Abstract

Recently determined lead-alpha and potassium-argon ages from northern southeastern Alaska indicate major plutonic events in the Paleozoic, Mesozoic, and Tertiary; in contrast, previous studies suggested that only one complex Jurassic and Cretaceous event occurred. The ages presented in this paper indicate the following Mesozoic and Tertiary plutonic events: Middle or Late Jurassic (144–164 m.y.); Early Cretaceous (103–117 m.y.); Eocene (42–48 m.y.); and Oligocene to Miocene (24–31 m.y.). The present data show no distinctive a real pattern for the Mesozoic plutons, but those of known Tertiary age are restricted to Baranof and Kruzof islands, a distribution that suggests a belt of Tertiary plutonism along the margin of the Pacific Ocean.

Stratigraphic evidence and radiometric ages indicate that Baranof Island and possibly Chichagof Island have been uplifted several kilometers since Miocene time, whereas Admiralty Island to the east appears to have been relatively stable since Paleocene time. This movement apparently took place on the north-striking Chatham Strait fault, which separates the islands, and probably also had a large right-lateral component. Northwest-striking faults in Chichagof and Baranof islands were probably active during at least part of the movement on the Chatham Strait fault. Movement on one of the northwest-striking faults, the Patterson Bay fault of Baranof Island, took place some time between the Eocene and the Miocene and produced a 5-km, right-lateral separation.

The inferred uplift of Baranof Island relative to Admiralty Island is based on the present-day exposure on Baranof Island of mesozonal Tertiary plutons, which were probably intruded at a depth of several kilometers, contrasted with the present-day exposure on Admiralty Island of continental sedimentary and volcanic rocks that were being deposited near sea level during the Tertiary. The uplift of the Baranof Island plutons to the surface in post-Miocene time contrasts sharply with the stable or weakly negative tectonic conditions that have prevailed on Admiralty Island since the Paleocene.

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