Confirmation of Imbricate Thrusting in California Coast Ranges: Geology of Mysterious Valley Area, Napa County

Abstract

End_Page 970------------------------------Most of the structural relief in the northern California Coast Ranges is generated by imbricate thrust faulting, although the region is undergoing major strike-slip deformation. In the Napa County Coast Ranges, the presence of subequal outcrop areas of Franciscan rocks and of Great Valley Sequence sediments and their underlying ultramafic basement, highlights the regional structures. The area is characterized by plunging folds which show relay patterns, by windows and klippen, and by vertical and lateral repetition of the stratigraphic section. Flat-lying faults are common. Individually these structures are best explained by thrust faulting; as an ensemble they are characteristic of the imbricate overthrust belts of compressional orogenes. Construction of a series of retrodeformable (balanced) cross sections, using a quantitative geometric theory of fault-bend folding, reveals a limited range of possible solutions to the regional deep structure. The structures include thrust faults rotated to high angles, anticlines formed by repeated small-scale imbrication, and regional back thrusts. The cross sections are characterized by thrust faults which rise southwestward cross strike and southeastward along strike from the Franciscan into the serpentinite and then into the Great Valley Sequence. Decollement horizons are recognizable in all three lithologies. The concept of imbricate thrusting provides a single unifying hypothesis with significant predictive ability which explains much of the map pattern and illuminates the three-dimensional structure of the eastern half of the Santa Rosa 1:250,000 map sheet. End_of_Article - Last_Page 971------------