Neocrystallization, fabrics and age of clay minerals from an exposure of the Moab Fault, Utah

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• 作者：John G. Solum ; Ben A. van der Pluijm and Donald R. Peacor
• 关键词：Clay gouge ; ⁴⁰Ar/³⁹Ar dating ; Moab Fault
• 刊名：Journal of Structural Geology
• 出版年：2005
• 出版时间：2005
• 年：2005
• 卷：27
• 期：9
• 页码：1563-1576
• 全文大小：1084 K

文摘

Pronounced changes in clay mineral assemblages are preserved along the Moab Fault (Utah). Gouge is enriched up to 40 % in 1M_d illite relative to protolith, whereas altered protolith in the damage zone is enriched 40 % in illite–smectite relative to gouge and up to 50 % relative to protolith. These mineralogical changes indicate that clay gouge is formed not solely through mechanical incorporation of protolith, but also through fault-related authigenesis. The timing of mineralization is determined using ⁴⁰Ar/³⁹Ar dating of size fractions of fault rocks with varying detrital and authigenic clay content. We applied Ar dating of illite–smectite samples, as well as a newer approach that uses illite polytypes. Our analysis yields overlapping, early Paleocene ages for neoformed (1M_d) gouge illite (63±2 Ma) and illite–smectite in the damage zone (60±2 Ma), which are compatible with results elsewhere. These ages represent the latest period of major fault motion, and demonstrate that the fault fabrics are not the result of recent alteration.

The clay fabrics in fault rocks are poorly developed, indicating that fluids were not confined to the fault zone by preferentially oriented clays; rather we propose that fluids in the illite-rich gouge were isolated by adjacent lower permeability, illite–smectite-bearing rocks in the damage zone.