Work

Timing and Style of Tectonic Assembly and Exhumation of the McHugh Complex Within the Chugach‐Kodiak Accretionary Wedge, Alaska

Year: 2026

Type: article

Abstract: Accretionary complexes are key archives of subduction zone processes, preserving records of forearc growth, deformation, and tectonic reorganization. The Chugach–Kodiak accretionary complex of southern Alaska records Jurassic to Cretaceous underplating at shallow to intermediate subduction conditions. The McHugh Complex, an imbricated sequence of prehnite–pumpellyite facies oceanic and trench‐derived rocks forms a major component of this subduction margin. We present new structural data, U–Pb detrital zircon geochronology, zircon (U–Th)/He thermochronology, and Raman spectroscopy of carbonaceous material from two regional transects across the McHugh on the Kenai Peninsula. In the structurally highest thrust panels, maximum depositional ages young trenchward from 169 to 148 Ma, consistent with downward growth by underplating. A 54 Myr gap (148–94 Ma) is interpreted to bracket a period of subduction erosion. A similar gap 200 km to the northeast, attributed by previous studies to Early Cretaceous ridge subduction, suggests that this erosional event was regionally extensive. Accretion resumed by 95 Ma and involved re‐entrainment of Jurassic material. Zircon (U–Th)/He thermochronology shows that most of the McHugh remained buried until Paleocene exhumation, coincident with regional ridge subduction. The McHugh complex is organized into 2–4 km thick duplexes bounded by mélange shear zones interpreted as paleo–subduction interfaces. Coherent thermal histories within duplex panels, and temperature jumps across thrusts, suggest underplating of lenses with distinct thermal histories, later tilted and exhumed intact. These results provide new constraints on how accretionary margins evolve through alternating episodes of underplating, erosion, and forearc unroofing.

Source: Tectonics

DOI: 10.1029/2025tc009004