LATE CRETACEOUS UNDERPLATING OF THE KLUANE-COTTONWOOD-MACLAREN SCHIST BENEATH THE NORTHERN COAST MOUNTAINS ARC, YUKON AND ALASKA

The timing and structural vergence of the Wrangellia terrane collision with North America has resurfaced as a controversial topic in Cordilleran tectonics. The controversy appears to stem from an incomplete understanding of the ages and deformation characteristics of metasedimentary rocks in the suture. We present a synthesis of our recent work elucidating the protolith age, metamorphic path, and deformation history of the Kluane-Cottonwood-Maclaren (KCM) schist belt at the inboard margin of the Wrangellia terrane.

The KCM schist belt is package of amphibolite facies metapelitic and quartzofeldspathic rocks, which are dismembered by the dextral Denali fault in southern Alaska and southwestern Yukon. New and published detrital zircon age spectra from the KCM schist belt give maximum depositional ages of ca. 90-100 Ma for the sedimentary protolith. A strong correlation between age populations of Mesozoic detrital zircon grains from the KCM belt (n=2,032) and periods of high magmatic flux in the Coast Mountains arc suggests that the sedimentary protolith of the KCM belt was sourced primarily from the Coast Mountains arc. Lesser Paleozoic and Proterozoic grains record input from Yukon Tanana terrane igneous rocks and peri-Laurentian strata. Syn-kinematic intrusions in the Maclaren schist (Alaska) record prograde metamorphism and thrust burial of the schist belt from ca. 75-65 Ma. The prograde shear zones dip toward the continent and are interpreted as primary collisional structures. Ca. 50 Ma intrusions throughout the KCM belt predate dissection by the modern Denali fault and ca. 40-35 Ma syn-kinematic intrusions in the Alaskan schists were emplaced into a dextral shear regime along the modern Denali fault.

On the basis of our integrated dataset, we interpret that the protolith of the KCM belt was deposited into a contractional basin to the west of the northern Coast Mountains arc (Northern B.C. and Yukon) at ca. 90-100 Ma. Collapse of the basin between ca. 90 and 65 Ma caused the KCM belt to be underplated beneath the arc along east-dipping thrust shear zones. Eocene intrusions seal the collisional structures. Collectively, our analysis of the KCM belt supports a model of diachronous (generally south to north) early-to-late Cretaceous accretion of the Wrangellia terrane along west-vergent structures.