The Klamath Mountains province of northwestern California has long been regarded by geologists as a probable northern continuation of the Sierra Nevada province. Attempts at stratigraphic correlations between the two provinces date back to the turn of the century, but attempts at correlation of tectonic elements have been hindered by a lack of data. Clark (I960) proposed a general correlation of north-striking faults in the Klamath Mountains and in the western Sierra Nevada. High-angle faults of the latter area constitute the Foothills fault system, which Clark believed to be of probable strike-slip origin. Recent studies in the southeastern Klamath Mountains demonstrate that the major faults of that region are thrust faults along which thrust plates have moved relatively westward. The contrast in geometry between subhorizontal to moderately dipping Klamath faults and steeply dipping faults of the Foothills system does not refute correlation of the two but can be explained by an eastward steepening of the Klamath thrusts into faults continuous with those of the western Sierra Nevada.

Geologic relationships indicate that the Melones fault of the Foothills fault system extends northward into the eastern Klamath region as the Trinity thrust fault. Correlation of the two faults appears probable on the basis of (1) stratigraphic and structural similarities in their hanging wall blocks, (2) the presence of thick bodies of serpentinized peridotites below the Trinity thrust plate and below the hanging wall of the Melones fault in the northwestern Sierra Nevada, (3) permissive evidence for a Late Jurassic age of the two faults, and (4) a postulated curvilinear trace for the Trinity-Melones fault which is parallel to observed curvilinear structural trends farther west. It is believed that steep minor structures within fault zones of the Foothills system, cited by others as evidence for strike-slip faulting, are not incompatible with an origin for the system by thrust faulting. Three of the four fault-bounded Klamath subprovinces are interpreted as having stratigraphic and structural counterparts in the northwestern Sierra Nevada. The Klamath central metamorphic subprovince is regarded as a thrust slice of Devonian(?) metamorphic rocks, which thins northward and southward and is not present in the Sierran region.

Eastward steepening of the proposed Klamath-Sierran thrust faults is related to their increasing proximity to the Sierran synclinorium and to their rooting into its western flank. Formation of the thrusts occurred in the Jurassic during a lengthy interval of profound crustal downbuckling and shortening. Rooting of the thrusts into a crustal zone undergoing contemporaneous subsidence and shortening rules out an origin for them by gravitational tectonics from an orogenic high or by extrusion tectonics from a dilating zone. The geometry of Mesozoic orogenesis in the Klamath-Sierran region, including thrust faulting, can best be explained by the convergence of subcrustal convection cells along the axial zone of the Sierran synclinorium.

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