Aleutian arc 

(early Tertiary to Holocene) 

(Alaska Peninsula, Aleutian Islands, and Komandorsky Islands, units QTvf, QTvm, QTpf, mTvf, 
mTvm, mTpf, mTpm)

Major volcanic arc extending across southern Bering Sea from the Kamchatka Peninsula to Alaska. The 
western part of the arc in the Aleutian Islands is built on oceanic crust, and is subdivided into 
lower (37 to 55 Ma), middle (5.3 to 37 Ma), and upper series (5.3 Ma to Holocene) (Scholl and 
others, 1992; Miller and Richter, 1994; Vallier and others, 1994).  The lower series forms the base 
of the arc and consists predominantly of flows and volcaniclastic rocks. Igneous composition 
ranges from basalt to rhyolite, but is mainly calc-alkalic basalt and andesite. In the Aleutian 
Islands, the lower series is generally metamorphosed to greenschist through zeolite facies. 
Common associated with local hypabyssal and plutonic rocks.

The middle series is predominantly volcanic flow and volcaniclastic rocks, conglomerate, and 
dolomite. Igneous compositions are mainly andesite and dacite. The unit forms an extensive 
sediment and volcanic blanket on both forearc and backarc slopes. Associated with locally 
abundant associated sills, dikes, plugs, and large quartz diorite and granodiorite plutons.

The upper series is predominantly sedimentary and volcanic strata in offshore basins and along the 
flanks of volcanoes. The early Tertiary part of arc, named the Meshik arc on the Alaska Peninsula, 
consists chiefly of andesite and dacite flows and volcaniclastic rocks. The younger, middle 
Tertiary through Holocene part of the arc consists of about 37 subaerial stratocones composed 
predominantly of calc-alkalic andesite to dacite flows, tuffs, and volcaniclastic rocks.

The eastern part of the arc on the Alaska Peninsula and in the Alaska Range is built mainly on the 
Peninsular sequence of the Wrangellia superterrane (unit WRP). It consists of the 50 to 55 Ma 
Teklanika Formation and the 37 to 45 Ma Mount Galen, Sheep Creek, and Post River volcanic 
complexes and associated unnamed units that range in composition from basalt to rhyolite. 
Characterized by mantle-derived, high-alumina, calc-alkalic suite of igneous rocks. This part of 
the arc consists of the eroded remnants of stratovolcanos.

The western extremity of the arc in the Komandorsky Islands consists of: (1) Eocene and Miocene 
volcanic, tuffaceous, and sedimentary deposits including basalt, andesite-basalt and andesite and 
interbedded tuff (K-Ar ages of 37± 4 Ma); (2) overlying conglomerate with volcanic rock pebbles; 
(3) interlayered sandstone, tuff, clay mudstone, and diatomite of the Oligocene Kamenskaya suite 
with local coal layers and plant detritus; (3) unconformably overlying, thin subalkalic basalt and 
alkalic basal; (4) middle Miocene tuff and tuffaceous siltstone; and (5) cross-bedded sandstone 
and gritstone. The younger part of the arc locally contains extensive interlayered Quaternary and 
Holocene glacial deposits.

For paleomagnetic determinations, a sequence of Paleocene sedimentary rocks on the 
Komandorsky Islands yield grade A results and indicate a southerly displacement with respect to 
the Siberian platform of 23o+7o. Sedimentary rocks of Eocene age from two locations on Amlia 
and Umnak Islands yield grade A results. They indicate a zero displacement with respect to North 
America within their error limits.  REFERENCES: Gilbert and others, 1976; Decker and Gilbert, 
1977; Bundtzen and others, 1982; Stone and others, 1983; Ivashchenko and others, 1984; Wilson, 
1985; Harbert, 1987; Tsvetkov and others, 1990;. Moll-Stalcup, 1990, 1994; Bazhenov and 
others, 1992; Vallier and others, 1994