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

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