Structure and petrology of the Pacific Ridge area, Colusa and Lake Counties, California
Lallemant, Hans G. Avé
Master of Arts
The Pacific Ridge area, located in the northeastern Franciscan terrain, consists of a broad anticlinorium plunging southeast. The core of the anticlinorium is composed of glaucophane lawsonite schists and related rocks of the Goat Mountain Metamorphic Complex. Structurally overlying the Goat Mountain Complex, and separated from it by the Pacific Ridge Thrust, are faintly metamorphosed graywacke sandstones that are exposed on the flanks of Pacific Ridge and eastward to the Coast Range Thrust. Glaucophane-lawsonite schists of probable basalt parentage comprise most of the Goat Mountain Metamorphic Complex, with metachert, metagraywacke, and serpentinite also occurring. The glaucophane porphyroblasts in these rocks often contain relict cores of green hornblende. This, together with occasional occurrences of relict garnet and clinopyroxene indicates an earliest metamorphic event of amphibolite or eclogite facies (Ml), followed by retrograde metamorphismto blueschist facies (M2). The faintly metamorphosed sandstones on the flanks of Pacific Ridge show only faint zeolite facies metamorphism (M3). The earliest deformational event recognizeable in the Goat Mountain Complex (Dla) has produced a striking glaucophane foliation. The next three phases of deformation (Dlb, Die, and Did) have axial surfaces striking northwest to northeast and dipping steeply eastward. The folds are small and generally plunge to 6 degrees north. The D2 event is the first deformational event which has effected all the rocks in the Pacific Ridge area, and has resulted in open, southeast trending folds with nearly vertical axial surfaces. The D3 event has produced open, north trending folds with nearly vertical axial surfaces and is best developed adjacent to the Coast Range Thrust. The final event recognized in the area (D4) consists of open, east trending folds with nearly vertical axial surfaces . The Ml metamorphism is the earliest event to have effected the area and appears to predate all recognizeable structural events. The Dla deformation is contemporaneous with blueschist metamorphism (M2). All subsequent deformational phases are postmetamorphic. This sequence seems to represent a decreasing pressure and temperature environment which probably reflects transport of the rocks back up and out of the northerly striking subduction zone. The D4 event cross cuts the preceding structures and suggest a final phase of . northward directed compression which may be related to the cessation of subduction along western North America in midTertiary time.