Structural evolution of the central part of the Foothills Terrane, Sierra Nevada, California

Five distinct accreted terranes occur near Placerville in the central part of the Sierra Nevada Foothills. The southern and eastern portions of the area comprise at least five imbricate slices of Bathonian to Kimmeridgian sedimentary and volcanic rocks, including the Logtown Ridge and Mariposa formations. This Mother Lode Terrane (MLT) is bounded to the west by a vertical fault contact with the French Creek Terrane (FCT) (medium to high grade metasedimentary and metavolcanic rocks). These two terranes are overthrust to the north by the Mount Ararat Terrane (MAT) (serpentinite melange with blocks of Late Triassic red chert, and Early Jurassic basalt, basalt breccia and chert). MAT is bounded to the north by south-dipping thrust fault that places it over the Cool Quarry Terrane (CQT) and the American River Terrane (ART). The CQT is composed of Permian limestone and basalt and is thrust over the ART, an Aalenian olistostrome containing blocks of Late Triassic to Early Jurassic chert, Paleozoic limestone, and basalt, interbedded with andesite tuff, and overlain by early Callovian conglomerate and slate (Colfax formation).;Six age groups of radiolaria occur within chert and cherty argillite in the area; Late Permian, Late Triassic, Triassic-Jurassic, Early Jurassic, Early--Middle Jurassic, and Middle Jurassic. Distribution of fauna has shown that the base of the MLT is depositionally linked to unmetamorphosed Late Permian basalt and chert (interpreted as Permian seafloor basement). ;Terrane boundary faults are all post-Callovian, and most are post-Kimmeridgian. These faults are cut by later north-south trending high angle faults. All faults are stitched by a 143 Ma granodiorite to hornblende diorite pluton except the Melones Fault, which cuts and deforms it south of Placerville, and the "Bear Mountains Fault" which is west of it. Therefore these terranes were amalgamated as separate entities from the proto-Pacific, mostly during the period after the Kimmeridgian but before 143 Ma. Structural evidence suggests that the mechanism of accretion was crustal wedging, except for the MAT which was obducted as an overthrust nappe, and that the belt underwent multiple phases of compressional and extensional deformation.