--> 90 Anticline and Counting: A Mostly Unmapped Fold Belt Between the San Andreas Fault and the Coast Range Thrust in Northern California

AAPG Pacific Section Convention 2019

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90 Anticline and Counting: A Mostly Unmapped Fold Belt Between the San Andreas Fault and the Coast Range Thrust in Northern California

Abstract

Using viewing and navigational tools provided in Google Earth Pro over 90 anticlines have been identified in the extensively deformed fold belt which is north of the San Francisco Bay and bounded by major strike-slip and thrust fault features. Geomorphological features such as stream patterns, flat irons, and vegetation offer clues to major and sometimes more subtle structures throughout this trend. Some of these features are in excess of 30 miles in length and have length to width ratios in the range of three to ten. Most of the anticlines are breached and quite difficult to recognize without detailed analysis of stream patterns and flat iron orientation. The axis orientation of most of these folds lies parallel to subparallel with the San Andreas, Maacama, Rogers Creek, Bartlett Springs, Coast Range Thrust, Little Salmon, South Humboldt, and Russ Faults. However, there is an anomalous axis trend that is more or less perpendicular to the San Andreas and Maacama Fault traces extending from just south of Fort Bragg to about 25 miles north of Fort Bragg. Several tools and techniques were developed to aid in identification of the structural features. Stream patterns were digitized and organized into major and subordinate drainage systems that can be individually turned on and off while viewing the terrain in both vertical and oblique orientations. Vertical relief can be exaggerated by up to a factor of 3:1, thus aiding in recognition of flat irons and other features when viewed in oblique orientation. Feature recognition can, in some cases, be aided by drafting lines along stream divides. This process is also greatly aided by oblique viewing and varying the elevation exaggeration. There are numerous reasons why this extensive fold belt has not been recognized. The terrain is quite rugged, road access is very limited, and there have been only minimal efforts to do any detailed field mapping. Dibblee mapped some portions of the trend and the daunting nature of the topography is evidenced by the concentration of his strike and dip measurements along the more accessible stream beds with very little in between. The entire trend east of the San Andreas Fault up to the Mendocino triple junction and beyond is a mix of ophiolite terranes. The convergent subduction and accretionary process resulted in chaotic bedding and low temperature metamorphism, all of which combine to reflect small scale deformational features when viewed in the field. All of this small-scale deformation occurred long before the advent of strike-slip movement along the San Andreas and other major faults throughout the trend. The strike-slip and compressional deformation commenced with or after the failure of subduction south of the triple junction. The very large anticlinal features now recognized overprint the older small-scale structures. However, most of these anticlines are of such great size that they are literally the forest that is hidden by the trees. The Kettleman Hills North Dome, Middle Dome and South Dome trend on the west margin of the San Joaquin Valley is about 30 miles in length. Many of the mapped anticlines in this trend are that size or bigger. Some of the features have been mapped as anticlinoriums with included closures having axial lengths of 8-10 miles. Several anticlines in the area just north of San Francisco Bay are annotated in Bulletin 149 (Geology and Mineral Deposits of the Area North of San Francisco Bay, California, 1949). The Alton and Grizzly Bluff anticlines on the southern edge of the Eel River Tertiary basin have been mapped (Bulletin 164). However, for most of the twentieth century there were no anticline symbols between those features on published maps. The older Santa Rosa Quadrangle (having no anticlines indicated) was revised in 1988 and one anticline was annotated north west of the Geysers Geothermal Field at Pine Mountain. The author has found no other indications that a great number of anticlines exist in the trend. The geomorphological features that are evident on the Pine Mountain structure can be seen throughout the trend when viewed with the tools used in this study. In a larger sense, this study also illustrates the value of using tools such as Google Earth to visualize and identify geomorphological features as an aid in structural mapping on a regional scale.