THE ROLE OF FORELAND BASIN BURIAL IN THE PRODUCTION OF A HIGH-GRADE METAMORPHIC TERRANE: THE KAHILTNA ASSEMBLAGE AND THE MACLAREN GLACIER METAMORPHIC BELT

The Kahiltna assemblage (KA) is exposed within the Alaska Range suture zone. Northeast of the KA in the Eastern Alaska Range is the Maclaren Glacier metamorphic belt which has been interpreted as a metamorphosed continuation of the KA. A ductile thrust fault, the Valdez Creek shear zone (VCSZ), forms the boundary between the KA and the Maclaren Glacier metamorphic belt. Previous work has shown that the VCSZ is associated with a steep inverted metamorphic gradient. The KA varies from sub-greenshist facies to lower amphibolite facies structurally up section into the VCSZ. The hanging wall of the VCSZ is composed of amphibolite facies rocks. The thrust is mapped as the abrupt transition from phyllite and schist, which are part of the KA, to gneiss in the hanging wall. Microstructural analysis of rocks across the VCSZ show a transition from rocks with coaxial flattening fabrics in the footwall to non-coaxial reverse shear fabrics in the hanging wall and is consistent with loading during top to the south, thrust stacking. We report a single detrital zircon age spectrum from the northern limb of a map scale anticline that lies within the footwall of the VCSZ, and north of the Talkeetna fault. The sample yielded 250 zircon ages with the resultant age spectrum defining a single age peak at 91± 1 Ma, and a maximum depositional age of 81± 7 Ma. No zircons older than 145 Ma were observed suggesting a source within Wrangellia and younger intruded areas. The 74± 1 Ma Valdez Creek tonalite intrudes the hanging wall gneiss. Prior work has shown that the tonalite intruded syntectonically into already hot gneiss near the metamorphic peak and cooled below biotite Ar retention temperatures by 62 Ma (Davidson 1991). This implies that strata within the footwall of the VCSZ were being deposited at the same time prograde metamorphism was occurring within the gneiss. The presence of garnet-kyanite bearing mineral assemblages suggests burial to 25-35 km in the hanging wall, implying ~20 km of vertical motion between the hanging wall and footwall. The detrital zircon ages combined with isotopic ages of the gneiss suggest this occurred between ~81 Ma when the KA sample was deposited and cooling of the hanging wall of the VCSZ at 62 Ma. The VCSZ may be an example of deep burial of basinal strata generating high-grade metamorphic rocks over a short geologic time interval.