Kinematics of basin development during the transition from terrane accretion to strike-slip tectonics, Late Cretaceous-early Tertiary Cantwell Formation, south central Alaska

The Cantwell basin was formed during Late Cretaceous time in the suture zone between the Wrangellia composite terrane and the former continental margin of southern Alaska. The Late Cretaceous (similar to 80-70 Ma) lower Cantwell Formation represents the initial fill of this basin and includes similar to 4000 m of sedimentary rock. Between Maastrichtian and late Paleocene time (similar to 70-60 Ma), rocks of the lower Cantwell Formation were deformed by nearly east-west trending folds and north vergent thrust faults. This deformation can be linked to the northward accretion and suturing of the Wrangellia composite terrace to southern Alaska. During late Paleocene to early Eocene time (similar to 60-55.5 Ma), at least 2750 m of volcanic rocks of the upper Cantwell Formation were deposited above the folded lower Cantwell Formation. New 40Ar/39Ar ages for the upper Cantwell Formation range from 59.8+/-0.2 to 55.5+/-0.2 Ma and reveal that the unconformity between the lower and upper Cantwell Formations represents a 10-20 million year hiatus. Following volcanism the Cantwell Formation was crosscut by northeast trending folds, northwest trending normal faults, north trending left-lateral slip faults, northeast trending reverse faults, and east trending right-lateral slip faults. These structures are consistent with northwest-southeast compression in a zone of right-lateral simple shear dong a fault with the same trend (east-northeast) as that of the present McKinley strand of the Denali fault system. Cantwell basin kinematics are compatible with an escape tectonics model for south central Alaska whereby the Cantwell basin was formed, folded, and partially uplifted during northward terrane accretion and suturing and was deformed again during the westward transfer of terranes along right-lateral strike-slip fault systems.