Haeussler, P.J., 1991, Paleomagnetic and structural constraints on the accretion history of the Alexander terrane, southeastern Alaska: University of California, Santa Cruz, Ph.D. dissertation, 213 p., illust., maps.
The accretion of the Alexander terrane profoundly influenced the geologic history of southeastern Alaska. I studied the paleomagnetism of the Late Triassic Hound Island Volcanics (HIV), the Permian Pybus Formation, and 22 Ma gabbro to constrain the ancient location of the Alexander terrane and the structural geology of the Jura-Cretaceous Gravina belt to characterize the deformation that occurred during and after accretion. There are three components of magnetization in the HIV. A high-temperature component (A) is interpreted to be primary, and indicates a paleolatitude similar to those for Wrangellia, consistent with the geologic interpretation that joins the terranes before Late Triassic time. Component B is an overprint acquired at 22 Ma due to intrusion of gabbroic sills. Component C, an overprint found in both the HIV and the Pybus Formation, was probably acquired between 90 and 100 Ma, and yields a paleolatitude similar to slightly older plutonic rocks in the Coast Plutonic Complex. This suggests the Alexander terrane was at the latitude of Baja California in early Late Cretaceous time. The data from the 22 Ma gabbro indicate that any displacement or rotation of the Alexander terrane occurred prior to Miocene time. Finally, great-circle analysis of paleomagnetic data from the Pybus Formation suggests the Alexander terrane was in the northern hemisphere in Permian time. The Late Jurassic-Early Cretaceous Gravina belt lies along the eastern margin of the Alexander terrane in southeastern Alaska. This group of arc-derived sedimentary and volcanic rocks were deformed during early Late Cretaceous time due to the collapse of an ocean basin between the Alexander terrane and the Stikine terrane. From six geologic transects along a 230 km length of the Gravina belt I infer a structural history with two main events: (1) tight to isoclinal folding or thrust faulting, reflecting orthogonal shortening across the Gravina belt followed by (2) coast-parallel right-lateral strike-slip faulting with total offset on the order of several tens of kilometers. This kinematic interpretation of structures in the Gravina belt is consistent with models of plate convergence vectors and changes in plate motions.
Theses and Dissertations
