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Clendenen, W.S., 1991

Thermal history and vertical tectonics of the southern Alaska convergent margin

Bibliographic Reference

Clendenen, W.S., 1991, Thermal history and vertical tectonics of the southern Alaska convergent margin: Providence, Rhode Island, Brown University, Ph.D. dissertation, 177 p., illust., maps.


Accretionary prisms along convergent margins are constructed from material transferred from the subducting to the overriding plate either by offscraping at the leading edge of the prism or by underplating at depth to the base of the prism. Prisms grow laterally primarily by offscraping at the toe, but they may thicken vertically to maintain their critical shape by several mechanisms: (1) underplating, (2) penetrative strain, (3) 'out-of-sequence' faulting, or (4) sedimentation on top of the prism. The southern Alaska convergent margin is an ideal location to study an accretionary prism because it records convergence and accretion during the past 200 m.y. and has not been dismembered by subsequent collision or strike-slip faulting. The early Mesozoic history of the convergent margin is represented by a clastic-poor, blueschist-bearing section. In contrast, the Late Cretaceous and Tertiary history is dominated by three episodes of clastic sediment accretion and by the intrusion of an extensive suite of igneous rocks. Fission track and 40Ar/39Ar dating were used to constrain the thermal history of the accretionary prism. These data indicate that the blueschist-bearing rocks on the Kodiak islands were uplifted slowly over an extended period of time, whereas the younger, clastic-rich accretionary prism underwent rapid cooling during the middle Tertiary, probably during the underplating of a large volume of sediment. Field observations, micropaleontology, and backstripping were used to study the evolution of the convergent margin during the late Cenozoic episode of accretion, which continues today. A new unit, the 'Albatross formation,' was identified, which records the effects of erosion and sedimentation in the accretionary prism. The vertical tectonic pattern across the modern accretionary prism is complex, with areas of both uplift and subsidence. Erosion and sedimentation act in concert to redistribute material and maintain the shape of the accreting system. Sedimentation is responsible for a major portion of the vertical growth of the accretionary prism during the late Cenozoic.

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