Metamorphism and near‐trench plutonism during initial accretion of the Cretaceous Alaskan forearc

Abstract

Cenozoic uplift of the eastern Aleutian forearc has exposed the tectonic join between the Jurassic crystalline hanging wall of the arc and the oceanic assemblages of the footwall accreted during Cretaceous and Tertiary subduction, the Border Ranges fault system (BRFS). The early stages of accretion along the BRFS in the Knik River area were characterized by high‐temperature ductile thrusting and metamorphism at low to moderate P/T (P from 300–700 MPa, Tmax about 600°C). Peak metamorphic conditions at the currently exposed levels can be explained by heat carried to the area by tonalite and trondhjemite plutons. Generation of tonalite and trondhjemite melts within the Cretaceous forearc, however, requires a heat source within or near the BRFS. Ridge subduction cannot be ruled out as the heat source; however, simple thermal models presented herein indicate that the temperatures required for anatexis within the forearc can be explained by transient heating on the fault by shear heating of dry lithologies during the earliest stages of thrust motion. Water released from subducted oceanic crust would result in generation of partial melts in the hanging wall if shear heating is significant or if the initial thermal gradient within the hanging wall is high. Further accretion of oceanic sediment would quench the system and temperatures rapidly evolve to the high P/T conditions of normal subduction zones. Thermochronologic data from this study show that the duration of high‐T‐low P/T conditions was short, of the order of about 15 m.y. or less. Exhumed forearcs here and elsewhere commonly record a period of low to moderate P/T conditions during subduction initiation in which shear heating likely played an important role.