Abstract The Geology of Alaska summarizes the onshore and offshore geology, tectonic evolution, and mineral resources of Alaska and the adjacent continental margin. The volume was prepared at a particularly appropriate time because it follows a period during which there has been an explosive increase in the amount, quality, and regional coverage of earth science data collected in Alaska and because the unifying concepts of plate tectonics and accretionary terranes have become available as a framework for interpreting the data. These new concepts have led to recognition that all of Alaska, except possibly for one area that underlies less than one percent of the state, consists of lithotectonic terranes (also referred to as “suspect” or “tectonostratigraphic” terranes) that have been added to, displaced from, and/or rotated to varying degrees relative to autochthonous parts of the continental margin (Silberling and others, this volume, Plate 3). Thus, a first-order division of the geology of Alaska can be made into (1) the small area of probable autochthonous rocks in east-central Alaska; (2) terranes underlain by known or probable pre-Late Proterozoic continental crust that were part of the North American miogeocline; and (3) terranes, flysch basins, and overlap assemblages that have either been added to, or built along, the south and west margins of the miogeoclinal assemblages in a belt 550 to 700 km wide (Fig. 1). Much of the geologic research in Alaska during the past 15 years has focused on defining these lithotectonic terranes on the basis of their biostratigraphic, magmatic, metamorphic, structural

Abstract Alaska faces the Canada Basin of the Arctic Ocean along an arcuate continental margin, gently concave to the north, that stretches unbroken from the Mackenzie Delta, near 137°W to North wind Ridge of the Chukchi Borderland near 162°W. (Marine geographic features mentioned below can be found on Plates 1 and 11 of Grantz and others, 1990a.) This margin, with an arc-length of about 1,050 km, marks one side of a continental rift along which the Canada Basin opened by rotation about a pole in the Mackenzie Delta region during middle Cretaceous time. The rift-margin structures, which lie beneath the inner shelf and coastal plain in the eastern Alaskan Beaufort Shelf and beneath the outer shelf in the western Beaufort and Chukchi Shelf, are now buried by a thick middle Lower Cretaceous to Holocene progradational continental terrace sedimentary prism. We divide the Arctic continental margin of Alaska into three sectors of strongly contrasting geologic structure and physiographic expression. In the Barter Island sector (see Figs. 3 and 4) the structure is dominated by the effects of Eocene to Holocene convergence and uplift, and the continental slope is upwardly convex; in the Barrow sector the structure is dominated by the effects of middle Early Cretaceous rifting and continental breakup, and the continental slope is upwardly concave; and in the Chukchi sector the structure is controlled by an easterly trending middle Early Cretaceous rift, and the continental slope abuts the Chukchi Borderland. Physiographically, the Alaska continental margin is expressed by the Alaska continental rise and slope

Abstract This chapter describes the geology of northern Alaska, the largest geologic region of the state of Alaska. Lying entirely north of the Arctic Circle, this region covers an area of almost 400,000 km 2 and includes all or part of 36 1:250,000 scale quadrangles (Fig. 1). Northern Alaska is bordered to the west and north by the Chukchi and Beaufort seas, to the east by the Canadian border, and to the south by the Yukon Flats and Koyukuk basin. Geologically, it is notable because it encompasses the most extensive area of coherent stratigraphy in the state, and it contains the Brooks Range, the structural continuation in Alaska of the Rocky Mountain system. Northern Alaska also contains the largest oil field in North America at Prudhoe Bay, the world's second-largest zinclead- silver deposit (Red Dog), important copper-zinc resources, and about one-third of the potential coal resources of the United States (Kirschner, this volume; Magoon, this volume; Nokleberg and others, this volume, Chapter 10; Wahrhaftig and others, this volume).

Abstract Seward Peninsula (Fig. 1) may be divided into two geologic terranes (Fig. 2) on the basis of stratigraphy, structure, and metamorphic history. The Seward terrane, an area 150 by 150 km in the central and eastern peninsula, is dominated by Precambrian(?) and early Paleozoic blueschist-, greenschist-, and amphibolitefacies schist and marble, and intruded by three suites of granitic rocks. The York terrane, roughly 100 by 75 km, occupies western Seward Peninsula and the Bering Straits region; it is composed of Ordovician, Silurian, Devonian, Mississippian, and possibly older limestone, argillaceous limestone, dolostone, and phyllite, which are cut by a suite of Late Cretaceous tin-bearing granites. The boundary between the Seward and York terranes is poorly exposed but is thought to be a major thrust fault because of its sinuous map trace, a discontinuity in metamorphic grade, and differences in stratigraphy across the boundary (Travis Hudson, oral communication, 1984). The boundary between the Seward terrane and the Yukon-Koyukuk province to the east is complicated by vertical faults (the Kugruk fault zone of Sainsbury, 1974) and obscured by Cretaceous and Tertiary cover. The Seward Peninsula heretofore was thought to consist largely of rocks of Precambrian age (Sainsbury, 1972, 1974, 1975; Hudson, 1977). Microfossil data, however, indicate that many of the rocks considered to be Precambrian are early Paleozoic in age (Till and others, 1986; Dumoulin and Harris, 1984; Dumoulin and Till, 1985; Till and others, 1983; Vandervoort, 1985). It is likely that Precambrian rocks are a minor part of the stratigraphy of the Seward Peninsula.

Abstract The east-central Alaska region extends west from the Canadian border to the southeast edge of the Koyukuk basin, and north from the Yukon-Tanana upland to the southeast flank of the Brooks Range (Fig. 1). The region encompasses all or parts of 13 1:250,000 quadrangles and covers an area of about 140,000 km 2 . The physiography of the region (Wahrhaftig, this volume) is extremely diverse (Fig. 1). Steep, mountainous areas having high to moderate relief include parts of the Ogilvie Mountains, White Mountains, Yukon-Tanana upland, Ray Mountains, and southeastern Brooks Range. Areas of dissected plateaus and rolling hills with moderate to low relief include the Porcupine plateau and the Kokrines-Hodzana upland. Lowlands characterize the Yukon Flats basin and extend upstream into other parts of the Yukon River and Porcupine River drainage basins. Bedrock exposures are severely limited in most of the east-central Alaska region. Mountainous areas are locally glaciated, but except in the most rugged parts of the Ogilvie Mountains and parts of the Brooks Range, bedrock is extensively mantled by surficial cover and the colluvial products of alpine weathering processes. Stream cuts commonly offer the best exposures in generally forested or tundra-covered areas of moderate relief. Lowland areas have few pre-Cenozoic bedrock exposures because of extensive surficial cover and tundra. The pioneering reconnaissance studies by J. B. Mertie and his predecessors were carried out in the east-central Alaska region prior to 1940. A relatively small group of geologists conducted the basic geologic mapping during the 1960s and 1970s that defined the

Abstract East-central Alaska as described in this volume (Fig. 1) is a physiographically diverse region that includes all or parts of the following physiographic divisions (Wahrhaftig, this volume): Northern Foothills (of the Alaska Range), Alaska Range (north of the northernmost strand of the Denali fault system), Tanana- Kuskokwim Lowland, Northway-Tanacross Lowland, and the Yukon-Tanana Upland. The Northern Foothills are largely rolling hills in Pleistocene glacial deposits and dissected Tertiary nonmarine sedimentary rocks. The included part of the Alaska Range is composed of highly dissected terranes of metamorphic rocks that have been intruded by Cretaceous and Tertiary igneous rocks. Mountain peaks reach altitudes as high as 4,000 m, and relief is commonly more than 1,000 m. Glaciers have carved a rugged topography. The Tanana-Kuskokwim Lowland is covered with thick glacial, alluvial, and wind-blown deposits. The Northway-Tanacross Lowland consists of three small basins mantled with outwash gravel, silt, sand, and morainal deposits. The Yukon-Tanana Upland, the largest of the physiographic divisions, consists of maturely dissected hills and mountains with altitudes as high as 1,994 m, and relief ranging from a few to hundreds of meters. Some of the highest areas supported small alpine glaciers during the Pleistocene, and rugged topography resulted locally. With the exception of the Alaska Range, outcrops in eastcentral Alaska are commonly widely scattered and small, due to extensive surficial deposits and vegetation. The vegetation ranges from heavy spruce forests along large streams to tundra at elevations of approximately 1,000 m. The region is largely in the zone of discontinuous

Abstract West-central Alaska includes a broad area that stretches from the Bering and Chukchi seacoasts on the west to the upper Yukon-Tanana Rivers region on the east, and from the Brooks Range on the north to the Yukon-Kuskokwim delta on the south. It covers 275,000 km 2 , nearly one-fifth of the entire state—and all or parts of 29 1:250,000 scale quadrangles (Fig. 1). Rolling hills with summit altitudes between 300 and 1,000 m and isolated mountain ranges that rise to a maximum altitude of 1,500 m characterize the area (Wahrhaftig, this volume). The uplands are separated by broad alluviated coastal and interior lowlands that stand less than 200 m above sea level. Bedrock exposures are generally limited to elevations above 500 m and to cutbanks along the streams. The bedrock underlying this huge area consists of six pre-mid-Cretaceous lithotectonic terranes, which were assembled by Early Cretaceous time and were subsequently overlapped by mid- and Upper Cretaceous terrigenous sediments (Figs. 2 and 3; Jones and others, 1987; Silberling and others, this volume). The bedrock in the east-central part is composed of lower Paleozoic sedimentary rocks and Precambrian metamorphic rocks that belong to the Nixon Fork and Minchumina terranes. A broad mid-Cretaceous uplift, the Ruby geanticline (Fig. 4), borders the Nixon Fork terrane on the northwest and extends diagonally across the area from the eastern Brooks Range to the lower Yukon River valley. The core of the geanticline consists of the Ruby terrane, an assemblage of Precambrian(?) and Paleozoic continental rocks that was metamorphosed

Abstract The Bering Sea shelf south of the Bering Strait encompasses an area of 1,300,000 km 2 , more than the combined area of California, Oregon, and Washington (840,000 km 2 , Fig. 1). The shelf area lies between western Alaska and eastern Siberia. The outer shelf is underlain by three large basins, Bristol, St. George, and Navarin, filled with sedimentary rocks, as well as by three bedrock ridges that extend from the Alaska Peninsula to near Siberia (Figs. 1 and 2). The innermost part of the shelf, Norton Sound, is underlain by the large, sediment-filled Norton basin (Fig. 1; Fisher and others, 1982). A similar inner basin, Anadyr basin, underlies the Gulf of Anadyr along the western side of thee Bering shelf (Fig. 1).

Abstract Southwest Alaska lies between the Yukon-Koyukuk province to the north, and the Alaska Peninsula to the south (Wahrhaftig, this volume). It includes the southwestern Alaska Range, the Kuskokwim Mountains, the Ahklun Mountains, the Bristol Bay Lowland, and the Minchumina and Holitna basins. It is an area of approximately 175,000 km 2 , and, with the exception of the rugged southwestern Alaska Range and Ahklun Mountains, consists mostly of low rolling hills. The oldest rocks in the region are metamorphic rocks with Early Proterozoic protolith ages that occur as isolated exposures in the central Kuskokwim Mountains, and in fault contact with Mesozoic accretionary rocks of the Bristol Bay region. Precambrian metamorphic rocks also occur in the northern Kuskokwim Mountains and serve as depositional basement for Paleozoic shelf deposits. A nearly continuous sequence of Paleozoic continental margin rocks underlies much of the southwestern Alaska Range and northern Kuskokwim Mountains. The most extensive unit in southwest Alaska is the predominantly Upper Cretaceous Kuskokwim Group, which, in large part, rests unconformably on older rocks of the region. Volcanic rocks of Mesozoic age are common in the Bristol Bay region, and volcanic and plutonic rocks of latest Cretaceous and earliest Tertiary age are common throughout southwest Alaska. Two major northeast-trending faults are known to traverse southwest Alaska, the Denali-Farewell fault system to the south, and the Iditarod-Nixon Fork fault to the north. Latest Cretaceous and Tertiary right-lateral offsets of less than 150 km characterize both faults. The Susulatna lineament (or Poorman fault), north of the Iditarod-Nixon Fork

Abstract South-central Alaska is defined as the region bounded by the Kuskokwim Mountains to the northwest, the basins north of the Alaska Range to the north, the Canadian border to the east, and the Chugach Mountains to the south (Fig. 1). This region, hereafter called the study area, includes the Alaska Range, the Wrangell, Nutzotin, and Talkeetna mountains, the Copper River and the Susitna basins, the northern flank of the Chugach Mountains, the Aleutian Range, and the Alaska Peninsula. This chapter describes and interprets the bedrock geology of the region, which consists mostly of a collage of Paleozoic and Mesozoic tectonostratigraphic terranes (hereafter referred to as terranes), Mesozoic flysch basin deposits, late Paleozoic and Mesozoic plutonic rocks, and younger late Mesozoic and Cenozoic sedimentary, volcanic, and plutonic rocks. Cited published sources and new data and interpretations of the authors are utilized for the descriptions and interpretations. The terranes, flysch basin deposits, and younger Mesozoic sedimentary, volcanic, and plutonic assemblages are described first in a general northwest to southeast order. Major faults or sutures are described second. Stratigraphic linkages and structural and tectonic relations between terranes are described last. Definitions of the various stratigraphic, structural, and tectonic terms are stated at the end of this introduction. The three largest terranes in the study area are the Alexander, Peninsular, and Wrangellia terranes (Fig. 2) (Jones and others, 1981, 1984, 1987). Even though many boundaries between the three terranes are commonly faults, important stratigraphic linkages exist. These linkages suggest a common history since the

Abstract The Aleutian arc is the arcuate arrangement of mountain ranges and flanking submerged margins that forms the northern rim of the Pacific Basin from the Kamchatka Peninsula (Russia) eastward more than 3,000 km to Cook Inlet (Fig. 1). It consists of two very different segments that meet near Unimak Pass: the Aleutian Ridge segment to the west and the Alaska Peninsula- Kodiak Island segment to the east. The Aleutian Ridge segment is a massive, mostly submerged cordillera that includes both the islands and the submerged pedestal from which they protrude. The Alaska Peninsula-Kodiak Island segment is composed of the Alaska Peninsula, its adjacent islands, and their continental and insular margins. The Bering Sea margin north of the Alaska Peninsula consists mostly of a wide continental shelf, some of which is underlain by rocks correlative with those on the Alaska Peninsula. There is no pre-Eocene record in rocks of the Aleutian Ridge segment, whereas rare fragments of Paleozoic rocks and extensive outcrops of Mesozoic rocks occur on the Alaska Peninsula. Since the late Eocene, and possibly since the early Eocene, the two segments have evolved somewhat similarly. Major plutonic and volcanic episodes, however, are not synchronous. Furthermore, uplift of the Alaska Peninsula-Kodiak Island segment in late Cenozoic time was more extensive than uplift of the Aleutian Ridge segment. It is probable that tectonic regimes along the Aleutian arc varied during the Tertiary in response to such factors as the directions and rates of convergence, to bathymetry and age of the subducting

Abstract This chapter summarizes the tectonic setting, geology, and tectonic evolution of the southern Alaska margin south of the Border Ranges fault system, which extends 2100 km from the Sanak Islands on the west to Chatham Strait on the east and seaward to the base of the continental slope (Fig. 1). Mesozoic and Cenozoic rocks that make up the southern Alaska continental margin record a complex history of subductionrelated underplating, offscraping, and metamorphism, as well as transform-related large-scale strike-slip displacements. The region discussed in this chapter has an area of about 328,000 km 2 , of which almost 30% is onshore. The land area includes parts or all of 26 1:250,000 scale quadrangles. The mainland along the northern Gulf of Alaska margin consists of alluvium- and glacier-covered coastal lowlands, 0 to 40 km wide, backed by a belt as wide as 40 km of rugged foothills that rise to elevations of about 2000 m (Wahrhaftig, this volume). The foothills are bordered to the north by the exceedingly rugged Kenai, Chugach, and Saint Elias mountains. Average summit elevations are over 2000 m, and numerous peaks are over 5000 m; the highest peaks are Mt. Saint Elias in Alaska (5488 m) and nearby Mt Logan (5745 m) in Canada. All major drainages in the coastal mountains are occupied by glaciers except for the Alsek River, which drains across the Saint Elias Mountains from Canada, and the Copper River, which drains across the Chugach Mountains from the interior of Alaska. The Kodiak Islands group, the islands

Abstract Southeastern Alaska, an archipelago also known as the “panhandle” of Alaska, is an approximately 52,000-mi 2 area of intensely glaciated and heavily forested mountains that rise abruptly from a complex system of deep fiords and inland marine waterways. This area is underlain by a complex and heterogeneous assemblage of rocks, and is cut by an intricate network of thrust, normal, and strike-slip faults (Buddington and Chapin, 1929; Gehrels and Berg, 1992). Rocks in the panhandle record a long and complete geologic history beginning in the Proterozoic, representing every Phanerozoic period, and continuing into the Holocene. These rocks are herein subdivided into ten tectonic assemblages (Figs. 1, 2, and 3), five of which are terranes that apparently contain distinct geologic records, and five of which are lithic assemblages that are in depositional, intrusive, or unknown contact with the terranes. This chapter begins with a summary of the regional geology of southeastern Alaska derived primarily from the compilation of Gehrels and Berg (1992) and from more recent studies by us and many others. Next, we discuss the components and characteristics of each of the primary tectonic assemblages that make up south-eastern Alaska and then discuss constraints and speculations on the relations between the terranes. We then present a general overview of the tectonic evolution of the area.

Abstract Interior Alaska has historically been considered the onshore geographic area of Alaska between the Brooks Range and the Seward Peninsula on the north and west respectively, and the Alaska Range on the south (Fig. 1). It is an area of more than 600,000 km2 that covers the central one-third of Alaska and has been variously referred to as the “intermontane plateaus” (Wahrhaftig, this volume) and the “central plateaus” (Raisz, 1948). The Yukon River, the largest river in Alaska, approximately bisects the province. The topography of the region is generally subdued. Broad alluviated lowland areas are underlain by Cenozoic nonmarine sedimentary rocks. Extensive areas in the western and southwestern parts of the province are characterized by ridge and valley topography cut in complexly deformed Jurassic and Cretaceous flysch rocks. The Brooks Range and the Alaska Range were extensively glaciated during the Pleistocene; however, only a few very small glaciers were present in the higher mountains of the interior province. Muskeg and tundra at the lower elevations, willow and alder brush in the stream valleys, and spruce and birch forests up to the tree line, at about 750 to 900 m elevation, form a thick cover of vegetation so that bedrock exposures are generally limited to ridge tops above the tree line and river-cut bank exposures.

Abstract This chapter presents a summary of the major, regionally developed, metamorphic episodes that affected Alaska throughout the evolution and accretion of its many lithotectonic terranes. Plate 4 (map and table showing metamorphic rocks of Alaska, 2 sheets, 1:2,500,000 scale) accompanies this chapter. The metamorphic scheme (Zwart and others, 1967) used for the map (Fig. 1, Table 1) is based on the occurrence of pressure- and temperature-sensitive metamorphic minerals. Regionally metamorphosed rocks are divided into four facies groups, each of which reflects a different grade of metamorphism. In order of increasing temperatures of crystallization, they are: (1) lamontite and prehnite-pumpellyite facies (LPP), shown on Plates 4A and 4B in shades of gray and tan; (2) greenschist facies (GNS), shown in shades of green; (3) epidote-amphibolite and amphibolite facies (AMP), shown in shades of orange and yellow; and (4) two-pyroxene (granulite) facies (2PX), which occurs only on the Seward Peninsula, shown in reddish brown. Where possible, the greenschist-facies and the epidote-amphibolite- and amphibolitefacies groups are further divided on the basis of pressure of crystallization into three facies series: high-, intermediate-, or low-pressure series. These facies series are indicated by an H, I, or L in place of the final letter in the symbol used for the facies group. High-pressure greenschist-(blueschist) facies rocks, and rocks metamorphosed under blueschist-facies conditions that evolved to intermediate- or low-pressure greenschist-facies conditions during a single episode, are shown in shades of blue. The metamorphic facies symbol for each episode is followed by a symbol showing the age of

Abstract Studies during the past decade have revealed that much of Alaska consists of a collection of generally far-traveled tectonostratigraphic terranes, most of which were transported to their present locations and accreted to North America in late Mesozoic to early Tertiary time (Jones and others, 1984; Silberling and others, this volume). This collage results in the exceedingly complex geologic and tectonic framework that constitutes much of present-day Alaska. Magmatic activity in Alaska was influenced by a host of factors, including many involving plate interactions, such as the rate of subduction, the angle of dip, the motion of individual plates, and the composition, thickness, and age of material that was subducted beneath or collided with Alaska. The identification and interpretation of magmatic patterns as reflected by time of intrusion, areal distribution, and composition can therefore contribute to an understanding of the tectonic history of Alaska. The following overview focuses on plutonic rocks in mainland (excludes southeastern) Alaska emplaced from the Proterozoic into the earliest Tertiary. Plutonic rocks and belts emplaced during a specific time frame (e.g., the Early Cretaceous) are not necessarily everywhere related in terms of genesis or tectonic setting, and some related rocks may be shown in different temporal episodes. Postplutonic terrane movement and Cenozoic strike-slip faulting with large displacements have further complicated the identification and interpretation of plutonic events and patterns. Rocks assigned to specific plutonic belts are assumed to be cogenetic regardless of the mechanism of formation. The informal name given to a temporal episode of intrusive activity

Abstract This chapter describes and gives elemental abundances of many of the accreted volcanic rocks and of a few hypabyssal rocks of Alaska. These rocks range from early Paleozoic (or perhaps late Precambrian) to Eocene age. All formed prior to accretion of the terrane containing them and thus were generated either as primary features in the ancestral Pacific Ocean or on terranes or superterranes carried by plates underlying that ocean. These accreted volcanic rocks are important in terms of continental growth by accretion of oceanic rocks. Various workers have asserted that such growth is by accretion of intraoceanic island arcs. This assertion, however, must be appreciably modified for the ca. 400,000-km 2 region of southern and central Alaska that is underlain by accreted rocks. Though these rocks are not known in sufficient detail to yield a precise figure, I estimate that no more than 70 to 75 percent of this newly formed crust consists of former island arcs and arc-derived epiclastic sedimentary rocks. Most of the tectonostratigraphic (lithotectonic) terranes of Alaska have minor exposures of volcanic rocks. Accounts of local and regional geology of the state contain cursory to extensive descriptions of such rocks. However, a catalog of such occurrences is not considered appropriate for this volume, and we discuss here only rocks studied by modern methods. The particular terranes containing these rocks are shown on Plate 13 (Barker and others, this volume), whereas all tectonostratigraphic terranes of Alaska are shown on Plate 3 (Silberling and others, this volume). Though virtually all

Abstract Continental Alaska has been the site of widespread magmatism throughout much of the late Mesozoic and Cenozoic, but until recently, most of this magmatism was unrecognized due to the lack of modern geologic maps or isotopic age data for large tracts of Alaska. Although parts remain unmapped, progress in reconnaissance mapping and dating have enabled workers to identify major late Mesozoic and Cenozoic magmatic provinces outside the well-known Aleutian arc and to speculate as to their tectonic implications and origin (Wallace and Engebretson, 1984). This chapter defines major Late Cretaceous and Cenozoic magmatic provinces in Alaska outside the Aleutian arc (Kay and Kay, this volume; Vallier and others, this volume; Miller and Richer, this volume) and southeast Alaska (Brew, this volume), and discusses their distribution, age, petrology, and tectonic implications. The available data suggest that Late Cretaceous and Cenozoic magmatism in continental Alaska can be roughly divided into three periods: (1) latest Cretaceous and early Tertiary (76 to 50 Ma), (2) middle Tertiary (43 to 37 Ma), and (3) late Tertiary and Quaternary (6 Ma to the present). Late Cretaceous and early Tertiary calc-alkalic volcanism and plutonism were widespread over much of western, central, and southern Alaska and on the Bering Sea shelf. Middle Tertiary magmatism was characterized by the eruption of small volumes of calcalkalic rocks in interior Alaska, contemporaneous with the inception of a major pulse of magmatism in the Aleutian arc. Late Tertiary and Quaternary volcanism has been characterized by the eruption of voluminous basaltic magma at