PATHWAYS AND TIMING OF FLUID PULSES IN SHALLOW CRUSTAL FAULT SYSTEMS – H AND AR ISOTOPIC ANALYSIS OF NORMAL FAULTS IN THE SW US

Constraining the source(s) and timing of fluids is key to understanding crustal-scale hydrology, rock mechanics, mineral reactions and the origin of economic deposits. A significant role for meteoric fluids in exhumed fault rocks has been proposed in recent studies, notably in mylonites in low-angle normal fault (LANF) systems. However, the extent of meteoric infiltration and fluid pathways, and the timing of fluid pulses remain poorly known. The occurrence of clay neomineralization in shallow fault rocks has the potential to resolve these questions, as clay (trans)formation preserves host fluid information in its stable isotopic signatures, particularly H isotopes, and ages through radiogenic Ar analysis.

We obtained δ2H (‰ wrt SMOW) isotopic measurements from neo-formed clays in fault gouge and breccias that formed above major LANF detachments in the SW US, which show that clays in fault rocks in the brittle regime formed from exchange with isotopically light fluids. Illite δ2H measurements range from -142‰ to -107‰; smectite δ2H measurements range from -147‰ to -95‰; and chlorite δ2H measurements range from -108‰ to -97‰. Fluid compositions calculated from these data indicate that clays in fault gouge down to several km depth formed in the presence of near-pristine meteoric fluids. New Ar dating of fault gouge reveals multiple pulses of mineralizing fluids in the range 2.8-18.6 Ma.

The isotopic signatures of clays at multiple depths in LANFs of the Western US indicate that this crustal-scale normal fault system is permeable on geologic time scales, and that it is dominated by episodic, downward flow of surface waters during the Neogene. Based on the observed input of meteoric fluids in these shallow fault rocks, we propose a dynamic scenario for surface fluid infiltration that involves whole upper crust fluid circulation along interconnected, transient networks of brittle faults and fractures that are activated during local deformation pulses. We surmise that drawdown of meteoric fluids is facilitated by the opening and closing of spaces in these evolving fracture networks.