Tracking Deep Meteoric Fluid Circulation Along a Regional Detachment Fault Using Field Infrared Spectroscopy

Abstract

Abundant evidence exists for deep crustal penetration of meteoric fluids along faults, including emergence of hot, dilute, and isotopically light geothermal fluids in extensional settings; however, the nature of the fluid conduits supporting this rapid circulation from surface to the brittle-ductile transition and back remains mysterious. Metamorphic core complexes (MCCs) are the sites of rapid exhumation of rocks from that depth, and their associated detachment faults are known loci of fluid migration. This study utilizes spot infrared (IR) spectroscopy of drill core and outcrop to unravel the fluid history of the late-Neogene Silver Peak-Lone Mountain MCC and detachment fault (SPLMDF) in SW Nevada. That history begins with Mesozoic regional burial metamorphism of Paleozoic sediments, minor late Mesozoic contact metamorphism by silicic intrusives, followed by upwelling of hot metamorphic fluids after detachment initiation (11 MYA), later circulation of moderate-temperature meteoric-geothermal fluids, and young (< 5 MYA) hot epithermal fluids upwelling along detachment-cutting normal faults. Each of these stages is characterized by distinct changes in sheet silicate mineral crystallinity and hydration. These are conveniently summarized by maturity indicators based on IR absorption peak ratios, for example, illite spectral maturity (ISM). Burial metamorphism up to greenschist facies is indicated by steadily increasing ISM versus depth in core from a detachment-penetrating geothermal exploration borehole. A sharp decrease in ISM characterizes the detachment damage zone, accompanied by reappearance of smectite, zeolite, and abundant iron oxides, indicating much cooler alteration by meteoric-origin fluids. Low-ISM zones are concentrated in the damage zone ± 10 m from the fault, resulting from an accumulation of very narrow alteration bands (10–50 cm wide). About 1/3 of the SPLMDF fault trace exhibits this low-temperature circulation. Another third of the trace is overprinted by postdetachment epithermal alteration with extreme ISM, often in zones extending along the detachment near cross-cutting normal faults.