EXHUMATION, SUBSIDENCE, INVERSION, AND UNDERTHRUSTING OF AN INTRA-ARC RIFT BASIN IN THE NORTH FORK TERRANE OF THE KLAMATH MOUNTAINS, CA

New mapping of well-exposed, glaciated areas in the Trinity Alps and Marble Mountains provides evidence for the structural evolution of an intra-arc rift basin in the Mesozoic Klamath orogen and illustrates accretionary tectonics and melange creation in an arc and transform setting.

Ultramafic(10%)-mafic(90%) plutonic rocks exposed over 120 km² in the Trinity Alps are interpreted to be the intrusive floor of the intra-arc rift basin. This fault-bounded, Carboniferous-early Jurassic? batholith contains several intrusive complexes made up of hb- and cpx-rich stocks, dikes, and tabular intrusions. Intrusive contacts are marked by mineral reaction zones, intrusion breccia, hypersolidus strain, and inclusions of meta-ultramafic rock, amphibolite, metagabbro, metabasalt, and metachert. Deuteric and hydrothermal alteration is common. Lower greenschist facies parageneses are pervasive.

Prh-pmp and zeolite-bearing syn-rift volcanic, sedimentary, and hypabyssal rocks overlie plutonic rocks above chloritized, low-angle faults that cut bedding and intrusive contacts. Low-angle faults and upper plate normal faults are crosscut by undeformed hb- and cpx-phyric dikes. The ≈1 km thick upper plate is overlain by a 2-3 km thick basinal sequence of shallow water carbonate, late Triassic chert, and argillite. Regional, high-angle fault zones marked by sheared serpentinite cut igneous and basinal rocks and are intruded by undeformed mid-late Jurassic plutons. Previously mapped as terrane boundaries, some high-angle faults are interpreted to be reactivated rift-related structures.

In the Marble Mountains, arc volcanic rocks, rift-basin strata, and rift-margin coarse-clastic rocks are metamorphosed to low-medium P amphibolite facies. Amphibolites form nappes within a 3-7 km thick structural duplex that is wedged below ≈201-173 Ma greenschist facies arc volcanic and sedimentary rocks and above a regional décollement that is marked by quartzose amphibolite tectonites and gabbroic-dioritic intrusions that are partly ≈165-162 Ma in age. The décollement separates rocks of differing PT, fluid, and strain conditions and contains evidence for both contractional and extensional deformation. After ≈150 Ma, C-S and cataclastic fabrics formed along this structure at greenschist to sub-greenschist conditions.