Repository logo

Faulting in the Foam Creek Stock, North Cascade Mountains, Washington




Kahn, Adrian Walter, author
Magloughlin, Jerry, advisor
Ridley, John, committee member
van de Lindt, John, committee member

Journal Title

Journal ISSN

Volume Title


The Foam Creek Stock (FCS) is a tonalite pluton in the northwestern part of the Nason Terrane in Washington state, a region that has experienced a wide range of structural regimes. Faults cutting the FCS have been studied through field, microscope, fluid inclusion, and geochemical methods. The purpose of this study is to understand the nature of these faults, and their place in the regional tectonic history. The FCS is cut by two populations (P1 and P2) of small-scale faults that share similar orientations but are microstructurally and geochemically distinct. P1 faults are generally E-dipping and host a distinct, bleached alteration halo and a thin, highly altered fault core containing the secondary minerals adularia (K-feldspar), chlorite, albite, and actinolite, and remnant host minerals quartz and altered plagioclase. P1 fault cores are thin (<1 mm) and display small apparent offsets (<2 cm). P2 faults cut P1 faults, dip N, S, and E, have generally steeper dips and greater displacements (average 14.1 cm apparent offsets) than P1 faults, and host predominantly secondary adularia and fractured host rock within thicker (~3 cm), more well-developed cataclastic fault cores. P1 faults show microstructural evidence of grain boundary bulging in quartz, along with seams that are interpreted to have hosted diffusive mass transfer (DMT) within the cataclastic fault core, suggesting a component of aseismic deformation accommodated some of the strain. In contrast, P2 fault cores range from random fabric to weakly foliated cataclasite, and host aseismic DMT and coseismic pseudotachylyte, indicating strain was accommodated across a wide range of strain rates. Kinematic analysis using both outcrop and microscale observations indicates that P1 faults are reverse, whereas P2 faults are normal and sinistral. Chemical analyses of the fault cores of the two populations, using a portable XRF reveal geochemical changes accompanying faulting. The most significant changes include P1 faults are enriched in Pb (~100%), and depleted in Ti (~50%), Ca, Sr, and Zn, whereas P2 faults are enriched in K (~40%) and Rb, and depleted in Fe (~30%), Mn, Ca, Sr and Zn. Interpretation of textural relationships between primary and secondary minerals suggests the fluids that migrated through both fault populations may have been initially sodic, and a shift in composition produced a sequence of alteration reactions with the host FCS tonalite. The final product of this changing fluid-rock system was adularia precipitation within fault cores, which in turn served to strengthen the faults. Mechanical strengthening likely inhibited reshear of faults, and thus additional strain of the pluton was accommodated along newly nucleated fault planes through slip delocalization. Fluid inclusion microthermometry, combined with observed deformation mechanisms and secondary mineral assemblages, allowed for estimation of temperature of trapping of the fluids, and revealed temperature conditions of ~289 ± 24°C during P1 faulting, and ~262 ± 23°C during P2 faulting. When combined with T-t curves constructed using K-Ar and Ar-Ar data from previous studies, the estimated age of the faults is ~71.9 ± 3.5 Ma for P1 and 69.2 ± 3.5 Ma for P2. Using these ages, it is proposed that Late Cretaceous deformation of the FCS observed in this study records relative counterclockwise rotation of the converging Farallon Plate. The resultant shift from E-W compression to dextral transpression was locally expressed as reverse P1 faults followed by N-S P2 extension with a sinistral component during regional post-metamorphic uplift and dextral shear.


Zip file contains data spreadsheet.

Rights Access



Associated Publications