Deformation and Metamorphism of the Marble Mountain and Pony Camp Areas, Western Triassic and Paleozoic Belt, Central Klamath Mountains, Northwestern California

Abstract

The Western Triassic and Paleozoic belt (WTrPz) is a regionally extensive, composite terrane correlative with Cache Creek-affinity rocks, a major crust-forming lithotectonic entity of the North American Cordillera. New structural, stratigraphic, and petrologic data suggest that a large tract of greenschist to amphibolite-grade metavolcanic and metasedimentary rock, previously considered to consist of several separate oceanic terranes, is, instead, a single fault-bounded, volcanic island arc, the Sawyers Bar terrane. It represents a mid-Jurassic, relatively intact, recrystallized nappe complex 5 to 10 km thick, extending over 100 km along strike in the central Klamaths. Protoliths of the complex are interpreted to be Lower Triassic (?) to mid-Jurassic supracrustal, volcanic arc-related units deposited, deformed, and metamorphosed within a suprasubduction zone adjacent to the continental margin. Metamorphism increases monotonically with depth in the nappe, ranging from prehnite-pumpellyite to lower greenschist-grade in the Pony Camp area on the south, through greenschist-grade in the medial Sawyers Bar area, to low-pressure amphibolite-grade metamorphism in the Marble Mountains on the north. The Pony Camp area generally lacks penetrative deformation. In the Marble Mountains, peak metamorphism largely postdates intense deformation; nevertheless, folding of fabrics and brittle deformation are common. The complex is bounded by low-angle, W-vergent, crustal-scale, mid-Jurassic thrusts. The Soap Creek Ridge fault juxtaposes Stuart Fork blueschists over the Sawyers Bar complex. The lower thrust is not definitely established, but must be situated beneath tectonic levels postulated by earlier workers. It may coincide with the previously unrecognized brittle-plastic Isinglass shear zone in the Marble Mountains, and a poorly exposed, unnamed low-angle fault in the Virgin Buttes region west of Pony Camp. In this area, mapping indicates that the Twin Sisters fault is a relatively minor high-angle break within the WTrPz, rather than being a crustal-scale terrane suture. Synmagmatic, brittle extensional faults are common, as are syn and postmetamorphic, regionally extensive, high-angle faults that internally imbricate the WTrPz; the latter are marked by sheared serpentinite. Folds within the Sawyers Bar nappe complex are NE to NW-trending and W-vergent. Structural evidence suggests that W-vergent thrusting, E-W contraction, regional Siskiyou metamorphism, penetrative deformation, and crustal thickening occurred at ∼170 to 165 Ma, and preceded voluminous 167 to 162 Ma calc-alkaline plutonism. In the study areas, waning stages of Siskiyou deformation were characterized by thermal relaxation, uplift, extension, crustal thinning, and E-directed tectonic transport. Nevadan age contraction (≈155 to 150 Ma), prevalent to the west at lower structural levels of the WTrPz, is not recognized in the Sawyers Bar nappe; however, regionally developed open folding of Siskiyou metamorphic fabrics and rare superposed folding and axial-plane cleavage development in the Marble Mountains may reflect a Nevadan event. Brittle deformation that clearly post-dates Siskiyou folding is younger than 150 Ma, but is older than ∼130 Ma, the age of the oldest marine strata that overlie the Klamath province regionally. Kinematic evidence from the eastern Marble Mountains suggests sinistral transtension of possibly latest Jurassic-Early Cretaceous age. Late-stage brittle deformation is permissibly Cenozoic; the Sawyers Bar thrust sheet was tilted a maximum of 30° to the south along the flanks of the Condrey Mountain dome during Cenozoic uplift. The Sawyers Bar nappe complex is similar to other composite terranes in Phanerozoic convergent suture zones throughout the world. Like the Klamath Mountains, these areas also may represent different exposure levels within a single fault-bounded entity rather than an amalgam of disparate terranes.