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Published February 1983 | public
Journal Article

The ophiolitic North Fork terrane in the Salmon River region, central Klamath Mountains, California


The North Fork terrane is an assemblage of ophiolitic and other oceanic volcanic and sedimentary rocks that has been internally imbricated and folded. The ophiolitic rocks form a north-trending belt through the central part of the region and consist of a disrupted sequence of homogeneous gabbro, diabase, massive to pillowed basalt, and interleaved tectonitic harzburgite. U-Pb zircon age data on a plagiogranite pod from the gabbroic unit indicate that at least this part of the igneous sequence is late Paleozoic in age. The ophiolitic belt is flanked on either side by mafic volcanic and volcaniclastic rocks, limestone, bedded chert, and argillite. Most of the chert is Triassic, including much of Late Triassic age, but chert with uncertain stratigraphic relations at one locality is Permian. The strata flanking the east side of the ophiolitic belt face eastward, and depositional contacts between units are for the most part preserved. The strata on the west side of the ophiolitic belt are more highly disrupted than those on the east side, contain chert-argillite melange, and have unproven stratigraphic relation to either the ophiolitic rocks or the eastern strata. Rocks of the North Fork terrane do not show widespread evidence of penetrative deformation at elevated temperatures, except an early tectonitic fabric in the harzburgite. Slip-fiber foliation in serpentinite, phacoidal foliation in chert and mafic rocks, scaly foliation in argillite, and mesoscopic folds in bedded chert are consistent with an interpretation of large-scale anti-formal folding of the terrane about a north-south hinge found along the ophiolitic belt, but other structural interpretations are tenable. The age of folding of North Fork rocks is constrained by the involvement of Triassic and younger cherts and crosscutting Late Jurassic plutons. Deformation in the North Fork terrane must have spanned a short period of time because the terrane is bounded structurally above and below by Middle or Late Jurassic thrust faults. The North Fork terrane appears to contain no arc volcanic rocks or arc-derived detritus, suggesting that it neither constituted the base for an arc nor was in a basinal setting adjacent to an arc sediment source. Details of the progressive accretion and evolutionary relationship of the North Fork to other terranes of the Klamath Mountains are not yet clear.

Additional Information

© 1983 Geological Society of America. Manuscript received by the Society July 8, 1981; Revised manuscript received February 3, 1982; Manuscript accepted March 3, 1982. Ando's field work was supported by grants from the Society of Sigma Xi, the Geological Society of America Penrose Fund, and an internship with the California Division of Mines and Geology. Sincere thanks are extended to these organizations for their generous financial support and encouragement. Parts of this study were initiated in partial fulfillment of requirements for the Ph.D. degree at the University of Southern California. Support from National Science Foundation Grant EAR-7925998 for geochronological work by Saleeby is gratefully acknowledged. Reviews by A. W. Snoke and G. D. Harper and readings of a n earlier version of the manuscript by Richard Allmendinger, Gregory Davis, Gary Fuis, and Michael Rymer generated thoughtful comments and significantly improved the final version. Discussions among the authors of this paper did not always result in complete agreement, and the principal author (Ando) accepts responsibility for the interpretations presented herein.

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