INTRODUCTION

Rocks within the Chewelah quadrangle record a complex and varied structural history from the Late Proterozoic to the Eocene, one that is characterized by several discrete periods of igneous intrusion, folding, normal and reverse faulting, and repeated reactivation of faults during succeeding periods of faulting. The western third of the quadrangle includes a segment of the Kootenay arc, a highly faulted and folded allochthonous sequence of Middle and Late Proterozoic and Paleozoic rocks. Within the quadrangle these rocks have a consistent north-northeast strike. The Jumpoff Joe Fault sharply defines the east boundary of the Kootenay arc, juxtaposing it against coeval rocks of the Belt Supergroup and Paleozoic rocks that have distinctly different structural and lithostratigraphic characteristics. Much of the eastern half of the Chewelah quadrangle is underlain by the western part of the (informal) Priest River complex, which consists of intermixed, heterogeneous two-mica granitic rocks and highly recrystallized Middle Proterozoic rocks. The shallowly dipping Eocene Newport Fault separates the midcrustal crystalline rocks of the Priest River complex from an overlying, relatively thin, spoon-shaped, detached flap of essentially unmetamorphosed Belt rocks. Numerous Cretaceous plutons intrude the hanging-wall Belt rocks, and scattered, syntectonic Eocene nonmarine sedimentary and volcanic rocks are found chiefly in low-lying areas. Eocene normal faults associated with unroofing of the Priest River complex and development of the Newport Fault appear to be the youngest structures affecting rocks in the quadrangle. Granitic rocks that range in age from Triassic or Jurassic to Eocene and represent five discrete intrusive episodes form about 50 percent of the rocks in the area.