Landslides, threshold slopes, and the survival of relict terrain in the wake of the Mendocino Triple Junction

1 REGIONAL GEOLOGY 2 The northern California Coast Ranges are composed the Franciscan Complex, a penetratively 3 sheared set of metasedimentary rocks forming an accretionary prism. The Franciscan complex is 4 subdivided into three broad belts that young to the west: the Eastern, Central and Coastal belts, 5 reflecting the cumulative accretion of oceanic sediments to western North America (McLaughlin 6 et al., 2000). Our study area encompasses the Central and Coastal belts, described further here. 7 The Central belt consists predominantly of Franciscan Mélange (KJf in Fig. 1A and DR1), which 8 is a penetratively sheared, Jurassic-Cretaceous, argillitic mélange embedded with blocks of more 9 coherent sandstone and shale turbidites and occasional blocks of metabasalt and others. The 10 younger Coastal belt can be divided into several structural terranes. Most relevant for our study 11 area are the Yager terrane (EP in Fig. DR1), and Coastal terrane (TK in Fig. DR1). The Yager 12 terrane is composed predominantly of Paleocene to Eocene argillite, sandstone and conglomerate 13 that vary from well bedded to sheared. The Coastal terrane is compositionally similar to the 14 Yager, but is more penetratively disrupted and sheared and contains some additional lithologies 15 such as blocks of basalt. Rocks of the Coastal terrane span the late Cretaceous to Pliocene. 16