The Melones fault zone is the easternmosi major element of the Foothills Fault system (Clark, 1960, 1964) of the Western Sierra Nevada metamorphic belt, California. In the southern part of the belt, it commonly separates. Mesozoic rocks on the west from Paleozoic rocks on the east, but to the north, the fault is bounded on both sides largely by Paleozoic rocks. Accordingly, E. M. Moores considers the northern segment to be significantly older than that farther south (Schweickert and Cowan, 1975). Sense of displacement along the zone is uncertain, as is the role of the fault in the tectonic evolution of the belt. For example, the zone has been interpreted as being both of possible dip-slip origin (Knopf, 1929; Russell and Cebull, 1914) and strike-slip origin (Chandra, 1953; Clark, 1960; Baird 1962). Probably, movement was complex and includes both components of displacement (Cebull, 1972), which may be related either to a single oblique-slip motion or to changes of sense with time. Recently, the Melones fault has been interpreted in terms of regional plate-tectobics models. Schweickert and Cowan (1974, 1975) depict the Melones, with some of the other Foothills faults, as sutures between collided arcs. At one place along the Melones fault zone, along the north fork of the Yuba River west of Downieville (Fig. 1), Schweickert (1976) reports lawsonite blueschist and cites such rocks as supporting the proposal that the zone is one of suturing “between diverse terranes juxtaposed by subduction” (p. 409). According to Schweickert and Cowan (1975), the rock succession along the North Yuba River, from Downieville westward for about 30 km, is a melange. If this is correct, the Melones fault zone there represents only the easternmost portion of the melange succession (see Figure 1 of Schweickert and Cowan, (1975)

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