GSA Connects 2022 meeting in Denver, Colorado

Paper No. 102-4
Presentation Time: 9:00 AM-1:00 PM


BEARDEN, Alexander1, BRUESEKE, Matthew1, BENOWITZ, Jeffrey2, MIGGINS, Daniel P.3 and MANN, Michael Everett4, (1)Department of Geology, Kansas State University, 108 Thompson Hall, Manhattan, KS 66506, (2)Unaffilated, PO Box 753851, Fairbanks, AK 99775, (3)College of Ocean and Atmospheric Sciences, Oregon State University, 104 CEOAS Admin Bldg, Corvallis, OR 97331-5503, (4)Brown University, Providence, RI 02912

The Yakutat slab is an oceanic plateau that has subducted beneath North America since 30 Ma, resulting in the archetypal example of flat slab subduction/slab edge arc magmatism (e.g., the Wrangell Arc). Yakutat subduction generated a ~400 km region between the Aleutian and Wrangell Arcs that was considered to be amagmatic since ~25 Ma (the Denali gap). Three newly identified volcanoes within the Denali gap (e.g., the Maclaren River volcanic field or MRVF) are the focus of this investigation. These volcanoes lie above a recently scattered-wave-imaged slab tear that is located west of the Wrangell Arc and separates the shallow dipping western Yakutat slab (~20°) and the steeper dipping eastern segment (~45°) under the Wrangell arc. Lava from the first volcano is a subalkaline basaltic andesite with an 40Ar/39Ar age = 940 ± 27 ka. Tephra and lavas from the second volcano are alkali basalt to shoshonite-like with Sr (~2300-3100 ppm) and Ba (~2700-3200 ppm) enrichments, high Ni (308-331 ppm) and Cr (442-501 ppm), but with Mg# between 67-73, suggestive of primary mantle melts. Groundmass from 4 separate units yielded 40Ar/39Ar ages between 424.5 and 420.5 ka. A lava sample from volcano 2 yielded an age of 423.2 ± 2.4 ka. Fine-grained lavas are exposed at a small (~1 km2) third volcano located ~9 km south of volcano 2. These rocks are transitional basaltic trachyandesite or trachyandesite, have ol + plag + cpx + opx + amph, and show evidence of crustal interaction (plag + qtz xenocrysts). Rocks from volcano 3 display a slab melt affinity (e.g., adakite), with lower incompatible element abundances than other MRVF rocks (e.g., Sr and Ba < 1000 ppm; Sr/Y = 64-73; La/Yb = 21-22) and Mg# = 67-69. Lava from volcano 3 lava yielded an age of 422.7 ± 6.7 ka, demonstrating that volcano 2 and 3 erupted coevally. All rocks display LILE enrichments and HFSE depletions consistent with subduction. However, their location, S-N alignment along a ~20 km transect, and volcano 3’s slab melt affinity are consistent with magma production in a subduction-affected environment along a slab tear. Our study provides geological evidence that conclusively demonstrates the existence of a Yakutat slab tear. We suggest that this tear formed ca. ~1 Ma, coincident with jamming of the Wrangell Arc trench and brought on by subduction of the thickest (~30 km) part of the Yakutat slab.