167-6 High temporal resolution record of flare-up magmatism reveals tectonic and local controls on crustal thickening in the Cretaceous North Cascades continental arc

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

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Continental arcs exhibit variable magmatic flux, with long-lived systems experiencing short-lived flare-ups (10–15 Myr) separated by prolonged lulls (25–50 Myr). During flare-ups, magmatic flux can surge 15–20 times above background levels, reflecting significant crustal reworking and mass transfer between mantle-to-crust. This pattern is observed globally and often coincides with crustal thickening, driven by both regional and local tectono-magmatic processes. Garnet stability in crustal protoliths and hydrous arc melts is pressure-dependent, leaving distinct geochemical signatures (e.g., high Sr/Y, La/Yb, low MnO/MgO) that reflect crustal thickness. While previous studies have tracked crustal thickening during flare-ups, many rely on low-precision U-Pb zircon dating, limiting resolution of short-term versus long-term changes. Here, we combine whole-rock geochemistry with high-precision Chemical Abrasion-Isotope Dilution-Thermal Ionization Mass Spectrometry (CA-ID-TIMS) zircon geochronology to examine a Late Cretaceous flare-up (79–63 Ma) in the North Cascades arc to evaluate the episodic nature of flare-up magmatism and understand the drivers behind local vs. regional thickening. 

This study shows that magmatism during the flare-up was highly episodic, marked by short, discrete pulses (emplacement of a pluton) separated by quiescent intervals. Each magmatic peak aligns with the emplacement of an individual pluton, indicating localized magma focusing events rather than regionally continuous magmatism. Multiple pulses within single plutons further reflect incremental growths at specific centers. The observed increase in (La/Yb)ₙ ratios at the scale of individual plutons are interpreted to reflect progressive local thickening of the crust during the end-Cretaceous magmatic flare-up in the North Cascades. Along-strike variations in crustal thickness are evident from their temporal variation of (La/Yb)_n ratios. Further, comparison with geochemical data from Shea et al. (2018) shows no shift in (La/Yb)ₙ ratios between mid- and Late Cretaceous flare-ups, suggesting crustal thickness remained stable between them. This implies a greater role of sustained localized magma loading in driving crustal thickness than a regional tectonic shortening.

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

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