Structural evolution of an arc-basin: The Gravina Belt in central southeastern Alaska

摘要: The upper Middle Jurassic-Lower Cretaceous Gravina belt lies along the eastern margin of Alexander terrane in southeastern Alaska. This group turbidites and mafic to intermediate volcanic rocks was deformed during mid Late time closing a basin unknown size between on west Stikine east. Therefore structures largely reflect final accretion subsequent transport terrane. Six geologic transects across central (southern Mitkof Island northern Douglas Island) define structural history that includes (1) syndepositional soft-sediment folding faulting, possibly conjunction with block tilting extension; (2) tight isoclinal or thrust formation slaty cleavage (S1) striking ∼330°; (3) local coaxial refolding crenulation (S2); (4) development domainal folds (S3) oriented at large angle belt; (5) intrusion tonalitic plutons around 90 Ma; (6) right-lateral strike-slip displacement faults ∼330°. Finite strain measurements sedimentary suggest least twice its present width, normal foliation, before deformation. Subhorizontal margin-parallel fold axes, cleavage, margin-perpendicular stretching lineations orthogonal contraction basin, assuming oblique plate convergence not decoupled faults. After contractional deformation, indicate dextral (probably order several tens kilometers) respect terranes Domainal cleavages high change directions occurred while (presently surface) could still plastically deform. kinematic interpretation is consistent changes motions [Engebretson et al., 1985]. Before 100 Ma, Kula North America plates were continental margin, whereas after small margin. In addition, Kula-North America, Farallon-North direction most compatible N-S principal paleostress orientation derived from inversion fault data. relationship suggests it may have been drove northward America.