Donald H. Richter

Quaternary Faulting in the Eastern Alaska Range

Quaternary faulting is well displayed along the Denali fault system and the recently recognized and related Totschunda fault system in the eastern Alaska Range. The principal movement on both fault systems is right-lateral strike-slip. Offset glacial features of Wisconsin age indicate minimum Holocene slip rates of 1.1 to 3.5 cm per year along parts of the Denali fault system, and 0.9 to 3.3 cm per year along the Totschunda fault system. Strike-slip movement along the Denali fault system may be no older than early Pliocene and, southeast of the Totschunda fault system junction, may have terminated by the middle Pleistocene. The strike-slip Totschunda fault system, a much younger feature probably no older than middle Pleistocene, exhibits 9 to 10 km of right-lateral offset and 1,500 m of relative vertical movement. The Totschunda fault system is aligned with, and has the same sense of slip as, the Fairweather fault in the Gulf of Alaska. The Denali fault system and the Queen Charlotte Islands fault are part of a major transform fault system separating the North American and Pacific plates. Continental southern Alaska between the Aleutian arc and the Denali fault system is now largely coupled to the Pacific plate. The Totschunda-Fairweather alignment probably represents the beginning of a new transform fault by-passing the southeast part of the Denali fault system.

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.

Granitic Plutonism and Metamorphism, Eastern Alaska Range, Alaska

Plutonic rocks in the eastern Alaska Range were emplaced in Late Pennsylvanian time (282 to 285 m.y. B.P.) and during two distinct intervals in Cretaceous time (105 to 117 and 89 to 94 m.y. B.P.) Development of a large plutonic-metamorphic complex, consisting of diorite and quartz diorite intimately associated with banded gneiss and other metamorphic rocks, apparently occurred during Late Triassic to Middle Jurassic time (163 to 199 m.y. B.P.). A smaller plutonic-metamorphic complex is Miocene in age (17 m.y.). The younger Cretaceous plutons are recognized only in the regionally metamorphosed Devonian and older terrane north of the Denali fault. Plutons of the older Cretaceous and Pennsylvanian events are restricted to Pennsylvanian and younger terrane south of the Denali fault and are associated with coeval volcanic rock assemblages. The major plutonic-metamorphic complex is also restricted to the terrane south of the Denali fault and may relate to collapse of an upper Paleozoic volcanic arc in Triassic time followed by syntectonic magmatism in Jurassic time. The Miocene plutonic-metamorphic complex may reflect the time of initial movement along the Denali fault. Porphyry copper deposits are associated with the Cretaceous plutons south of the Denali fault. The source of the copper may be subjacent copper-rich basalt flows (Nikolai Greenstone) of Triassic age.

Eruptive history and petrology of Mount Drum volcano, Wrangell Mountains, Alaska

Mount Drum is one of the youngest volcanoes in the subduction-related Wrangell volcanic field (80x200 km) of southcentral Alaska. It lies at the northwest end of a series of large, andesite-dominated shield volcanoes that show a northwesterly progression of age from 26 Ma near the Alaska-Yukon border to about 0.2 Ma at Mount Drum. The volcano was constructed between 750 and 250 ka during at least two cycles of cone building and ring-dome emplacement and was partially destroyed by violent explosive activity probably after 250 ka. Cone lavas range from basaltic andesite to dacite in composition; ring-domes are dacite to rhyolite. The last constructional activity occured in the vicinity of Snider Peak, on the south flank of the volcano, where extensive dacite flows and a dacite dome erupted at about 250 ka. The climactic explosive eruption, that destroyed the top and a part of the south flank of the volcano, produced more than 7 km3 of proximal hot and cold avalanche deposits and distal mudflows. The Mount Drum rocks have medium-K, calc-alkaline affinities and are generally plagioclase phyric. Silica contents range from 55.8 to 74.0 wt%, with a compositional gap between 66.8 and 72.8 wt%. All the rocks are enriched in alkali elements and depleted in Ta relative to the LREE, typical of volcanic arc rocks, but have higher MgO contents at a given SiO2, than typical orogenic medium-K andesites. Strontium-isotope ratios vary from 0.70292 to 0.70353. The compositional range of Mount Drum lavas is best explained by a combination of diverse parental magmas, magma mixing, and fractionation. The small, but significant, range in 87Sr/86Sr ratios in the basaltic andesites and the wide range of incompatible-element ratios exhibited by the basaltic andesites and andesites suggests the presence of compositionally diverse parent magmas. The lavas show abundant petrographic evidence of magma mixing, such as bimodal phenocryst size, resorbed phenocrysts, reaction rims, and disequilibrium mineral assemblages. In addition, some dacites and andesites contain Mg and Ni-rich olivines and/or have high MgO, Cr, Ni, Co, and Sc contents that are not in equilibrium with the host rock and indicate mixing between basalt or cumulate material and more evolved magmas. Incompatible element variations suggest that fractionation is responsible for some of the compositional range between basaltic andesite and dacite, but the rhyolites have K, Ba, Th, and Rb contents that are too low for the magmas to be generated by fractionation of the intermediate rocks. Limited Sr-isotope data support the possibility that the rhyolites may be partial melts of underlying volcanic rocks.

LAVA TREE MOLDS OF THE SEPTEMBER 1961 ERUPTION, KILAUEA VOLCANO, HAWAII

Well-developed lava tree molds were formed during the September 1961 eruption along the east rift zone of Kilauea Volcano. The upright molds were produced where fluid lava, flowing through dense tropical forest, became chilled against the larger trees and tree ferns and later drained away. Where the lava ponded temporarily in a structural valley, tree molds more than 14 feet high mark the high level attained by the flow.