Deconstruction of the Franciscan Complex Central Terrane Mélange and re-evaluation of Franciscan mélanges and architecture of the northwestern San Francisco Bay Area, California, USA

Abstract

Various mélange types occur within the Franciscan accretionary Complex of western California. The largest mélange body, called the Central Belt Mélange (or similar names) served earlier as the type example for the orogen-long, subduction channel model, yet in the northwestern San Francisco Bay Area the name does not accurately reflect the geology. The mélange designation was commonly applied where resistant exotic and native blocks of rock are scattered across a relatively smooth terrain. Detailed mapping shows that many blocks experienced post-accretion transport. Areally large rock masses previously designated as Central Belt Mélange consist of multiple units and less than 30% of the tectonostratigraphy is mélange. Weakly metamorphosed sandstone–mudrock broken to dismembered formational units and similarly deformed sandstone–mudrock submarine fan facies dominate the tectonostratigraphy. Subordinate mélanges of the northwestern San Francisco Bay Area are of tectonic or sedimentary origin. The sedimentary bodies represent submarine mass flow deposits. Tectonic mélanges mark Mesozoic subduction-zone faults or Cenozoic strike-slip faults. Discriminating between mélange types and their origins, and reconstructing tectonostratigraphic columns for major fault blocks, clarifies the primary accretionary complex architecture and reveals significant along-strike variations in the Franciscan subduction accretionary Complex.