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Gehrels, G.E., 1986

Geologic and tectonic evolution of Annette, Gravina, Duke, and southern Prince of Wales islands, southeastern Alaska

Bibliographic Reference

Gehrels, G.E., 1986, Geologic and tectonic evolution of Annette, Gravina, Duke, and southern Prince of Wales islands, southeastern Alaska: Pasadena, California, California Institute of Technology, Ph.D. dissertation, 329 p., illust., maps (2 folded in pocket).


Annette, Gravina, Duke, and southern Prince of Wales Islands belong to the Alexander terrane, which is a coherent tectonic fragment that underlies much of southeastern Alaska and the Saint Elias Mountains region of British Columbia and Yukon. Geologic mapping combined with U-Pb (zircon) geochronologic studies indicate that these islands are underlain primarily by pre-Ordovician metasedimentary and metavolcanic rocks of the Wales metamorphic suite, Middle and Late Cambrian metaplutonic rocks, arc-type volcanic and sedimentary rocks (Descon Formation) and coeval dioritic to granitic intrusive bodies of Early Ordovician through Early Silurian age, Late Silurian trondhjemite and subordinate granite and sodic leucodiorite, Lower Devonian clastic strata and limestone (Karheen Formation) and coeval basaltic–andesitic volcanic rocks, and Upper Triassic volcanic and sedimentary rocks (Hyd Group) and a coeval gabbro body. Regional tectonic events recorded by these rocks include the Middle Cambrian–Early Ordovician 'Wales orogeny,' the middle Silurian–earliest Devonian Klakas orogeny, and a latest Paleozoic(?)–Triassic uplift and erosional event. The geologic history of this region, combined with relations reported by other workers from the central and northern parts of the terrane, indicate that the Alexander terrane evolved: (1) within an ensimatic volcanic arc environment prior to Middle Devonian time, (2) in a tectonically quiescent environment in which shallow-marine clastic strata and limestone were deposited in most regions during Middle Devonian through Early Permian time, and (3) in a rift environment during latest Paleozoic(?)–Triassic time. Comparison of the geologic framework, tectonic history, paleomagnetic record, and paleobiogeographic affinity of fossils from the Alexander terrane with the characteristics of other Paleozoic orogenic systems suggests that the terrane may have: (1) evolved along the Australian margin of the paleo-Pacific basin during early Paleozoic time, (2) migrated eastward across the basin near the paleo-equator during late Paleozoic time, and (3) been located near northwestern South America during Triassic time. Geologic relations record accretion of the Alexander terrane to western North America during Late Cretaceous–early Tertiary time.

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