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.
Highlights
The Franciscan Complex of California and southwestern Oregon (Figure 1) is widely considered to be the archetypal subduction accretionary complex [1,2,3]
Given the definitions set out above, the primary purpose of this paper is to review the characteristics of several relatively well-described Franciscan mélanges and relate the descriptive features to the proposed origins of the mélanges
This review focuses attention on some conflicting interpretations of available data and on the contrasting emphases placed on particular criteria in determining the origins of particular Franciscan mélanges
Summary
The Franciscan Complex of California and southwestern Oregon (Figure 1) is widely considered to be the archetypal subduction accretionary complex [1,2,3]. Hsu [16] posited that fragmentation of rocks and mixing of different rock types were the critical processes in formation of these block-in-matrix structures. These are known to form via tectonic, sedimentary, and diapiric processes [23,24,25]. For mélanges of deformational origin in which fragmentation and mixing is primarily a tectonic process (tectonic mélanges), the most definitive features are sheared or deformed contacts, the presence of S-C and P-R fabrics, and the presence of a pervasive scaly microfabric that may or may not be accompanied by microbreccia, pseudotachylite, or both (Table 1).
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
