Metamorphic convergence of the upper and lower plates of the Vincent Thrust, San Gabriel Mountains, Southern California, USA

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Authors:Jacobson, C. E.
Author Affiliations:Primary:
Iowa State University, Department of Geological and Atmospheric Sciences, Ames, IA, United States
Volume Title:Journal of Metamorphic Geology
Source:Journal of Metamorphic Geology, 15(1), p.155-165. Publisher: Blackwell, Oxford, United Kingdom. ISSN: 0263-4929
Publication Date:1997
Note:In English. 49 refs.; illus., incl. 1 plate, 1 table, geol. sketch maps
Summary:This thrust separates eugeoclinal Pelona schist from overlying Precambrian-Mesozoic igneous and metamorphic rocks of North American continental affinity, and is generally considered to be synmetamorphic because of similarities in structural orientations and mineral assemblages between the Pelona schist and the mylonites at the base of the upper plate. Amphibole in schist is mostly actinolite to actinolitic hornblende with a high Na/Al ratio, indicating relatively high-P/low-T metamorphism; individual grains are zoned, with Na and Al both decreasing from core to rim. Pre-mylonitic amphibole in the upper plate is hornblende, tschermakite and pargasite with compositions indicative of low- or medium-P metamorphism. During mylonitization this amphibole was replaced by actinolite to actinolitic hornblende with a similar range of Na and Al as amphibole rims in the Pelona schist, but with slightly lower Na/Al ratio, considered to be the result of an inverted thermal gradient during thrusting. Convergence of K and Ti contents of amphibole and An content of plagioclase also occurs. These features are considered to confirm that mylonitization of the upper plate was closely related in space and time to metamorphism of the Pelona schist and therefore that the Vincent thrust is a remnant of a primary fault along which the Pelona schist and correlative units were subducted beneath North America. [R.A.H.]
Subjects:Amphibole group; Chain silicates; Crystal chemistry; Faults; Feldspar group; Framework silicates; Geochemistry; Metamorphism; Pelona Schist; Petrography; Plagioclase; Precambrian; Silicates; Syntectonic processes; Thrust faults; California; Los Angeles County California; San Gabriel Mountains; Southern California; United States; Vincent Thrust
Abstract Numbers:97M/3373
Record ID:1997018044
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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