Alaska Margin Research Articles

Chronology of ophiolite crystallization, detachment, and deeper parts of subduction zones. emplacement: Evidence from the Brooks Range, Alaska

{sup 40}Ar/{sup 39}Ar data from early Middle Jurassic ophiolites (187-184 Ma) in the western Brooks Range, Alaska, indicate that detachment - related metamorphism occurred {approximately}20 m.y. after crystallization and {approximately}20 m.y. before emplacement onto the Arctic Alaska margin. High-temperature metamorphic rocks along the basal surfaces of many ophiolites have ages that are contemporaneous with ophiolite crystallization, suggesting that detachment and thrust faulting occur while the lithosphere is young ({lt}10 Ma) and relatively hot. From these relations it has been generally assumed that detachment and initial overthrusting of oceanic lithosphere occur near the site of generation, such as a marginal basin or mid-ocean ridge. The new data from the Brooks Range ophiolites confirm previous indications that some ophiolites have much longer intervals between crystallization, thrust-related metamorphism, and emplacement. On the basis of these new results and data from other ophiolites, the authors propose that ophiolites originating within large ocean basins will generally have longer crystallization-to-emplacement intervals than the more commonly recognized marginal basin-type ophiolites.

Sediment Transport and Fan Deposition in the Gulf of Alaska; Effects of Transform Motion on Deep Sea Sedimentation: ABSTRACT

GLORIA side-scan sonar images and two channel seismic profiles recently collected in the Gulf of Alaska reveal a major site of late Miocene to Recent terrigenous sediment accumulation on the oceanic plate adjacent to the Fairweather-Queen Charlotte transform and the Yakutat Terrane. Sediment moving across this margin has formed several large channel dominated fan systems that blanket the entire gulf and spill westward onto the Tufts Abyssal Plain. The Surveyor Fan, fed by the glaciers of the Yakutat Terrane and insulated from transform sediment source offset by the Terrane, has maintained a single channel course over the entire life of the fan. The Chirikov and Baranof fans receive their sediment supply from glaciofluvial point sources along the SE Alaska margin, separated from the fans by an active transform. The fans show a southward younging of channel ages consistent with the sense of plate motion. Early (late Miocene) deposition within the gulf was limited to the structural basin between the continental margin and the Kodiak-Bowie seamount chain. The geometry of these early depositional systems is poorly known, but available data suggest their channels were oriented NW-SE. Subsequent establishment of a depositional slope between the margin and the seamount chain, coupled withmore » the filling of the basin, led to a reorganization into SW-NE channel systems. The fan bodies of the Gulf of Alaska are members of a distinct class of fans that are characterized by long distributary channels which persist to near the fan limits. This type of fan morphology is most often attributed to a predominantly fine-grained sediment supply. This is difficult to reconcile with the obvious proximal glacial source for much of the sediment supplied to these fans.« less

Paleogene anatexis along the Gulf of Alaska margin

Early Tertiary plutons of biotite tonalite, granodiorite, and granite are found in a curvilinear 2,000-km-long belt along the margin of the Gulf of Alaska. These plutons intrude flyschoid rocks that were accreted to the continental margin during Late Cretaceous and/or early Tertiary time. Field, petrologic, age, and Rb-Sr data on plutonic and metamorphic rocks in eastern Chugach Mountains suggest that the granitic magmas were produced by partial melting of deeper parts of the accretionary prism after it was deformed against the continent. Such magmas may be a common product of heating at the termination of certain major accretionary episodes; they are not subduction-related magmas from subcrustal sources.

Uyak Complex, Kodiak Islands, Alaska: A Cretaceous subduction complex

The Uyak Complex is a chaotic assemblage of gray chert and argillite, wacke, greenstone, radiolarian chert, and gabbroic and ultramafic rocks. The simplest interpretation of these rocks is that the gabbroic and ultramafic rocks and greenstone represent basal oceanic crust upon which the radiolarian chert, gray chert and argillite, and wacke were deposited, respectively, at a mid-ocean rise, on the abyssal ocean floor, and in an oceanic trench. Fossils are scarce in the complex and range in age from mid-Permian to mid–Early Cretaceous. The Uyak Complex was emplaced by underthrusting to the northwest beneath lower Mesozoic metamorphic, igneous, and sedimentary rocks. During underthrusting, brittle rock types were broken into phacoids of all sizes and suspended in the less competent matrix of gray chert and argillite with their longest dimensions aligned subparallel to cataclastic foliation. Prehnite and pumpellyite developed extensively in lithologies of suitable composition. The Uyak Complex correlates to the northeast with a similar assemblage of deep-sea rocks on the Barren Islands and with the McHugh Complex on the Kenai Peninsula and near Anchorage. The Uyak-McHugh belt defines a probable subduction complex trending northeast for at least 600 km along the margin of southwestern Alaska. The time of emplacement of this melange is uncertain, but fossils present indicate that it occurred after mid–Early Cretaceous time. To the southeast, the Uyak is underthrust by deformed turbidites of the Kodiak Formation, which are interpreted to have been deposited in an oceanic trench and accreted to the Alaskan margin in Late Cretaceous time. The relationship between the Uyak Complex and Kodiak Formation is uncertain, but they may represent two phases, or two facies, of Late Cretaceous accretion. Two large bodies of schist, referred to as the Kodiak Islands schist terrane, occur along the northwest border of the Uyak. The Kodiak Islands schist terrane is a blueschist-bearing metamorphic belt that has yielded mainly Early Jurassic K-Ar mineral ages. Similarities in K-Ar ages, metamorphism, and tectonic setting support a correlation between the Kodiak Islands schists and the Seldovia schist terrane on southern Kenai Peninsula. The Upper Triassic Shuyak Formation structurally overlies the Kodiak Islands schist terrane and Uyak Complex but is separated from them by a long, narrow pluton emplaced in Early Jurassic time. The Shuyak is a little-deformed formation of volcanic and volcaniclastic rocks. It correlates with similar rocks on the Alaska and Kenai Peninsulas, which together outline a lower Mesozoic forearc basin; K-Ar ages show that much of the Alaska-Aleutian Range batholith was intruded coeval with deposition in this forearc basin. A likely interpretation of these rocks is that the Kodiak-Seldovia schists are the only vestige of a subduction complex emplaced along the margin of southwestern Alaska during the prominent early Mesozoic volcanoplutonic activity recorded on the Alaska and Kenai Peninsulas and the Kodiak Islands.

Orientation of underthrusting during latest Cretaceous and earliest Tertiary time, Kodiak Islands, Alaska

The Uganik thrust and correlative faults constitute a major structural boundary between a melange and a deformed turbidite sequence exposed along the continental margin of southwestern Alaska. During under-thrusting between 60 and 70 m.y. B.P., the lower plate of this thrust was completely disrupted as much as 1 km or more below the fault, whereas the upper plate was overprinted by a cleavage inclined to the thrust surface. A new technique utilizing the geometry of the cleavage-thrust intersection provides the basis for determining 37 slip vectors that indicate that the lower plate of the fault was underthrust along a vector that now trends 334° ± 7° and plunges 42°NW. The movement of the Uganik thrust probably was caused by underthrusting of the sea floor. At the 95% confidence level the direction of movement is similar to slip vectors determined from adjacent accreted terranes and confirms the subduction polarity inferred from regional geology.

Mesozoic Magmatism and Accretion, Southwest Alaska Margin: ABSTRACT

Mesozoic Magmatism and Accretion, Southwest Alaska Margin: ABSTRACT

Historic Earthquakes and Future Earthquake Hazards along Gulf of Alaska Margin: ABSTRACT

Historic Earthquakes and Future Earthquake Hazards along Gulf of Alaska Margin: ABSTRACT

Tectonic Framework of Petroliferous Rocks in Alaska: ABSTRACT

Alaska, comprising 3.6 × 106 sq km (about 28%) of the land, shelf, and upper continental slope of the United States, has been estimated by the U.S. Geological Survey to contain about 20% of total petroleum liquids and natural gas resources of the nation. Some 15 billion bbl of petroleum liquids and about 31 trillion cu ft of natural gas have been discovered. In northern Alaska, Paleozoic and Mesozoic shelf and slope deposits and some ophiolitic rocks of the Brooks Range orogen were thrust northward over the depressed south margin of the Paleozoic and Mesozoic Barrow platform, on which a foredeep (the Colville geosyncline) developed in Early Cretaceous time. Cretaceous and Tertiary sediments from the Brooks Range filled this foredeep and prograded northwest and northeast to form the Chukchi and Colville delta systems and to fill the Camden coastal basin. A series of arc-trench systems developed on oceanic rocks in southern Alaska during the Jurassic and Cretaceous. These arcs were subparallel with the Mesozoic continental margin of southern Alaska. Between the arcs and the metamorphic (continental) terranes of east-central Alaska and the southern Brooks Range, a large marginal ocean basin received thick Jurassic and Cretaceous volcanic and detrital deposits. These deposits were extensively deformed and disrupted by widespread mid-Jurassic to Tertiary plutonism, Late Cretaceous and early Tertiary (Laramide) oroclinal bending, wrench faulting, and arc-related compression. The Laramide events continentalized the late Mesozoic marginal basin deposits and welded them to the older continental terranes. Subsequent sedimentation was more localized and nonmarine, except in basins along the Pacific, Arctic, and Bering coasts where thick mixed marine and nonmarine sections are present. The active Aleutian arc and associated Queen Charlotte transform-fault system were superimposed obliquely across the southern continental margin of Alaska in early Cenozoic time and have since dominated structural and depositional patterns in southern Alaska. The largest petroleum reserves in Alaska and the best prospects for additional large discoveries are in northern Alaska, where an extensive terrane is underlain by upper Paleozoic to Tertiary shelf and slope clastic and carbonate deposits. The pre-Tertiary arc and marginal-sea deposits in southern and interior Alaska are either too intensely deformed or too low in porosity to offer more than modest local prospects. The Tertiary coastal onshore and offshore basins with End_Page 1446------------------------------ thick marine and nonmarine clastic rocks, and locally many large folds, are attractive for exploration. These little explored basins are known to be petroliferous on Bristol Bay and the Gulf of Alaska and to contain major accumulations of oil and gas at Cook Inlet. End_of_Article - Last_Page 1447------------

Yakutat Group, an Upper Mesozoic Flysch and Melange Sequence in Southern Alaska: ABSTRACT

The Yakutat Group, part of a belt of highly deformed late Mesozoic rocks deposited in deep water and extending for more than 1,600 km along the Gulf of Alaska margin, underlies an area 200 km long by 15-30 km wide of the Saint Elias Mountains foothills and adjacent coastal plain. Bedded rocks of the Yakutat Group are dominantly Cretaceous graywacke and pelite with local pebble-cobble conglomerate and sparse pods of oolitic chert-nodule limestone. Interspersed with the bedded rocks are melanges composed of large blocks of competent rocks up to several kilometers in size engulfed in a pervasively sheared matrix of pelite or tuffaceous pelite. These clasts include externally derived or exotic greenstone, marble, metagraywacke, metachert, and diorite that are at least in part of Triassic(?) and Middle Jurassic age. Following deposition, the Yakutat was subjected to (1) pre-late Eocene compressive folding and thrusting, (2) regional zeolite to low greenschist-facies metamorphism with emplacement of early Tertiary granitic plutons, and (3) disruption by large-scale dextral shearing. Available data suggest that the Yakutat was deposited in an oceanic trench associated with a volcano-plutonic arc. Large exotic clasts were presumably incorporated in the melange by tectonic disruption and submarine sliding of older rocks exposed along the inner wall of the trench. A source for the sediments and exotic clasts does not exist in contiguous parts of the Saint Elias Mountains. The most probable provenance lies roughly 240 km southeast in the Chicago-Baranof-Admiralty Islands area of southeastern Alaska. Large-scale post-middle Eocene dextral slip on the Fairweather fault is postulated to explain separation of the Yakutat Group from its source area. End_of_Article - Last_Page 800------------

Uplifted Trench Sediments: Southwestern Alaska-Bering Shelf Edge

Cretaceous turbidites are discontinuously exposed for 1700 kilometers along the continental margin of southwestern Alaska and the Bering shelf edge. Paleocurrent flow parallel to exposure patterns and the abundance of primary andesitic volcanic detritus suggest deposition in an oceanic trench. This Cretaceous trench bordering the continent was superseded by the Aleutian arc-trench in the earliest Tertiary.

Geologic History of Alaska Peninsula: ABSTRACT

The Alaska Peninsula area is of particular geologic interest because it is part both of the Aleutian volcanic arc and the continental margin of southwestern Alaska. Topographically, the peninsula is a ridge, rising above the general level of a broad marine platform consisting of the Bering Sea shelf and the Shumagin-Kodiak shelf. However, the structural and stratigraphic history of these shelves appears to be separate from that of the Alaska Peninsula. The islands of the Shumagin shelf consist largely of a thick flysch sequence of late Mesozoic turbidites and volcanic rocks containing ultramafic bodies and are intruded by earliest Tertiary quartz diorite plutons. Similar rocks comprise the Kenai and Chugach Mountains. The oldest dated rocks of the Alaska Peninsula are Permo-Triassic carbonate and volcanic rocks and Lower Jurassic volcanic debris, both of which were intruded by an Early Jurassic granitic batholith. Uplift and erosion of these rocks caused the appearance of the Alaska Peninsula, and the accumulated arkosic debris now constitutes a thick Middle Jurassic to Lower Cretaceous sequence. Middle Cretaceous deformation was relatively small-scale, but rocks of this age are absent from the Alaska Peninsula. Uppermost Cretaceous strata constitute a thin, but widespread, transgressive sequence. Marine and non-marine volcanic rocks and debris accumulated to great thicknesses throughout the early Tertiary, especially in the outer parts of the Alaska Peninsula; lesser amounts were deposited on the newly uplifted Shumagin shelf. These were deformed gently at the time of mid-Tertiary plutonic intrusions along the present Pacific shore. Miocene debris from older rocks, as well as new volcanic material, accumulated in great thickness, but Pliocene strata occur only as thin patches of volcanic rocks in the mountains and as isolated bodies of marine sediments near the present coast. Both the Pliocene volcanic and sedimentary rocks rest discordantly on older rocks. All of the prominent structural features of the Alaska Peninsula were formed by post-Miocene deformation. The Alaska Peninsula thus may have existed as early as Middle Jurassic time. The Shumagin-Kodiak shelf was formed during the earliest Tertiary. The Aleutian volcanic arc and trench are no older than Tertiary, and the trench may be relatively young. The greatest thickness of Tertiary sediments accumulated in isolated depressions that were only partly controlled by earlier structure, e.g., in the Gulf of Alaska, Cook Inlet, Bristol Bay, and at the outer parts of the Alaska Peninsula. End_of_Article - Last_Page 645------------