167-5 Deciphering early shear zone slip: combined titanite petrochronology and microstructural analysis from the Ross Lake fault system, North Cascades, Washington

Session: Integrating Geochronology and Geochemistry to Decipher the Tectonic Evolution of Orogenic Belts

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Abstract:

Shear zones in the mid-to-deep crust play a major role in accommodating strain during both crustal thickening and extension. The end stages of fault activity can often be temporally constrained using cross-cutting relationships. However, the timing of early slip is more difficult to decipher. Here we use a combination of field data and LA-ICP-MS dates and EBSD data from variably deformed titanite to determine the timing of activity of the Ross Lake fault system, a strike-slip fault system which represents the northeastern boundary to the Late Cretaceous–Eocene North Cascades magmatic arc, Washington. Several magmatic intrusions were emplaced into or near the fault system throughout its history, including the ca. 90 Ma Black Peak intrusive complex. The western margin of this intrusion becomes mylonitic along the central part of the Ross Lake fault system. Large, euhedral titanite grains in a weakly deformed Black Peak sample are randomly oriented, and EBSD maps show minimal to no internal strain; LA-ICP-MS results define a lower-intercept regression age of ca. 89 Ma. In comparison, titanite from a mylonitic Black Peak sample are elongate parallel to foliation, and grains are bent and have low-angle boundary microstructures; U-Pb results are more scattered, with 207-corrected age peaks at ca. 88 and 82 Ma. Titanite from a strongly deformed ca. 64 Ma orthogneiss adjacent to the Black Peak and from an orthogneiss found farther south, along strike, from within a different part of the fault system also contain recrystallized titanite that are aligned parallel to foliation. Titanite from these samples yield dates of ca. 66 Ma and ca. 63–59 Ma, respectively. The titanite in the three deformed samples likely recrystallized due to deformation and fluid flow during shear zone motion. The titanite show that earliest dextral-reverse-slip displacement in the fault system was ca. 82 Ma, earlier than previously documented, and that fault activity continued through ca. 59 Ma. Emplacement of a ca. 49–48 Ma pluton documents the end of fault activity. From ca. 65–49 Ma, substantial vertical slip occurred, with dextral–reverse followed by dextral–normal slip documented in different parts of the fault system. The combined geochronology and microstructural titanite analysis are the only known record of the timing of early motion within this convoluted shear zone system.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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