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FOLDING AND UPLIFT [c4, p97-98]

Typically, Quaternary folds are more intense and better defined in thick Tertiary marine sequences than near or above such basement blocks as the granitic and metamorphic complexes of the Santa Lucia, La Panza, and Gabilan Ranges or the Franciscan core of the Diablo Range. Thus, some of the best expressed Quaternary folds are in the western Transverse Ranges (Bailey and Jahns, 1954, p. 95-98), the west side of the San Joaquin Valley (Harding, 1976), the Santa Maria Basin (Woodring and Bramlette, 1950, p. 109-116), and the southeast flank of the Santa Lucia Range (Compton, 1966). Harding (1976) used subsurface geologic data from oil fields northeast of the San Andreas fault to demonstrate that folding has moved outward from the San Andreas over time-the more northeasterly folds along the west side of the San Joaquin Valley being the youngest.

Rates of Quaternary folding in the westernmost Transverse Ranges exceed those elsewhere in the fault system. On the coast near Ventura (lat 34�07' N., long 119�18' W.), Pleistocene basin deposits more than 4 km thick have been intensely folded and faulted during the past 200 ka, attaining dips of 32�-45� in the southern limb of the Ventura Avenue anticline ( Figure. 4.15); uplift rates of about 1 cm/yr (Yeats, 1977; Lajoie and others, 1982) characterize the axis and southern limb of this fold. Uplift and folding elsewhere in the western Transverse Ranges may be slower but probably still exceeds deformation rates elsewhere in the fault system.

Evidence for Quaternary uplift in this part of the San Andreas fault system is abundant and varied. Highstanding ranges (Diablo, Santa Lucia, La Panza, and Transverse) emerge from a cover of their own eroded debris, represented by the Paso Robles Formation of the Coast Ranges, the Tulare Formation of the San Joaquin Valley and northern San Emigdio Mountains, and the Saugns Formation (and equivalent marine deposits) of the western Transverse Ranges. These upper Tertiary and Quaternary strata consist chiefly of alluvial debris, which has been deposited in fans, channels, and flood basins. Although the stratified deposits have been tilted, uplifted, and eroded (Bailey and Jahns, 1954; Christensen, 1965; Compton, 1966; Yeats, 1977; Davis, 1983), the map pattern of their remnants documents regional patterns of uplift.

Christensen (1965) employed such evidence ( Figure. 4.16) to document uplift of 600 m in the Diablo, Santa Lucia, and La Panza Ranges and to establish general system-wide uplift, north of the Transverse Ranges, of about 300 m. This broad pattern of late Pliocene and Quaternary uplift is reversed chiefly in the Salinas and Santa Maria Valleys (lat 36�09' N., long 121�09' W., and lat 34�34' N., long 120�15' W.), Kettleman Plain (lat 35�55' N., long 120�05' W.), and southern San Joaquin Valley, where elongate deep (100-900 m) but local basins have formed at the edge of basement blocks.

Late Pliocene and Quaternary uplift of the northern San Emigdio Mountains totals about 1,800 m (Davis, 1983, p. 299-300), the present average height of a Pliocene erosion surface preserved on accordant ridge crests near the summit of the mountains. Relatively rapid uplift of the western Transverse Ranges was also accompanied by local downwarping of narrow basins-for example, the offshore Santa Barbara Basin and the Santa Clara River valley ( Figure. 4.15) east of Ventura.

Evidence that uplift is still continuing is found in raised, paired stream terraces and youthful drainage patterns in the Santa Lucia, Diablo, and western Transverse Ranges, in elevated Holocene marine terraces near Ventura (Lajoie and others, 1982), and in the radially segmented slopes of Holocene alluvial fans that record progressive episodes of deformation along the east front of the Diablo Range (Bull, 1964).