Origin of Mélanges of the Franciscan Complex, Diablo Range and Northern California: An Analysis and Review

Abstract

The Franciscan Complex of California is characterized in part by the presence of mélanges. In general, mélange origins are attributed to sedimentary, tectonic, or diapiric processes—or a combination of these. Published reviews list the main features of mélanges characteristic of each type of origin. In this review, particular diagnostic features typical of sedimentary, tectonic, and diapiric mélanges are used to assess 15 specific mélanges, which in some cases have been interpreted in contrasting ways in the literature. The data do not support the view that most Franciscan mélanges were formed by sedimentary processes, but rather that both tectonic and sedimentary processes are important. There is little evidence that diapirism contributed significantly to Franciscan mélange genesis. Tectonic features present in most mélanges of subduction accretionary complexes create challenges in assessing mélange-forming processes. Notably, although tectonic overprints commonly mask the primary diagnostic fabric of sedimentary mélanges, some diagnostic features—such as depositional contacts, fossils in mélange matrix, and interlayering of mélange and non-mélange units—are critical to recognition of mélanges of sedimentary origin.

Terminal Suturing Between the Tarim Craton and the Yili‐Central Tianshan Arc: Insights From Mélange‐Ocean Plate Stratigraphy, Detrital Zircon Ages, and Provenance of the South Tianshan Accretionary Complex

“…F. Zhang et al, 2002), providing evidence of deep subduction. The ophiolites emplaced as slivers associated with incorporated fragments of OPS are comparable to those in the Franciscan complex (Raymond, 2019b;Wakabayashi, 2015). They represent slices of ancient South Tianshan oceanic crust together with fragment of seamounts, oceanic islands, or oceanic plateaus that were accreted to the toe of an accretionary complex.…”