Abstract

Deformation of metamorphic rocks, the tonalite of Blue Canyon, several bodies of granite and granodiorite, and the leucotonalite of Ward Mountain within an area of about 600 km2 in the western foothills of the Sierra Nevada, just south of the south end of the western metamorphic belt, is attributed to emplacement of the leucotonalite in a nearly solid state. The east flanks of two plutons of leucotonalite of Ward Mountain in the west-central part of the deformed area dip gently beneath the area of deformed rocks, whereas their west flanks are vertical or dip steeply east. Cata-clastic planar and linear structures in the metamorphic rocks and in the tonalite of Blue Canyon form coherent and related patterns. The planar structures generally strike parallel to contacts with the leucotonalite and dip outward; between the two plutons of leucotonalite, they form a synform. Lineations generally plunge outward from the leucotonalite but trend parallel to contacts within the synform. Heat and possibly volatiles that accompanied emplacement of the rising and westward-moving mass of leucotonalite may have increased the susceptibility of the overlying rocks to attenuate normal to the planar structures and to stretch in the direction of the linear structures. Isotopic U-Pb and K-Ar determinations indicate that the age of the tonalite of Blue Canyon is about 114 m.y. and that the age of the leucotonalite of Ward Mountain coincides with the time of deformation at about 113 m.y. ago. The isotopic age of the granodiorite of Knowles, which truncates cataclastic structures in the tonalite of Blue Canyon and the leucotonalite of Ward Mountain, is 110 m.y.

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