Journal
JOURNAL OF STRUCTURAL GEOLOGY
Volume 43, Issue -, Pages 2-32Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.jsg.2012.05.004
Keywords
Fault rock; Gouge; Clay; Cordillera; Normal faults
Categories
Funding
- National Science Foundation [EAR-0738435, EAR-1118704]
- GDL Foundation
- University of Michigan
- GSA
- AAPG
- Directorate For Geosciences [0738435, 1118704] Funding Source: National Science Foundation
- Division Of Earth Sciences [0738435, 1118704] Funding Source: National Science Foundation
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Neoformed minerals in shallow fault rocks are increasingly recognized as key to the behavior of faults in the elasto-frictional regime, but neither the conditions nor the processes which wall-rock is transformed into clay minerals are well understood. Yet, understanding of these mineral transformations is required to predict the mechanical and seismogenic behavior of faults. We therefore present a systematic study of clay gouge mineralogy from 30 outcrops of 17 low-angle normal faults (LANFs) in the American Cordillera to demonstrate the range and type of clay transformations in natural fault gouges. The sampled faults juxtapose a wide and representative range of wall rock types, including sedimentary, metamorphic and igneous rocks under shallow-crustal conditions. Clay mineral transformations were observed in all but one of 28 faults; one fault contains only mechanically derived clay-rich gouge, which formed entirely by cataclasis. Clay mineral transformations observed in gouges show four general patterns: 1) growth of authigenic 1 M-d illite, either by transformation of fragmental 2M(1) illite or muscovite, or growth after the dissolution of K-feldspar. Illitization of fragmental illite smectite is observed in LANF gouges, but is less common than reported from faults with sedimentary wall rocks; 2) 'retrograde diagenesis' of an early mechanically derived chlorite-rich gouge to authigenic chlorite smectite and saponite (Mg-rich tri-octahedral smectite); 3) reaction of mechanically derived chlorite-rich gouges with Mg-rich fluids at low temperatures (50-150 degrees C) to produce localized lenses of one of two assemblages: sepiolite + saponite + talc + lizardite or palygorskite +/- chlorite +/- quartz; and 4) growth of authigenic di-octahedral smectite from alteration of acidic volcanic wall rocks. These transformation groups are consistent with patterns observed in fault rocks elsewhere. The main controls for the type of neoformed clay in gouge appear to be wall-rock chemistry and fluid chemistry, and temperatures in the range of 60-180 degrees C. (C) 2012 Elsevier Ltd. All rights reserved.
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