REPEATING PATTERNS OF MINERAL TRANSFORMATIONS IN CLAY GOUGES, WITH EXAMPLES FROM LOW-ANGLE NORMAL FAULT ROCKS IN THE WESTERN USA

Neoformed (authigenic) clay minerals in fault rocks in the brittle crust are increasingly recognized as being key to the both the mechanical behaviour of faults in the brittle crust. Understanding the controls on these mineral transformations in fault zones is necessary to better understand both the mechanical and sealing behaviour of faults. Outcrop-based studies of clay mineral transformations in fault zones have typically been limited to a single outcrop or series of outcrops of a single fault, and so the observations of neoformed minerals in fault gouges is piecemeal, and a synthesis of occurrences and controls of these low-friction, low-permeability materials is lacking.

To better understand the degree to which clay mineral transformations that form clay-rich gouges are pervasive (or not) in fault zones, we present a systematic study of clay gouge mineralogy from a suite of 30 outcrops of 17 low-angle normal faults (LANF’s) in the American Cordillera to demonstrate the range and type of 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 mechanically derived clay-rich gouge, which formed entirely by cataclasis. We then synthesize these findings with reports of neoformed clays in faults elsewhere to build a picture of clay growth in brittle faults as a fundamental process governed by temperature at the time of faulting, wall rock composition, and fluid availability.

Clay mineral transformations observed in gouges show 3 general patterns: 1) growth of authigenic 1M_d illite, either by transformation of fragmental 2M₁ illite or muscovite. 2) growth of 1M_d illite after the dissolution of K-feldspar. Illitization of fragmental illite-smectite is observed in sedimentary basin fault gouges, but is less common in LANF gouges. 3) 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 ºC.