Aleutian Ridge Research Articles

Bowers Swell: Evidence for a zone of compressive deformation concentric with Bowers Ridge, Bering Sea

Bowers Swell is a newly discovered bathymetric feature which is up to 90 m high, between 12 and 20 km wide, and which extends arcuately about 400 km along the northern and eastern sides of Bowers Ridge. The swell was first revealed on GLORIA sonographs and subsequently mapped on seismic reflection and 3.5 kHz bathymetric profiles. These geophysical data show that the swell caps an arcuate anticlinal ridge, which is composed of deformed strata in an ancient trench on the northern and eastern sides of Bowers Ridge. The trench fill beneath the swell is actively deforming, as shown by faulting of the sea floor and by thinning of the strata across the crest of the swell. Thinning and faulting of the trench strata preclude an origin for the swell by simple sediment draping over an older basement high. We considered several models for the origin of Bowers Swell, including folding and uplift of the underlying trench sediment during the interaction between the Pacific plate beneath the Aleutian Ridge and a remnant oceanic slab beneath Bowers Ridge. However, such plate motions should generate extensive seismicity beneath Bowers Ridge, which is aseismic, and refraction data do not show any remnant slab beneath Bowers Ridge. Another origin considered for Bowers Swell invokes sediment deformation resulting from differential loading and diapirism in the trench fill. However, diapirism is not evident on seismic reflection profiles across the swell. We favour a model in which sediment deformation and swell formation resulted from a few tens of kilometers of low seismicity motion by intraplate crustal blocks beneath the Aleutian Basin. This motion may result from the translation of blocks in western Alaska to the south-west, forcing the movement of the Bering Sea margin west of Alaska into the abyssal Aleutian Basin.

Was the Aleutian Basin More Enclosed, Its Deeps Oxygen Deficient, and the Geothermal Gradient Steepened in the Middle Tertiary? Implications for the Petroleum Potential of the Bering Sea: ABSTRACT

Circulation of north Pacific waters through the Aleutian Basin of the deep Bering Sea (3,500-4,000 m) is presently vigorous and affects all depths. Energetic currents enter and exit the Bering Sea via deepwater (1,000-4,000 m) passes in the Aleutian Ridge. But geologic evidence suggests that in the middle Tertiary (or earlier), the basin was possibly stagnant or poorly ventilated. A sequence of undeformed sedimentary deposits typically more than 4 km thick underlies the south-central bathymetric deep of the basin. The basinal sequence includes hundreds of velocity-amplitude anomalies (VAMPs), which are deep-water bright spots associated with underlying velocity pull-downs. The VAMPs are thought to record the pooling of thermogenic gas at subbottom depths of 800 to 1,000 m. Because VAMPs are largely confined to the basin's bathymetric low, subsurface source beds that supplied the gases must be located here. Favorable preservation of organic matter over the basin deep may reflect a former oxygen-poor bottom-water environment. Tectonic reconstructions indicate that prior to the middle Tertiary a more laterally continuous Aleutian Ridge restricted circulation with the north Pacific, a circumstance that would have favored incomplete deep basin ventilation. New information concerning the paleoceanographic history of the Bering Sea, the prospect that organic-rich sourcemore » beds have been deposited, and the likelihood that an episode of early Tertiary backarc spreading enhanced the basin's geothermal gradient, combine to improve the petroleum potential of the Aleutian basin.« less

The evolution of forearc structures along an oblique convergent margin, central Aleutian Arc

Multichannel seismic reflection data were used to determine the evolutionary history of the forearc region of the central Aleutian Ridge. Since at least late Miocene time this sector of the ridge has been obliquely underthrust 30° west of orthogonal convergence by the northwestward converging Pacific plate at a rate of 80–90 km/m.y. Our data indicate that prior to late Eocene time the forearc region was composed of rocks of the arc massif thinly mantled by slope deposits; the forearc region probably lacked both major depositional basins and a tectonically attached accretionary prism of offscraped oceanic deposits. Beginning in latest Miocene or earliest Pliocene time, a zone of outer‐arc structural highs and a forearc basin began to form. Formation of these companion intraarc structures may be linked to the late Neogene growth of an accretionary wedge that formed as the result of the deposition of a thick turbidite wedge in the Aleutian Trench. Initial structures of the zone of outer‐arc highs formed as the thickening wedge underran, compressively deformed, and uplifted the seaward edge of the arc massif above a landward dipping backstop thrust. Forearc basin strata ponded arcward of the elevating zone of outer‐arc highs. However, most younger structures of the zone of outer‐arc highs cannot be ascribed simply to the orthogonal effects of an underrunning wedge. Oblique convergence created a major right‐lateral shear zone (the Hawley Ridge shear zone) that longitudinally disrupted the zone of outer‐arc highs, truncating the seaward flank of the forearc basin and shearing the southern limb of Hawley Ridge, an exceptionally large antiformal outer‐arc high structure. Slivers of forearc basement rocks and overlying strata have been transported along the shear zone that is flanked by differentially elevated structures attributed to localized transpressive and transtensional processes. Uplift of Hawley Ridge may be related to the thickening of the arc massif by westward directed basement duplexes. In addition, the forearc is disrupted by structures transverse to the margin that occur where unusually high‐stress accumulations have resulted in the rupture of repeated great earthquakes. It is likely that many ancient active margins evolved in tectonic and depositional settings similar to those of the central Aleutian Ridge. Great structural complexity, including the close juxtaposition of coeval structures recording compression, extension, differential vertical movements, and strike‐slip displacement, should be expected, even within areas of generally kindred tectonostratigraphic terranes.

The origin of summit basins of the Aleutian Ridge: Implications for block rotation of an arc massif

It is proposed that many summit basins along the Aleutian Arc form from the clockwise rotation of blocks of the arc massif. Summit basins are arc‐parallel grabens or half‐grabens formed within the arc massif and are commonly located near or along the axis of late Cenozoic volcanism. Geomorphically, the Aleutian Arc appears to consist of contiguous rhombic blocks of varying size, tens to hundreds of kilometers in length. The boundaries between adjacent blocks are delineated by fault‐controlled canyons that cut the southern slope of the arc transverse to its regional trend. Evidence that these blocks have rotated clockwise is provided by the triangular‐shaped summit basins bordering the blocks to the north, oblique physiographic trends, offsets in the summit platform, and broad deflections in the southern slope of the arc. We present a model for block rotation that involves translation of blocks parallel to an arc. It is suggested that block rotation, which appears to have accelerated in late Cenozoic time, is linked to (1) a shift in the Euler pole for the Pacific plate, (2) the consequential start‐up of late Cenozoic volcanism, (3) improved interplate coupling instigated by sediment flooding of the Aleutian Trench, and (4) westward subduction of northeast striking segments of the inactive Kula‐Pacific Ridge.

Evolution and petroleum geology of Amlia and Amukta intra-arc summit basins, Aleutian Ridge : Geist, E.L., J.R. Childs and D.W. Scholl, 1987. Mar. Petrol. Geol., 4(4):334–352

Evolution and petroleum geology of Amlia and Amukta intra-arc summit basins, Aleutian Ridge : Geist, E.L., J.R. Childs and D.W. Scholl, 1987. Mar. Petrol. Geol., 4(4):334–352

Structural and Stratigraphic Evolution of Aleutian Convergent-Margin Basins--Ridge Crest to Trench Floor: ABSTRACT

The Aleutian Ridge lies along nearly 2,000 km of the north Pacific's obliquely converging plate boundary with North America. Since middle Eocene time, convergent-margin basins have repeatedly formed here, typically as summit basins along the ridge crest, and as forecarc basins on the landward trench slope. Thick (1-4 km) sequences of terrigenous, hemipelagic, and biogenic debris have accumulated in these depressions, which are mostly intra-arc structures floored by arc-basement rocks. Summit and forearc basins formed as a consequence of plate-boundary coupling and the application of compressional and right-lateral shear stresses to the arc massif. Basins typically evolved along shear zones in response to transtensional processes, and as trailing-edge grabens behind rotating blocks of arc massif. In the late Cenozoic, high rates of trench sedimentation led to the growth of an accretionary complex that underthrust forearc basement. Wedging and improved plate coupling elevated and laterally shifted blocks of outer forearc rocks, creating much of the structural framework of the regionally extensive Aleutian Terrace forearc basin. Changes in plate-boundary conditions that affected the ridge's volcanic activity and regional elevation importantly influenced basinal sedimentation. Changes of greatest significant were a major shift in convergence direction and rate about 42 Ma (reduced volcanism), ridgemore » underthrusting by increasingly younger ocean crust in Oligocena and Miocene time (arc elevation), and the combination of more orthogonal underthrusting and the subduction of a dead spreading center 5-120 Ma (arc subsidence).« less

Evolution and petroleum geology of Amlia and Amukta intra-arc summit basins, Aleutian Ridge

Amlia and Amukta Basins are the largest of many intra-arc basins formed in late Cenozoic time along the crest of the Aleutian Arc. Both basins are grabens filled with 2–5 km of arc-derived sediment. A complex system of normal faults deformed the basinal strata. Although initial deposits of late Micocene age may be non-marine in origin, by early Pliocene time, most of the basinfill consisted of pelagic and hemipelagic debris and terrigenous turbidite deposits derived from wavebase and subaerial erosion of the arc's crestal areas. Late Cenozoic volcanism along the arc commenced during or shortly after initial subsidence and greatly contributed to active deposition in Amlia and Amukta Basins. Two groups of normal faults occur: major boundary faults common to both basins and ‘intra-basin’ faults that arise primarily from arc-parallel extension of the arc. The most significant boundary fault, Amlia-Amukta fault, is a south-dipping growth fault striking parallel to the trend of the arc. Displacement across this fault forms a large half-graben that is separated into the two depocentres of Amlia and Amukta Basins by the formation of a late Cenozoic volcanic centre, Seguam Island. Faults of the second group reflect regional deformation of the arc and offset the basement floor as well as the overlying basinal section. Intra-basin faults in Amlia Basin are predominantly aligned normal to the trend of the arc, thereby indicating arc-parallel extension. Those in Amukta basin are aligned in multiple orientations and probably indicate a more complex mechanism of faulting. Displacement across intra-basin faults is attributed to tectonic subsidence of the massif, aided by depositional loading within the basins. In addition, most intra-basin faults are listric and are associated with high growth rates. Although, the hydrocarbon potential of Amlia and Amukta Basins is difficult to assess based on existing data, regional considerations imply that an adequate thermal history conducive to hydrocarbon generation has prevailed during the past 6-5 my. The possibility for source rocks existing in the lower sections of the basins is suggested by exposures of middle and upper Miocene carbonaceous mudstone on nearby Atka Island and the implication that euxinic conditions may have prevailed during the initial formation of the basins. Large structures have evolved to trap migrating hydrocarbons, but questions remain concerning the preservation of primary porosity in a sedimentary section rich in reactive volcaniclastic debris.

Seismicity and tectonics of the far Western Aleutian Islands

Study of focal mechanisms of earthquakes in the Near and Komandorsky Islands indicate that there are several distinct zones of tectonic activity. South of the Near Islands, normal faulting occurs in the trench east of 172°E and low-angle thrusting dominates the Aleutian ridge. Mechanisms indicate underthrusting as far west as Mednyy Island with strike-slip faulting restricted to the south and west of Beringa Island. A zone of northeast striking left-lateral faulting near 1645.°E is proposed to separate the Aleutian Ridge from Kamchatka Peninsula. This motion, as well as faulting north of the Komandorsky Islands, may be related to the existance of a buffer plate comprising the Aleutian Ridge in the Komandorsky Islands. Active subduction terminates near 173°E and the faulting north of the Komandorsky Islands may, in part, be due to the bouyancy of a remnant slab. Depth phase modelling indicates bulletin-reported depths are overestimated due to a misidentification of depth phases.

Petrology and provenance of deep sea drilling project sand and sandstone from the north pacific ocean and the bering sea

Sand and sandstone compositions from different types of basins reflect provenance terranes governed by plate tectonics. One hundred and one thin sections of Upper Miocene to Holocene sand-sized material were examined from DSDP/IPOD Sites in the North Pacific Ocean and the Bering Sea. The Gazzi-Dickinson point-counting method was used to establish compositional characteristics of sands from different tectonic settings. Continental margin forearc sands from the western North America continental margin arc system are clearly different from backarc/marginal-sea sands from the Aleutian intraoceanic arc system. The forearc sands have average QFL percentages of 29-42-29, LmLvLst percentages of 32-34-34, 3 Fmwk%M and 0.82 P/F. Aleutian backarc sands have average QFL percentages of 8-22-69. LmLvLst percentages of 9-85-6, 0.5 Fmwk%M and 0.96 P/F. A trend of increasing QFL%Q and decreasing LmLvLst%Lv westward in the backarc region of the Aleutian Ridge reflects the influence of the Asiatic continental margin. Aleutian backarc sands without continental influence have average QFL percentages of 1-20-79, LmLvLst percentages of 1-98-1, 0 Fmwk%M and 0.99 P/F. Of the continental margin forearc samples, sands on the Astoria Fan (west of the Oregon—Washington trench) contain the highest LmLvLst%Lv and lowest P/F; sands from mixed transform-fault and trench settings (Delgada Fan and Gulf of Alaska samples) have slightly higher Qp/Q (0.03); and sands from the Pacific-Juan de Fuca-North America triple junction have the highest Fmwk%M. Delgada Fan and Gulf of Alaska sands have average QFL percentages of 27-38-35, LmLvLst percentages of 37-26-37, 2 Fmwk%M and 0.86 P/F. Astoria Fan sands have average QFL percentages of 35-41-24, LmLvLst percentages of 30-47-23, 3 Fmwk%M and 0.74 P/F. The triple-junction sands have average QFL percentages of 28-59-13, LmLvLst percentages of 25-26-49, 9 Fmwk%M and 0.87 P/F. The petrologic data from the modern ocean basins examined in this study can provide useful analogs for interpretation of ancient oceanic sequences. Our data suggest some refinements of, but generally substantiate, existing petrologic models relating sandstone composition to tectonic setting.

Sedimentation and deformation in the Amlia Fracture Zone sector of the Aleutian Trench

A wedge-shaped, landward thickening mass of sedimentary deposits composed chiefly of terrigenous turbidite beds underlies the west-south west-trending Amlia sector (172°20′–173°30′W) of the Aleutian Trench. Pacific oceanic crust dips northward beneath the sector's sedimentary wedge and obliquely underthrusts (30° off normal) the adjacent Aleutian Ridge. The trench floor and subsurface strata dip gently northward toward the base of the inner trench slope. The dip of the trench deposits increases downsection from about 0.2° at the trench floor to as much as 6–7° just above basement. The wedge is typically 2–2.5 km thick, but it is thickest (3.7–4.0 km) near the base of the inner slope overlying the north-trending Amlia Fracture Zone and also east of this structure. Slight undulations and relatively abrupt offsets of the trench floor reflect subsurface and generally west-trending structures within the wedge that are superimposed above ridges and swales in the underlying oceanic basement. The southern or seaward side of some of these structures are bordered by high-angle faults or abrupt flexures. Across these offsets the northern side of the trench floor and underlying wedge is typically upthrown. West-flowing turbidity currents originating along the Alaskan segment of the trench (1200 km to the east) probably formed the greater part of the Amlia wedge during the past 0.5 m.y. The gentle northward or cross-trench inclination of the trench floor and underlying wedge probably reflects regional downbending of the oceanic lithosphere and trench-floor basement faulting and rotation. Much of the undulatory flexuring of the trench wedge can be attributed to differential compaction over buried basement relief. However, abrupt structural offsets attest to basement faulting. Faulting is associated with extensional earthquakes in the upper crust. The west-trending basement offsets are probably normal faults that dip steeply south or antithetic to the north dip of the subducting oceanic crust. Up-to-arc extensional faulting can be attributed to the downbending of the Pacific plate into the Aleutian subduction zone. The rupturing direction and dip is controlled by zones of crustal weakness that parallel north Pacific magnetic anomalies, which were formed south of a late Cretaceous—early Tertiary spreading center (Kula—Pacific Ridge). The strike of these anomalies is fortuitously nearly parallel to the Amlia sector. The up-to-arc fracturing style may locally assist in elevating blocks of trench deposits to form the toe of the trench's landward slope, which is in part underlain by a compressionally thickened accretionary mass of older trench deposits. Compressional structures that can be related to underthrusting are only indistinctly recorded in the turbidite wedge that underlies the trench floor.

Multichannel seismic evidence bearing on the origin of Bowers Ridge, Bering Sea

Bowers Ridge is a large, arcuate sub-marine ridge that extends north and west from the Aleutian Ridge and separates the abyssal Aleutian and Bowers Basins in the Bering Sea. Two multichannel seismic-reflection lines recorded in 1976 over Bowers Ridge and the adjacent basins confirm the existence of 8- to 10-km-thick sediment wedges on the north side of Bowers Ridge and at the base of the Bering continental margin. Deformed sediment within the Bowers wedge indicates that subduction of the adjacent ocean crust beneath the ridge probably occurred prior to middle Cenozoic time. Flat-lying reflectors near the bottom of the trench suggest that a bathymetric trough and large ridge existed in Mesozoic time. The major period of underthrusting, subsidence, and in-filling of the trench probably occurred from Mesozoic to early Tertiary time. Small amounts of underthrusting may have continued after the early Tertiary development of the Aleutian Ridge; however, by middle Miocene time, the formerly subaerial Bowers Ridge had subsided below sea level. The multichannel seismic data do not show evidence for a buried spreading center within the eastern Aleutian Basin. Consequently, the sediment wedges (trenches) at both Bowers Ridge and the Bering continental margin are believed to be the consequence of subduction that occurred during the convergence of the ridge and the margin. The large size of Bowers Ridge suggests that a large amount of convergence has occurred since Mesozoic time. If Bowers Ridge was a large feature in Mesozoic time, as suggested by the apparent bathymetric trough, then the ridge may be as old or older than the Aleutian Ridge to which it connects.

Early evolution of the Bering Sea by collision of oceanic rises and North Pacific subduction zones

Three major bathymetric features exist in the Bering Sea: Shirshov Ridge, Bowers Ridge, and Umnak Plateau. New refraction data over Umnak Plateau and previous geophysical data across Bowers Ridge indicate that a thickened welt of crustal material is present beneath both features. The crustal structure is transitional between oceanic and continental types. Various models for the origin of these features have been investigated. One that has not been proposed previously assumes that the protostructures of Bowers Ridge and Umnak Plateau could have formed outside of the present Bering Sea. According to this model, before formation of the Aleutian Ridge in late Mesozoic or earliest Tertiary time, these protostructures moved into their present Bering Sea positions. Prior to the arrival of these two structures in the Bering Sea, oceanic crust was subducted along the Bering continental margin connecting Alaska and Siberia. The collision of the Umnak Plateau protostructure with the southeastern edge of the margin may have caused subduction to terminate here and move southward. The new southerly position of subduction beneath the Aleutian Ridge was therefore controlled by late Mesozoic or early Tertiary locations of Umnak Plateau, Bowers Ridge, and possibly, the north-trending Shirshov Ridge farther to the west.

Neogene sedimentation on the outer continental margin, southern Bering Sea

Neogene sedimentary rocks and sediments from sites on the outer continental margin in the southern Bering Sea and on the Alaska Peninsula are dominated by volcanic components that probably were eroded from an emergent Aleutian Ridge. A mainland continental source is subordinate. Most sediment in the marine environment was transported to the depositional sites by longshore currents, debris flows, and turbidity currents during times when sea level was near the outermost continental shelf. Fluctuations of sea level are ascribed both to worldwide glacio-eustatic effects and to regional vertical tectonics. Large drainage systems, such as the Yukon and Kuskokwim Rivers, had little direct influence on sedimentation along the continental slope and Unmak Plateau in the southern Bering Sea. Sediments from those drainage systems probably were transported to the floor of the Aleutian Basin, to the numerous shelf basins that underlie the outer continental shelf, and to the Arctic Ocean after passing through the Bering Strait. Environments of deposition at the sites along the outer continental margin have not changed significantly since the middle Miocene. The site on the Alaska Peninsula, however, is now emergent following shallow-marine and transitional sedimentation during the Neogene.

Resource Potential and Plate-Margin Geology of Frontier Basins of North Pacific and Bering Sea: ABSTRACT

Few of Alaska's offshore frontier basins have been explored by drilling. However, regional geologic data and tectonic considerations can be used to assess the likelihood that commercial volumes of oil and gas are present in the basins. Basins of the northern margin of the Gulf of Alaska, and the contiguous Aleutian Ridge on the west, have formed along the Aleutian subduction zone, a tectonic terrane 3,600 km long that separates the Pacific and North American plates. The eastern gulf shelf is underlain by Cenozoic deposits that are as much as 10 km thick, but adequate reservoir beds are thought to be absent in Neogene and younger beds. However, the discovery that potential reservoir and source beds of early Tertiary age underlie the continental slope enhances the oil and gas prospects of the eastern gulf margin. Basins of the central gulf shelf (Kodiak Island area) contain upper Cenozoic beds that are as much as 5 to 6 km thick. These beds are broadly deformed and unconformably overlie more deformed rocks of Paleogen and Cretaceous age. Grabens (unusual for the gulf margin) filled with 6 to 8 km of Neogene and younger beds are present beneath the western gulf shelf (Sanak Island). Publicly available data imply that reservoir and End_Page 1279------------------------------ source beds adequate to form large hydrocarbon deposits are probably absent from the central and western gulf. Farther west the lower Tertiary igneous core of the Aleutian Ridge is overlain by broadly deformed Neogene and younger deposits. These beds are 2 to 4 km thick in summit basins, and probably much thicker below the Aleutian terrace along the ridge's southern flank. Although these basins include diatomaceous and turbidite sequences, the probable abundance of readily altered volcanic detritus cautions against optimistic expectations of large quantities of oil and gas along the Aleutian Ridge. Five extensive (25,000 sq km) basins filled with as much as 15 km of mostly Cenozoic beds are present beneath the Beringian shelf, and, therefore, north of the Aleutian subduction zone. Except near Siberia, the deposits in these basins are little deformed. Elongate St. George and Navarin basins, along the southern or outer edge of the shelf, have formed on a collapsed foldbelt of miogeoclinal rocks that include beds of Jurassic and Cretaceous age. Subsidence of the foldbelt occurred after subduction of oceanic crust ceased beneath the Beringian margin (60 to 70 m.y. ago) and shifted south to the Aleutian Ridge. In contrast, Norton basin, which underlies the inner or northern edge of the shelf, is floored by Paleozoic and older rocks of Brooks Range affinity that subsided in response t Cenozoic strike-slip faulting in western Alaska. A speculative reading of the geologic history of the Beringian basins implies that some of them could harbor commercial volumes of oil and gas. South of the Beringian margin, the abyssal floor (3 to 4 km) of the Bering Sea basin is underlain by 4 to 10 km of undeformed deposits chiefly of Cenozoic age. Drilling results, and the detection of deep-water bright spots (VAMPs), suggest that hydrocarbon deposits (of unknown volume) occur in the basin. Its basement of Lower (?) Cretaceous oceanic crust was presumably separated from the north Pacific by the formation of the Aleutian Ridge in latest Cretaceous or earliest Tertiary time. Since early Mesozoic time, the evolution of the structural framework of the north Pacific margin has been controlled by the subduction of more than 10,000 km of oceanic lithosphere. However, recognition that segments of the margin are underlain by deeply submerged miogeoclinal rocks of Mesozoic and early Tertiary age, and the results of DSDP drilling at Pacific margins, attest that the evolution of Alaskan and Bering Sea margins is not adequately described by models of accretionary tectonics or back-arc spreading. Little understood aspects of subduction and post-subduction tectonics that cause and control marginal uplift and subduction are thought to hold important clues to the economic potential of the frontier basins of the north Pacific and Bering Sea regions. End_of_Article - Last_Page 1280------------

Episodes of Aleutian Ridge Explosive Volcanism

Earlier workers have overlooked deep-sea bentonite beds when unraveling the Cenozoic volcanic history of an area. In the North Pacific, identification of Miocene and older volcanic episodes is possible only if both altered (bentonite) and unaltered ash beds are recognized. Our study, which includes bentonite beds, shows that volcanism on the Aleutian Ridge and Kamchatka Peninsula has been cyclic. Volcanic activity seems to have increased every 2.5 x 10(6) years for the past 10 x 10(6) years and every 5.0 x 10(6) years for the time span from 10 to 20 x 10(6) years ago. The middle and late Miocene and the Quaternary were times of greatly increased volcanic activity in the North Pacific and elsewhere around the Pacific Basin. The apparent absence of a volcanic record before the late Miocene at Deep Sea Drilling Project site 192 is the result not of plate motion, as suggested by Stewart and by Ninkovich and Donn, but rather of the diagenesis of ash layers. Major, apparently global volcanic episodes occurred at least twice in the last 20 x 10(6) years. Yet, only one major glacial epoch (the Pleistocene) has occurred. Therefore, even though glaciation coincided with an increase in Quaternary volcanism, the increased volcanism itself may not have been the primary cause of global cooling.

Neogene Volcaniclastic Sediments from Atka Basin, Aleutian Ridge

Atka basin is a major depression in the Aleutian Terrace, south of the Andreanof Islands in the Aleutian chain (174°W). Sediments flooring the basin were recovered at Site 186 of the Deep Sea Drilling Project on the southern edge of the basin. Most of the sedimentary fill is diatomaceous silty clay. Interbedded in the silty clay are numerous layers of turbidite sand and vitric volcanic ash. The sand layers are mostly volcanic-rock fragments and volcanic glass, although some feldspar and quartzose debris, including chert, is present. Most of the sediment in the basin must have originated by erosion and volcanic eruptions in the adjacent central Aleutian arc. Neogene sedimentation rates at Site 186 exceeded 180 m/m.y. and in the early Pliocene may have been more than 300 m/m.y. These high rates were caused by extensive shelf erosion perhaps associated with extensional rifting and volcanism in the Miocene and early Pliocene. The sediments of Atka basin are structurally and temporally part of the late series of the Aleutian Ridge and are probably representative of the vast volumes of Tertiary debris that has accumulated on the ridge and in adjacent parts of the Bering Sea. The reactive nature of the turbidite sediments in Atka basin suggests that other basins along the Aleutian Ridge may not be particularly favorable locations for hydrocarbon accumulation.

Meiji sediment tongue: North Pacific evidence for limited movement between the Pacific and North American plates

The Meiji sediment tongue is an elongate southeast-trending sedimentary body in the northwestern corner of the Pacific. The tongue is approximately 800 km long, 300 km wide, and as much as 1,800 m thick. It is thickest immediately south of Kam­ chatka Strait, which separates Kamchatka from the west end of the Aleutian Ridge, and it thins southeastward away from the strait. Studies of the sediment cored at Deep Sea Drilling Project site 192 indicate that the tongue began to form in earlies t Miocene time and that most of it is clay-size terrigenous debris derived from eastern Siberia. Pelagic beds, chiefly diatomaceous debris, make up the remainder. Today, day-size erosional detritus is presumably swept into the northwestern Pacific by the Kamchatka Current, which flows from the Bering Sea southwestward through Kamchatka Strait. A large part of this flow is presumably deflected to the southeast along the axis of the sediment tongue. The tongue therefore signifies that the current, and presumably the strait, formed at least by earliest Miocene time. In middl e Miocene time, the accumulation rate of terrigenous clay (now claystone) at DSDP site 192 (the summit of Meiji Guyot) was 40 to 45 m/m.y. This rate implies that the guyot was near the strait in early Miocene time. Because the guyot is now near the strait, little of the tongue has been subducted beneath, or scraped off against, the Kamchatka continental margin. The Meiji sediment tongue is evidence that dur­ ing the past 16 (probably 22) m.y. no more than 300 to 400 km of Pacific lithosphere has been subducted beneath Kamchatka (that is, American plate). Another sediment body in the North Pacific, the turbidite beds of the Aleutian Abyssal Plain, signifies that during the past 50 m.y. convergence be­ tween the Pacific and American plates has not exceeded about 500 km.

Episodic Aleutian Ridge igneous activity: Implications of Miocene and younger submarine volcanism west of Buldir Island

Extrusive rocks of Miocene and younger age have been dredged from the submerged insular slopes of the arcuate, 2,220-km-long Aleutian Ridge. Hornblende dacite porphyry recovered at station 70-B29 (lat 52.6/sup 0/N, long 174.8/sup 0/E; depth, 700 m) was extruded less than 610,000 yr ago. The dacite crops out approximately 80 km west of Buldir Island, the westernmost volcanic edifice of the 2,550-km-long Aleutian volcanic chain. The submerged dacite extends the westward limit of this chain of eruptive centers, which are the product of a distinct phase of late Cenozoic (chiefly early Pliocene to present) volcanism. This part of the ridge is not associated with a north-dipping Benioff zone, a fact that may imply that arc-type calc-alkalic magma can be emplaced along sectors of the ridge either obliquely underthrust by the Pacific plate or in strike-slip contact with it (western 800 km).

Summit Basins of Aleutian Ridge, North Pacific

The crestal region of the 2,200-km-long (and mostly submerged) Aleutian Ridge is commonly an irregular tableland or summit platform. Rising above this platform, which generally lies at a depth shallower than about 200 m, are the Aleutian and Komandorsky Islands. Deeper areas of the summit platform include elongate and roughly rectangular shaped basins. Three of the larger summit basins, Amukta and Amlia basins and Pratt depression, lie along the northern edge of the summit platform and are aligned geomorphically with the loci of late Cenozoic volcanic centers. These basins are underlain by thick, slightly deformed sequences of late Cenozoic beds. Amukta basin (171.7°W) and neighboring Amlia (173°W) basin have a combined area of about 4,700 sq km (1,800 sq mi), and their floors are covered by about 1,000 m of water. They are underlain by an estimated 4,000 m of gently folded late Miocene through Holocene beds. This estimate is based on interpretations of magnetic, gravimetric, and seismic-reflection data. The deposits are dislocated along normal and high-angle reverse faults (growth types), and deformation appears to be most severe adjacent to a prominent normal fault bordering the northern sides of these basins. The floor of Pratt depression (179°E), approximately 850 m deep, also may be underlain by a thick sequence of late Cenozoic beds. A major fault scarp borders this basin on the south. The summit basins are grabens or half-grabens that began to form near the end of the Miocene, but perhaps not until the Pliocene. Andesitic volcanism accompanied their formation, and, we suspect, the basins may have subsided in response to a slight distension of the ridge crest caused by the generation of deep-seated magmas. Contemporaneous with subsidence and faulting, the basins filled with erosional debris derived from the adjacent ridge crest. The crest is underlain by deformed, altered, and plutonically intruded sedimentary and igneous rocks of Eocene through early Miocene age. Volcanic ash and diatomaceous debris are probably important minor constituents of the late Cenozoic basin fills. Seismic-reflection records suggest that much of the detritus reaching the central areas of t e basins was transported there by turbidity currents. Gaps or outlets in the northern walls of Amlia and Amukta basins allowed some of this debris, especially that of Pleistocene age, to bypass the basin and excavate large submarine canyons in thick late Cenozoic beds underlying the ridge's northern insular slope. In contrast, Pratt depression, which does not have outlets, may include a thicker sequence of Pleistocene strata.

Aleutian Arc

The Aleutian Arc is the orogenic link, some 3200 km long, between the American and Asian continents and separates off the Bering Sea Basin from the Pacific Ocean. There is clear evidence of the antiquity—Cretaceous or earlier—and of the stages of development—submarine, basic (ophiolitic) volcanism followed by subaerial, andesitic volcanism—of the arc. Deformation occurred between these stages and, in contrast to the western Pacific arcs, an outer non-volcanic arc is absent, unless represented by the Aleutian Terrace. Nor are paired metamorphic belts present here. Segment : this is a 400 km long section close to the centre of the Arc, chosen to include the Andreanof Islands (Fig. 1). The Arc as a whole varies in width from 125 km to 235 km, averaging 165 km. Zones : four zones, three entirely submarine, are recognized. On the south, the well-defined Aleutian Trench (zone 1) is over 7000 m deep. The Aleutian Terrace (zone 2) is a wide bench feature at around 4000 m beneath sea level from which the Aleutian Ridge (zone 3) rises steeply. The latter includes all the islands of the Arc. To the north of the Arc lies the Aleutian Basin (zone 4), a broad submarine plain at around 3500 m beneath sea level. History : the oldest rocks exposed are the submarine, ophiolitic, early Marine Series (late Cretaceous to early Miocene); the initiation of the volcanism must have occurred earlier, but the date is unknown. In M. Miocene times volcanism ceased, arching and faulting took place and plutonic rocks