SHALLOW CRUSTAL FAULTING IN THE LATE CRETACEOUS FOAM CREEK STOCK, WASHINGTON

The Foam Creek Stock (FCS) is a small, biotite tonalite to granodiorite pluton in the northwestern Nason Terrane of Washington state that likely experienced SW-NE shortening (~96-73 Ma), dextral transpression (~73-55 Ma), and dextral transtension (<55 Ma) during exhumation (Paterson et al., 2004).

The FCS is cut by quartz veins and two populations of small-scale brittle sinistral faults that share similar orientations but are microstructurally and geochemically distinct. P1 faults are ~1 mm thick with minor offsets (~5 cm) and a white alteration halo involving saussuritization of Pla, and Bio into Chl, typically ~10 cm wide. The younger P2 faults are typically ~3 mm, displaying offsets up to 30 cm, with a green fault core containing visible clasts of Pla and Qtz.

Microstructurally, both populations display a fault core hosting cataclastic grain size reduction and thin, discontinuous pressure solution seams. The damage zone is almost exclusively brittlely fractured; quartz displays little evidence for plastic strain but abundant fracturing. Gas-rich fluid inclusion planes adjacent to both fault populations show a near-vertical, near E-W σ₁-σ₂ paleostress plane. P2 fault cores range from random fabric cataclasites to foliated with bands of ultracataclasite, and one sample displays thin pseudotachylyte (PST) seams overprinting cataclasite fault core.

Chemical analyses using handheld XRF show that P1 faults are enriched in Pb (~100%), and depleted in Ti (~50%), Ca, Sr, and Zn. P2 faults are enriched in K (~40%), Rb, and V, and depleted in Fe (~30%), Mn, Ca, Sr and Zn.

Distinctly different fluids accompanied the two populations, evidence for which includes quartz veins, pressure solution, metasomatism, contrasting elemental changes in the two populations, and abundant fluid inclusion planes. We will report estimates of temperatures of fluid flow based on microthermometry. Deformation mechanisms, pressure solution, and PST indicate deformation ranged from aseismic creep to coseismic slip. Biotite cooling dates from previous studies in the region used in conjunction with likely <300°C brittle microstructures indicate faulting occurred <70 Ma. E-W paleostress orientations and P1-P2 relationships suggest both sets of faults formed during a dextral transpressional regime, and underwent clockwise block rotation.