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 zon e between the Wrangellia composite terrane and the former continental margi n of southern Alaska. The Late Cretaceous (similar to 80-70 Ma) lower Cantw ell Formation represents the initial fill of this basin and includes simila r to 4000 m of sedimentary rock. Between Maastrichtian and late Paleocene t ime (similar to 70-60 Ma), rocks of the lower Cantwell Formation were defor med by nearly east-west trending folds and north vergent thrust faults. Thi s deformation can be linked to the northward accretion and suturing of the Wrangellia composite terrace to southern Alaska. During late Paleocene to e arly 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 Cant well Formation. New Ar-40/Ar-39 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 hia tus. Following volcanism the Cantwell Formation was crosscut by northeast t rending folds, northwest trending normal faults, north trending left-latera l slip faults, northeast trending reverse faults, and east trending right-l ateral slip faults. These structures are consistent with northwest-southeas t 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 escap e tectonics model for south central Alaska whereby the Cantwell basin was f ormed, folded, and partially uplifted during northward terrane accretion an d suturing and was deformed again during the westward transfer of terranes along right-lateral strike-slip fault systems.