cm/yr for the late Pleistocene. Interpretation (Matti and others, 1982) of the displacement history across strands of the Cucamonga fault, as discussed above, also results in an uplift rate of about 0.3 cm/yr for the latest Pleistocene and Holocene at the south front of the range. Together, these consistent estimates of uplift rate sample a 78-km segment of the San Gabriel front. Regional uplift rates to the north and northwest and within the main body of the Transverse Ranges are probably somewhat higher; rates for the western Transverse Ranges, as discussed in the section above entitled "Central Section of the San Andreas Fault System," are locally as high as 1 cm/yr near Ventura.

Elevated wave-cut platforms and marine-terrace deposits document Quaternary uplift of the southern California coast from the Transverse Ranges southward to San Diego. Mappable, well-dated (450-85 ka) marine terraces, which correlate with major worldwide highstands of the sea, show nearly uniform coastal uplift at rates that range from about 0.01 to 0.05 cm/yr (Lajoie and others, 1979).

Similar uplift rates (approx 0.02 cm/yr) can be interpreted from the elevations and estimated ages of Quaternary erosional surfaces on the Perris structural block, which lies between the Elsinore and San Jacinto faults. Late Quaternary drainage systems on the Paloma erosional surface are presumed to be superimposed from the Gavilan-Lakeview surface (Woodford and others, 1971), which is nearly 200 m higher and an estimated 1 Ma older than the Paloma surface. Persistent uplift of the Perris block must be geologically recent because a late Miocene erosional surface, overlain by basalt dated at 8.3+/-0.5 Ma (J.W. Hawkins, in Woodford and others, 1971), is at an altitude intermediate between the two younger surfaces. The nearly horizontal remnants of the late Miocene erosional surface and the relatively undeformed detrital and eruptive strata of Neogene age attest to long-term vertical stability of the Perris structural block.

Together, the Quaternary geomorphic and geologic evidence south of the Transverse Ranges implies systemwide Pleistocene and Holocene uplift, but at higher rates toward the northeast than on the Perris and coastal blocks.