Till and 3 co-authors, published this report and 1:500,000-scale geologic map in 2011 as the U.S. Geological Survey Scientific Investigations Map 3131 , which describes the bedrock geology of Seward Peninsula, western Alaska, on the North American side of the Bering Strait. The map encompasses all of the Teller, Nome, Solomon, and Bendeleben 1:250,000-scale quadrangles, and parts of the Shishmaref, Kotzebue, Candle, and Norton Bay 1:250,000-scale quadrangles (sheet 1 and sheet 2).
Rolling, tundra-covered hills cover most of Seward Peninsula; trees and shrubs are present only in limited areas in the southern and eastern parts of the peninsula. Broad lowlands underlie large areas in central and northern Seward Peninsula. Part of the central area and large areas in northernmost and easternmost Seward Peninsula are an extensive, unvegetated lava field. Four glacially carved mountain ranges, the York, Kigluaik, Bendeleben, and Darby Mountains, reach heights of 1,000- 1,300 m (sheet 2); the highest point on the Peninsula, Mount Osborn, is in the Kigluaik Mountains.
The York Mountains of western Seward Peninsula and hills adjacent to them on the east are underlain by bare, unvegetated carbonate rocks. The map area covers 74,000 km2, an area slightly larger than West Virginia or Ireland.
Most of Seward Peninsula is underlain by rocks that were part of a Late Proterozoic to early Paleozoic continental margin; remnants of that margin are also present in the Brooks Range. Rocks of the Chukotsk Peninsula, 85 km west of the Seward Peninsula, across Bering Strait (sh.1, 2), probably were part
of the same continental margin as well (Dumoulin and others, 2002). The extensive high-pressure metamorphic rocks exposed in central Seward Peninsula (Nome Complex), the southern Brooks Range, and the Ruby geanticline are thought to record subduction of that continental margin during the Jurassic
(Patton and Box, 1989; Christiansen and Snee, 1994; Roeske and others, 1995; Gottschalk and Snee, 1998).
During the Early Cretaceous, contractional deformation ceased within the subduction zone, moved into the subjacent
continental crust, and propagated to shallower levels. As a result, the continental margin was involved in a mountain-building episode, which involved thickening of the continental crust and movement of thrust sheets northward. The authors refer to this post-subduction contractional deformation of continental crust as the Brooks Range orogeny. Some exhumation of the Nome Complex blueschists by thrust faulting may have been part of this episode. Rocks in the York Mountains of Seward Peninsula likely experienced thrust faulting and shallow burial at this time, based on thermal indices (conodont color alteration indices; table A–2) from carbonate rocks and outcrop-scale structures in siliciclastic rocks.
Metamorphic fabrics in the Nome Complex have been related to subduction zone deformational processes (Thurston, 1985; Patrick, 1988) and extensional collapse of the Brooks Range orogen after the mountain building and crustal thickening episode (Hannula and others, 1995; Amato and others, 2002). 40Ar/39Ar and fission-track data indicate that significant cooling of the high-pressure metamorphic rocks now exposed on Seward Peninsula occurred in the period 120–70 Ma (Hannula and McWilliams, 1995; Dumitru and others, 1995); extensional deformation is thought to have occurred during that
time span.
Amphibolite to granulite facies metamorphism overprinted the Nome Complex and other rocks during the mid-Cretaceous; at about the same time, a protracted period of magmatism began on Seward Peninsula and lasted until the end of the Cretaceous (fig. 2, on sheet 2). At least one suite of plutonic rocks formed in an island arc setting (Amato and Wright, 1997). During the Cenozoic, normal faulting created small mountain ranges and
adjacent sedimentary basins, and mafic volcanic rocks accumulated in the central and northern parts of the peninsula.
In this report the term “metabasite” refers to metamorphosed mafic rocks of any nature, including originally intrusive and extrusive varieties. Based on lithology, age, tectonic affinity, and metamorphic history, rocks of Seward Peninsula are divided into several tectonic subdivisions, illustrated on sheet 2.
Igneous Rocks
Granitoid rocks intruded the Nome Complex during the Cretaceous (fig. 2, on sheet 2). Based on early reconnaissance work and K-Ar ages, three suites of plutons were recognized: an early alkalic group, 110 to 96 Ma; a calc-alkaline group, 95 to 80 Ma; and a tin granite group, 80 to 69 Ma (Till and others,
1986). These characterizations are in flux. The alkalic rocks are part of a belt that extends east of Seward Peninsula into the Yukon-Koyukuk basin (Miller, 1989; Arth and others, 1989; Amato and Wright, 1998). The calcalkalic plutons core the Kigluaik Mountains and are common on eastern Seward Peninsula (Till and others, 1986; Amato and Wright, 1998). Magmatism continued with intrusion of plutons and alkalic dike swarms (Amato and others, 2003c; Gottlieb and Amato, 2008).
During latest Cretaceous time, a suite of tin-bearing granites intruded both the Nome Complex and adjacent rocks in western Seward Peninsula (Hudson and Arth, 1983). Magmatic events on Seward Peninsula are coeval with similar events on Saint Lawrence Island, Alaska, approximately 250 km southwest of Seward Peninsula, and in the Okhotsk/Chukotsk volcanic belt on the Russian side of Bering Strait, and formed in the same general tectonic regime (Amato and Wright, 1998; Miller and others, 2002; Amato and others, 2003b). Better delineation of crystallization ages using U-Pb zircon geochronology and a deeper understanding of petrogenetic events using modern geochemical tools was initiated by Arth and others (1989) and Amato and others (1994).
Continued study of the age and nature of petrogenetic events on Seward Peninsula is underway (for example, Amato and Wright, 1997; Gottlieb and Amato, 2008); the earlier delineation of plutonic suites is in the process of being revised. In central, eastern, and northern Seward Peninsula, extensive plateau basalts, cinder cones, and very large maars formed during several episodes of volcanism as old as the Oligocene, and as young as the Holocene (Hopkins and others, 1971; Turner and Swanson, 1981).
Tertiary and Active Deformation
Geomorphologic, structural, and thermal data indicate active deformation on Seward Peninsula occurred in Tertiary to Recent times. Along the coastline south of the York Mountains, terraces were uplifted during the late Pleistocene (Sainsbury, 1967). Active normal faults, which are marked by sag ponds,
offset streams, scarps and other features, bound the Kigluaik and Bendeleben Mountains (Kaufman, 1985; Plafker and others, 1994), and expose the high-grade metamorphic rocks. Apatite fission-track data show that significant slip on these faults
started in the Eocene (Dumitru and others, 1995).
A transition from Nome Complex detritus to detritus that also includes high-grade minerals was recognized in wells drilled in Tertiary (Paleogene) offshore basins (Forbes and others, 1984; Tolson, 1987). Based on gravity data, basins sit below McCarthy Marsh, south of the Bendeleben Mountains, and the Imuruk basin, north of the Kigluaik Mountains (Barnes and Hudson, 1977; Barnes and Morin, 1988); these basins are likely the same age as those offshore.
Tertiary activity along the Kugruk Fault Zone included reverse faulting (Retherford and others, 1986) as well as early Eocene graben formation (Dickinson and others, 1987). Death Valley, in the northern Darby Mountains, is likely a Tertiary feature.
The Digital Data for the report on the Bedrock Geology of the Seward Peninsula, Alaska
U.S. Geological Survey Open-File Report 2009-1254 (here).
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