Geology topics
Nora B. Shew
Publications and source records attributed to Nora B. Shew.
Map showing potassium-argon ages from the Mount Katmai and adjacent parts of the Naknek and Afognak Quadrangles, Alaska Peninsula, Alaska
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Age and progression of volcanism, Wrangell volcanic field, Alaska
The Wrangell volcanic field covers more than 10 000 km2 in southern Alaska and extends uninterrupted into northwest. Yukon Territory. Lavas in the field exhibit medium-K, calc-alkaline affinities, typical of continental volcanic arcs along convergent plate margins. Eleven major eruptive centers are recognized in the Alaskan part of the field. More than 90 K-Ar age determinations in the field show a northwesterly progression of eruptive activity from 26 Ma, near the Alaska-Yukon border, to about 0.2 Ma at the northwest end of the field. A few age determinations in the southeast extension of the field in Yukon Territory, Canada, range from 11 to 25 Ma. The ages indicate that the progression of volcanism in the Alaska part of the field increased from about 0.8 km/Ma, at 25 Ma, to more than 20 km/MA during the past 2 Ma. The progression of volcanic activity and its increased rate of migration with time is attributed to changes in the rate and angle of Pacific plate convergence and the progressive decoupling of the Yakutat terrane from North America. Subduction of Yakutat terrane-Pacific plate and Wrangell volcanic activity ceased about 200 000 years age when Pacific plate motion was taken up by strike-slip faulting and thrusting. ?? 1990 Springer-Verlag.
Potassium-argon age determinations from the Port Moller and Stepovak Bay quadrangles, Alaska Peninsula
No abstract available.
Apparent episodicity of magmatic activity based on radiometric age determination: A section in The United States Geological Survey in Alaska: Accomplishments during 1980
Results of recent potassium-argon age studies in the Chignik region, Alaska, (Wilson, 1980; Wilson and others, 1982) have suggested a distinct episodicity in igneous activity during Tertiary time. To date work on the Aleutian magmatic arc indicates that plutonic activity took place along the present outer Pacific margin and in the northern Alaska-Aleutian Range batholith (Reed and Lanphere, 1973; Kienle and Turner, 1976; DeLong and others, 1978) in latest Cretaceous and earliest Tertiary time (70-58 m.y.) and was followed by a hiatus lasting until late Eocene ( 45 m.y~) time. Late Eocene to earliest Miocene ( 45-20 m.y.) magmatic activity was followed by a middle Miocene hiatus (10 m.y.). Since that time, magmatic activity in the Aleutian arc has been continuous.
Preliminary results of potassium-argon age determinations from the Ugashik quadrangle, Alaska Peninsula: A section in The United States Geological Survey in Alaska: Accomplishments during 1980
Early and preliminary results of potassiumargon dating work on samples from 12 sites in the Ugashik quadrangle indicate a continuation of the geologic trends seen in the Chignik and Sutwik Island quadrangles to the south (Wilson, 1980). Tertiary volcanic and hypabyssal rocks apparently fall into two age groups: early Tertiary-late Eocene to earliest Miocene and late Tertiary and Quaternary-late Miocene to Holocene (fig. 53).
Radiometric age file for Alaska: A section in The United States Geological Survey in Alaska: Accomplishments during 1980
The Alaska radiometric age file of the Branch of Alaskan Geology is a computer-based compilation of radiometric dates from the state of Alaska and the western parts of the Yukon Territory and British Columbia. More than 1800 age determinations from over 250 references have been entered in the file. References date back to 1958 and include both published and unpublished sources. The file is the outgrowth of an original radiometric age file compiled by Don Grybeck and students at the University of Alaska-Fairbanks (Turner and others, 1975).