ALTERNATING EXTENSION AND CONTRACTION IN LATE PERMIAN? TO EARLY JURASSIC ARC-RELATED BASINS IN THE NORTH FORK TERRANE, KLAMATH MTNS, CA

New mapping, structural, and stratigraphic data from well-exposed glaciated areas in the North Fork terrane are integrated with the mapping of Ernst (1998), Irwin (1994), Donato (1985), Mortimer (1984), Welsh (1983), and Ando (1983), and published detrital zircon, radiometric, and paleontologic age data to identify 3 fault-bounded, volcanoplutonic/sedimentary basinal sequences of 1) late Permian?-mid-Triassic, 2) late Triassic, and 3) early Jurassic age. Each sequence has a structural thickness between 2-4km, was weakly metamorphosed at ≤350°C, ≤3kb, contains lithologically and geochemically similar rock types, and contains structural and stratigraphic evidence for synkinematic deposition in arc-related extensional basins. Although complexly deformed, younger sequences unconformably overlie older sequences locally. Lithostratigraphic, provenance, and structural ties between the sequences suggest that a series of three superposed basins developed with older deformed basinal rocks forming basement for younger basins. Plutonic rocks in the oldest sequence contain xenoliths of metaultramafic rock, amphibolite, and oceanic supracrustal rocks interpreted to be original basement. Sequence-bounding faults have cataclastic to phyllonitic fabrics and are interpreted to originally have been low- to steeply-dipping, syn-depositional extensional and transtensional faults. These faults were reactivated and/or crosscut during basin inversion, contraction, and transpression that occurred in the late mid-Triassic, latest Triassic-early Jurassic, mid-Jurassic (Siskiyou orogeny), and during multiple episodes of post-Siskiyou extension and contraction. Sequence 1 and 2 were juxtaposed with the Eastern Hayfork terrane to the west by at least latest Triassic-early Jurassic time. Within the juxtaposed terranes, movement on a proto-Browns Meadow fault is interpreted to have formed an early Jurassic half graben that was filled with ≈4km of arc tholeiites and basinal sediments of sequence 3. Complexly deformed amphibolite nappes in the Marble Mtns are interpreted to be a structural duplex of sequence 1 and 2 rocks that overlies a previously unrecognized regional décollement which formed during the contractional event that ended basin formation in the North Fork terrane between ≈175-165 Ma.