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Abstract Phanerozoic lithologic assemblages associated with specific plate-tectonic regimes are reasonably well understood, especially the siting of igneous and sedimentary rock suites. Less appreciated is the relationship between differential plate motions and subsolidus phase equilibria. Metamorphism is a complex but quantifiable function of the conditions attending recrystallization; in turn, the thermal-depth parameters of contrasting segments of the crust are related to individual plate-tectonic histories.

Detailed mapping of greenschist-facies rocks in the central part of the Western Triassic and Paleozoic Belt near Sawyers Bar, California, demonstrates the stratal continuity and interlayered nature of intricately folded but unbroken, chemically distinctive mafic metavolcanic and metaturbiditic units. This section passes southward without break into the contiguous, weakly recrystallized Salmon River/North Fork amalgamated terrane, and northward, apparently along depositional contacts, into structurally lower, more intensely metamorphosed eastern(?) Hayfork metaclastic units. Although previously regarded as portions of several disparate terranes, these supracrustal rocks all belong to a single lithostratigraphic belt, technically emplaced beneath the more easterly Stuart Fork terrane. Because of the controversial nature of the correlations, this complex is referred to informally as the Sawyers Bar terrane. Contacts in the mapped area are depositional rather than faulted, except for the east-dipping thrust fault bringing the Stuart Fork over the Sawyers Bar terrane. No other terrane boundaries are recognized in the mapped area. This conclusion is supported by petrologic, structural/stratigraphic, and geochemical studies that demonstrate the local continuity of mineralogic, metamorphic, tectonic/stratigraphic, and bulk-rock compositional trends. Evidently this portion of the Western Triassic and Paleozoic Belt formed and evolved as a single, possibly parautochthonous accretionary complex, rather than as a collage of unrelated, exotic crustal entities.

Abstract The Pacheco Pass-San Luis Reservoir area is located in the central Diablo Range along the crest and eastern flank of the uplift. The site is transected by California State Highway 152, a four-lane, divided roadway near and east of the summit (elevation 1,368 ft; 410 m). The area is situated about 120 mi (200 km) southeast of San Francisco and 285 mi (450 km) northwest of Los Angeles. The location is illustrated in Figure 1. Access is provided by Highway 152, and a portion of the former road; four of the six outcrops referred to in this report are roadcuts. Because of heavy automobile and truck traffic on Highway 152, special care must be exercised when examining the rocks, and vehicles should be parked in turnouts or on the shoulder well off the pavement.