Cataclastic rocks of the San Gabriel Fault; an expression of deformation at deeper crustal levels in the San Andreas fault zone

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doi: 10.1016/0040-1951(83)90296-2
Authors:Anderson, J. Lawford; Osborne, Robert H.; Palmer, Donald F.
Author Affiliations:Primary:
Univ. S.C., Dep. Geol. Sci., Los Angeles, CA, United States
Other:
Kent State Univ., United States
Volume Title:Tectonophysics
Source:Tectonophysics, 98(3-4), p.209-251. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0040-1951
Publication Date:1983
Note:In English. 77 refs.; illus. incl. 4 tables, sketch maps
Summary:The cataclastic rocks along the San Gabriel fault show more comminution than observed for gouge along the San Andreas; average grain diameter for San Andreas gouge ranges 0.01-0.06 mm. For the San Gabriel cataclastic rocks, it ranges 0.0001-0.007 mm. The ultra-fine grain matrix of the San Gabriel cataclasite is composed of a mosaic of equidimensional, interlocking grains. The cataclastic rocks along the San Gabriel fault show more mineralogical changes compared to gouge from the San Andreas fault. At the expense of biotite, amphibole, and feldspar, there is some growth of new albite, chlorite, sericite, laumontite, analcite, mordenite (?), and calcite. The highest grade of metamorphism is laumontite-chlorite zone (zeolite facies). Mineral assemblages and constrained uplift rates allow T and depth estimates of 200 + or - 30°C and 2-5 km, thus suggesting a geothermal gradient of approx 50°C/km. These results suggest that the previously observed lack of heat flow coaxial with the fault zone may be the result of dissipation rather than low stress. Porphyroclasts, even in the most deformed rocks, consist of relict plagioclase (oligoclase to andesine), alkali feldspar, quartz, biotite, amphibole, epidote, allanite, and Fe-Ti oxides. Whole-rock major- and trace-element chemistry coupled with mineral compositions show that mixing within the zone of cataclasis is not uniform, and that originally micaceous foliated, or physically more heterogeneous rock units may contribute a disproportionally large amount to the resultant intrafault material. Chemical mobility is not a major factor in the formation of cataclastic rocks of the San Gabriel fault. There is a marked mobility of Li, however, which is a probable result of the alteration and formation of new mica minerals. The gouge of the San Andreas and San Gabriel faults probably formed by cataclastic flow. There is some indication that the fine-grained matrix of the cataclasite from the San Gabriel fault formed in response to superplastic flow. [C.N.]
Subjects:Breccia; Cataclasis; Cataclasites; Chemical composition; Clastic rocks; Data; Deformation; Effects; Faults; Field studies; Gouge; Grain size; Metamorphic rocks; Mineral composition; Mylonites; Petrology; Sedimentary rocks; SEM data; Structural geology; Textures; X-ray data; California; Los Angeles County California; San Andreas Fault; San Gabriel Fault; United States
Coordinates:N334500 N344500 W1174000 W1185000
Abstract Numbers:87M/3255
Record ID:1984000141
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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