Structural Evolution Of The Central Part Of The Foothills Terrance Sierra Nevada California

Author : Russell Walter Graymer
Publisher :
ISBN 13 :
Total Pages : 355 pages
Book Rating : 4.:/5 (828 download)

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Download or read book Structural Evolution of the Central Part of the Foothills Terrane, Sierra Nevada, California written by Russell Walter Graymer and published by . This book was released on 1992 with total page 355 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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). $\sp{40}$Ar/$\sp{39}$Ar studies show rocks of the FCT have a metamorphic age of about 190 Ma, and are probably related to rocks west of the "Bear Mountains Fault." Study of a clast from the base of the MLT yielded a similar age, so the FCT is interpreted as one sedimentary source for the MLT. Geochemical study of igneous pyroxenes in the MLT have shown other similar rocks should be included in the Logtown Ridge formation, and that conglomerate within the MLT contains detrital pyroxenes derived from the Logtown Ridge formation and therefore postdates it. 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.