Oceanic faulting and fault‐controlled subseafloor hydrothermal alteration in the sheeted dike complex of the Josephine Ophiolite

Based upon detailed mapping (1:10 and 1:100) of a large water‐polished outcrop of the sheeted dike/gabbro transition zone in the Josephine ophiolite of NW California and SW Oregon, the following history of alternating episodes of magmatic, structural, and hydrothermal events has been documented using crosscutting relationships, petrography, geochemistry, and strontium and oxygen isotopic data: (1) crystallization of gabbro and later subvertical mafic dike injection, (2) amphibolite facies metamorphism, (3) extensional faulting and tilting of dikes, (4) continued faulting, tilting, and dike injection associated with retrograde metamorphism at greenschist facies conditions, (5) continued extensional faulting and tilting synchronous with the development of a variety of hydrothermal veins at decreasing temperature and increasing fluid/rock ratios, and (6) subvertical injection of a highly fractionated dike which truncates all previous features (features 1–5). Trace element geochemistry indicates the highly fractionated dike (feature 6) is genetically related to the other dikes and thus was intruded at or near the paleospreading axis. Hence, all previous events (features 1–5) can be constrained to have occurred at the rift axis, including large‐scale tilting (∼ 50°) of the sheeted dikes and extensional faulting. The [ 87 Sr/ 86 Sr] initial ratios from recrystallized whole rocks and from hornblende, epidote, and prehnite separates from veins display a systematic increase with relative age, from 0.7033 for altered gabbro screens and mafic dikes to 0.7049 for prehnite in the youngest oceanic fault rock. Calculated oxygen isotope fluid compositions for the same suite of samples range from δ 18 O fluid = +5 to −1 with time, indicating a change to a seawater‐dominated hydrothermal system with time, consistent with observed increased permeability due to seafloor extensional faulting. The crosscutting relationships, alteration mineral assemblages, and isotopic data suggest (1) an early stage of high‐temperature (≥450°C) hydrothermal alteration with low permeability (i.e., grain‐scale flow), followed by (2) a decrease in temperature (∼350to ≤200°C), and an increase in permeability due to faulting and accompanied by tectonic tilting at the rift axis. The consistency of these crosscutting relationships at similar pseudo‐stratigraphic levels at different localities in the Josephine ophiolite suggests that alternating magmatic and structural extension with synchronous retrograde alteration is common in crust formed at similar rates of spreading (slow‐ to intermediate‐spreading centers), such as the modern Mid‐Atlantic Ridge.