DETACHMENT FAULT-RELATED MASS TRANSFER IN THE SHALLOW CRUST: FIELD AND STABLE ISOTOPE EVIDENCE FROM EAST-CENTRAL NEVADA

Field mapping using GPS at a scale of 1:3000 and seismic reflection profiling demonstrate that the Oligocene-Miocene Currant Gap fault (CGD) is an upper crustal detachment that coalesces into the White Pine detachment (WPD) at the southern White Pine Range, Nevada (Francis et al., 2021). Brecciation and stratigraphic attenuation/omission characterize these faults rooted in argillaceous ductile units. Massive amounts of mass transfer of Si⁴⁺ and CaCO₃ along the CGD is evidenced by silica replacement of carbonate that intensifies towards the detachment and widespread carbonate veins. The heat that drove fluid flow was provided by the intrusion of numerous felsic sills into the lower plate that were altered by meteoric-hydrothermal fluids as indicated by δD_biotite ~ –150 and non-equilibrium quartz (~11‰) and plagioclase (4 to 10‰) δ¹⁸O values. Low vein carbonate δ¹⁸O values (down to –4.4‰) that display a trend toward higher values (up to 19‰) downward into the lower plate suggest this detachment fault system as a recharge zone for downward circulating fluids due as an up-temperature decrease in carbonate ion solubility exists. Host-rock carbonates display a narrower range of δ¹⁸O values, the product of a much lesser degree of fluid interaction and a channelized, fracture-controlled fluid system. Silica replacement zones along the CGD indicate deposition of hydrothermal quartz from meteoric-hydrothermal fluids (δ¹⁸O = –13 to 2‰) that migrated upward as silica solubility decreased down temperature. These silica replacement zones include, from low to high structural level: (1) thin veins (<5 mm; δ¹⁸O <2.5) at the top of the Cambrian Upper Pole Canyon limestone, (2) a fault-bounded ‘spine’ rock (δ¹⁸O = 2-11), a brecciated amalgam of dolomite, limestone, and quartzite in an Si matrix, (3) complete Si-replacement with clasts of Ordovician Eureka quartzite and earlier phases of Si matrix formed during previous episodes of silicification (δ¹⁸O <2), and (4) jasperoid breccia (δ¹⁸O >5). Quartz δ¹⁸O values from numerous klippe knobs mapped by Moores et al. (1968) as Eureka quartzite delineated hydrothermal silica (<10‰) from this sedimentary unit (>11‰). Low quartz fluid inclusion δD values (–165 to –130‰) from Si-replacement units attest to the pervasive nature of meteoric-hydrothermal fluid circulation through the CGD.