Thermal modeling of the Chugach Metamorphic Complex

Abstract

Heating from hot, young, subducting oceanic lithosphere and from a subducting spreading center has been proposed to explain the metamorphism and associated plutonism observed in former accretionary prism sedimentary rocks in the Gulf of Alaska. We evaluated the thermal effects on the overlying accretionary wedge by heat conduction from a hot, young, subducting slab through numerical modeling. The temperature‐limiting parameter in the modeling is the age of the subducted crust. The rate of convergence, angle of subduction, amount of radiogenic heating, and values of thermal conductivity are of secondary importance in determining temperatures in the accretionary prism. A possible former setting for the subduction of young oceanic crust is recorded in the Valdez metasedimentary rocks in the Chugach Metamorphic Complex of the Chugach terrane in southern Alaska. Petrologic data indicate that rocks now at the surface were at 10‐km depth when they were metamorphosed under greenschist facies conditions (350–400°C) and subsequently intruded by felsic plutons. Using parameter values appropriate for Chugach terrane rocks, our results indicate that, for accretionary prisms less than 20 km thick, oceanic lithosphere aged 1.0 Ma or less must be subducted in order to obtain temperatures greater than 300°C at 10‐km depth within the prism. We also modeled the transient heating of the accretionary prism due to the cessation of subduction of young oceanic lithosphere. Recent seismic refraction surveys from the Trans‐Alaska Crustal Transect have been interpreted to show underplated sedimentary rocks and relict oceanic crust beneath the Chugach Mountains. Transient heating due to cessation of subduction or underplating of hot oceanic crust does not appreciably relax the requirement of subducting oceanic crust aged 1.0 Ma or less in order to attain temperatures greater than 30O°C at 10‐km depth. However, at depths greater than 25 km, it may provide the heat necessary to generate hot fluids and felsic magma, which can augment the temperatures at shallow depths by advection of heat, as well as produce the postmetamorphism or synmetamorphism emplacement of the anatectically derived plutons observed in the Chugach Mountains. Our modeling requires subduction of such young crust that subduction of a spreading ridge was probable, although underplating of extremely young oceanic crust would also have provided sufficient heat to cause the formation of the Chugach Metamorphic Complex.