ALASKA AND THE YUKON 



623 



of the Aleution Islands, when more information is available, will resemble 

 that presented by Gutenberg and Richter for the structurally similar Japanese 

 are (1941). The distribution of deep and intermediate earthquake foci will 

 probably fall along an active zone or surface, which will be shown to reach the 

 surface of the crust along the northern slope of the Aleutian Trench and to 

 dip northward at a moderate angle (Coates, 1950). 



Age of Aleutian Arc 



The southern part of Kiska and the nearby islands of Attu, Agattu, 

 and the Semichis at the west end of the Aleutian arc lack the young 

 stratovolcanoes characteristic of the central and eastern islands. Instead, 

 they are composed of pre-middle Tertiary rocks and subordinate amounts 

 jof Upper Tertiary coarse clastic sediments and subaerial lava flows. On 

 jTanaga and Oglinga islands of the west-central Aleutians smoothly 

 rounded boulders in gravel beds on a wave cut platform appear to repre- 

 sent the oldest rocks of the region. The rock types are hornfels, horn- 

 'blende gneiss, slate, schist, granulite, granodiorite, biotite granite and 

 hornblende granite. The bedrock from which the boulders were derived 

 was not discovered. Judging from the lack of directional characters 

 in the granites, they are presumed to be intrusive into the other meta- 

 raorphic rocks. 



A sequence of basalt flows, tuft-breccia, and agglomerate, intruded by 

 llarge masses of gabbro and small masses of rhyolite, underlies most of 

 the island of Adak and are known as the Finger Bay volcanics (Coates, 

 jl956). These rocks have generally been greatly deformed and hydro- 

 jthermally altered, although in no way metamorphosed like the meta- 

 imorphic types in the boulders, which are therefore considered older. 



A third sequence of basalt flows and tuffs, gray, hard argillite, and 

 gray-green, coarse graywacke, seen on Attu and Shemya islands, has 

 'been intensely sheared and may be of intermediate age between the 

 boulder rocks and the Finger Bay volcanics. 



A plant fossil was found in the Finger Bay volcanics and identified as 



ate Paleozoic in age (Coates, 1956). Therefore, the intermediate basalts 



nd graywackes and the metamorphic and granitic rocks of the boulders 



ire regarded as Paleozoic. Coates regards the gneiss, schist, granulite, 



granodiorite, and granite as continental types, and concludes, therefore, 



that a continental land area stood nearby from which the boulders were 

 derived. This poses a difficult tectonic problem because the Aleutian 

 Islands in this segment rise from a rather narrow welt which is Hanked 

 on each side, most probably, by oceanic crust. It seems possible to the 

 writer that in the evolution of a great volcanic island arc from the oceanic 

 crust that deep-seated metamorphism is possible, and that granitic tvpe 

 magmas can originate there by fractional crystallization. These acidic 

 differentiates will not be large in volume such as those that arise in the 

 master eugeosynclinal belts of the continental margin. 



Although the evidence is preponderant that the Aleutian arc as we 

 now see it is Cenozoic in age, we must recognize some much older 

 aspects in its evolution. These are certainly not clear to us in their tectonic 

 relations. As will be postulated under a later heading, the main tectonic 

 elements of continental Alaska are believed to veer northwestward to 

 the Anadyr Gulf and Chukotski peninsula of the Siberian mainland, 

 holding within the confines of the Bering Sea shelf. See Fig. 39.2. 



SIBERIAN TECTONIC CONNECTIONS 



Aleutian Projection 



Since the structures of the Alaska Bange extend in a smooth curve into 

 the Aleutian Bange of the Alaska peninsula, and since the adjacent 

 geanticlines and basins, including the Aleutian trench, project in the 

 same direction, the natural inference has been that the Xevadan and 

 Coast Bange orogenic belts run out to sea and mostly die out abruptly 

 or continue as a single geanticline concealed by Tertiary volcanics. This 

 is the main assumption of Carey (195S) in the presentation of his theory 

 of the Alaskan orocline. 



Anadyr-Chukotski Projection 



In 1955 Payne showed on a tectonic map of Alaska the Colville basin 

 and Brooks Bange geanticline to project northwestward under the shallow 

 waters of the shelf off Siberia toward Wrangell Island, and this view is 

 reiterated by D. J. Miller (1959), who conceived the Seward and Tigara 



