Structural chronology, oroclinal deformation, and tectonic evolution of the southeastern Klamath Mountains, California

Abstract

Devonian to Jurassic volcanic, sedimentary, and minor intrusive rocks of the Redding section (RS) accumulated in an intermittently active island arc setting. General stratigraphic concordancy and the downsection continuity of metamorphic facies series indicate that these rocks shared a common postdepositional geologic history. Burial metamorphism produced secondary mineral assemblages of the zeolite, prehnite‐pumpellyite, pumpellyite‐actinolite, and greenschist facies. Close correspondence between pressures calculated from existing stratigraphic overburden and those predicted from the observed metamorphic facies' series indicates that less than 3 km of strata have been eroded from the presently exposed top of the Early‐Middle Jurassic Potem Formation. Shortly after deposition of the Potem Formation, compressional deformation (F1) produced open to isoclinal folds, foliations, and small thrust faults. K‐Ar dates from post‐tectonic intrusions indicate that F1 deformation was completed before 168 Ma. This minimum age of F1 deformation is corroborated by a postfolding paleomagnetic overprint acquired at about 171 Ma. A second suite of intrusions dated at 120–130 Ma provided a thermal pulse that variably reset the K‐Ar system in F1 fabric‐defining celadonite and other relatively Ar‐unretentive materials. The Nevadan orogeny (Late Jurassic) did not significantly deform rocks of the RS, if at all, nor is evidence for Permo‐Triassic (Sonoman) deformation apparent. The geometry of F1 structures varies systematically with location: in the western Bollibokka Quadrangle (BQ), F1 folds are upright, with subvertical axial planes and associated foliations. In the eastern BQ, F1 folds are overturned to the east, with axial planes and associated foliations dipping as little as 40° to the west. Regional relationships suggest that F1 may have been a response to coeval tectonism in the western Klamaths, wherein crustal thickening during regional westward thrusting induced gravitational detachment of the RS from its partly serpentinized ophiolitic basement. F1 structures subsequently were broadly warped about vertical axes, producing variable paleomagnetic declination anomalies that range from 0 to greater than 100° clockwise. The timing and cause of this F2 deformation event are as yet relatively unconstrained but were almost certainly post‐Jurassic and may have been associated with development of the Columbia Embayment. Paleomagnetic data indicate North American paleolatitudes for the RS since at least the Permian, and apparently weigh against an allochthonous or “exotic” origin for this terrane. Paleontologic data indicate strong faunal similarities with North America and to a lesser extent with a Tethyan province. The significance of this apparent dual provinciality remains uncertain, but extreme endemism of Tethyan fusulinids, corals, brachiopods and pelecypods is questionable.