STRUCTURE OF THE NORTHERN SIERRA NEVADA, CALIFORNIA
SCHMIDT, WILLIAM JAY
Doctor of Philosophy
Structural features on megascopic, mesoscopic, and microscopic scales have been examined in a study area located in the central portion of the northern Sierra Nevada (west of Lake Tahoe). Structures observed within the study area are correlated with regional deformational events of the northern Sierra Nevada. Although at least four regionally extensive deformational events have affected the rocks of the northern Sierra Nevada, the effect of only three events are recognized in the study area. Structures associated with post-Ordovician/Silurian and pre-Late Devonian deformation were not identified due to overprinting of later deformational events. There is ample evidence supporting extensive late Paleozoic-early Mesozoic deformation in the study area. Structures from this event formed as the result of compressive deformation along a northwest-striking, east-dipping convergent plate boundary along the western margin of the Sierran province and may be related to the accretion of an exotic terrane, Sonomia. Although a significant component of strike-slip motion may have existed along the plate boundary, the structures appear to be related to the normal component of convergence. The intense, short-lived Nevadan orogeny deformed rocks throughout the study area. This latest Jurassic event is thought to be the result of an arc-continent collision. Nevadan structures, which have an anomalous north to north-northeast trend in the study area, are often indistinguishable from late Paleozoic-early Mesozoic structures due to similarities in style and orientation. Analysis of strain from quartz microfabrics indicates that Nevadan deformation is, at least locally, non-coaxial. The non-coaxial deformation is probably related to left-lateral oblique convergence. The last deformational event that affects rocks in the study area occurred in the Cretaceous. Cretaceous structures have a consistent northwest trend throughout the northern Sierra Nevada. The anomalous trend of Nevadan structures in the study area is most likely related to Cretaceous deformation. In the northern Sierra Nevada, post-Nevadan, dextral oroclinal folding, which occurred prior to or during the Cretaceous deformational event, is the result of right-lateral oblique convergence.