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 The arcuate New Ireland Basin is 150 km wide and trends northwest for 600 km between Feni and Mussau islands in northern Papua New Guinea. Multichannel seismic reflection data combined with refraction data show that much of the basin is a simple structural downwarp filled with up to 7 km of strata. Island outcrops and offshore reflection data indicate that New Ireland Basin contains thick sequences of Eocene to earliest Miocene volcanic rocks, Miocene shallow marine volcaniclastic rocks, Miocene shelf limestones, and latest Miocene to Holocene pelagic carbonates and volcaniclastic turbidites. Exploration for hydrocarbons in New Ireland Basin has been limited to geophysical surveys and to a few dredge and core stations. Offshore surveys show depocenters that contain more than 1000 m of Miocene elastics, overlain by about 2000 m of Miocene shelf carbonates and 2000 m of younger volcaniclastic rocks and bathyal carbonates. Counterparts of these units are exposed on New Ireland, where outcrop geology suggests, if these units are deeply buried offshore, that they may have some potential as both source and reservoir rocks for oil and gas. Possible hydrocarbon traps offshore include reeflike buildups and fore-reef deposits in the Miocene limestones as well as anticlines, normal faults, and stratigraphic pinch-outs along the basin margins. One multichannel seismic reflection line revealed a flat high-amplitude reflection, or “bright spot,” within the core of an anticline some 20 km east of New Ireland. The bright spot is about 2 km wide and occurs 1.2 s (1700 to 1800 m) beneath the sea floor in water depths of 2500 to 2600 m. Quaternary calderas developed during late-stage volcanism in each of the four island groups along the Tabar-to-Feni islands. Recent exploration within one caldera on Lihir Island, Luise, has revealed a sizable gold discovery. The gold occurs within pyrite in volcanic rocks adjacent to a 350,000-year-old monzonite intrusion. Exploration by drilling in the caldera indicates about 18.4 million ounces of gold in 167 million tons of host rock, which averages 3.4 g/t and excludes any host rock with less than 1.5 g/t.

The general aspects of the structural evolution of the Aleutian–Bering Sea region can be described in terms of plate tectonics. Involved in this model is the formation of the Aleutian Ridge in latest Cretaceous or earliest Tertiary time. The ridge is presumed to have formed in response to a southward relocation in the convergence zone of the Pacific oceanic plate, a shift away from the Beringian continental margin connecting Alaska and Siberia to an oceanic location at the Aleutian Trench. Prior to the formation of the ridge, Pacific crust is presumed to have directly underthrust the northeast-trending Koryak-Kamchatka coast. The middle and late Mesozoic eugeosynclinal or thalassogeosynclinal masses that underlie this segment of the Pacific fold belt are highly deformed, thrust faulted, and intruded by ultramafic bodies—characteristics that can be ascribed to the mechanical and magmatic consequence of plate underthrusting. This model implies a similar orogenic process for the formation of the stratigraphically and structurally similar Mesozoic rocks underlying the northeast-trending continental margin of southern Alaska. Less intense underthrusting may have occurred along the northwest-trending Pribilof segment of the Beringian margin connecting Alaska and Siberia. This margin may have been more parallel to the approximate direction of relative motion between the oceanic and continental plates. Nonetheless, fold belts, possibly intruded by ultramafic masses, formed along this segment of the Beringian continental margin in Late Cretaceous and perhaps earliest Tertiary time. The folds have since subsided below sea level—their eroded tops presently underlying as much as 3 km of virtually undeformed Cenozoic deposits. Our model relates pre- and postorogenic deposits underlying the Beringian margin and adjacent coast to the time of formation of the Aleutian Ridge, which marked the cessation of continental underthrusting and the beginning of island-arc underthrusting. Our model also implies that the ridge formed near or at its present location and that oceanic crust of late Mesozoic age underlies the Aleutian Basin of the Bering Sea. Since formation of the ridge this basin has received from 2 to 10 km of sedimentary fill. Although the model we suggest broadly explains the observed changes in tectonic style, magmatic history, and sedimentation for the Aleutian–Bering Sea region, it also implies that thousands of kilometers of oceanic crust underthrust the Kamchatka, Beringian, and Alaskan margins between Late Triassic and Late Cretaceous time, and hundreds of kilometers underthrust the Aleutian Ridge in Cenozoic time. The enormous masses of pelagic and volcanic offscrapings that would be indicative of extensive or long-term crustal underthrusting are not apparent as mappable units. Thus, while our model may be stylistically adequate, it paradoxically predicts quantities of rocks and structures that we are not able to find. Presumably they have been subducted.

Abstract The general mechanism of global tectonics implies that deep-sea deposits should be either scraped off and folded against the inner walls of trenches, or subducted along with oceanic plates. However, evidence that these tectonic processes are taking place in modern trenches is extremely difficult to recognize. For example, seismic reflection records have shown structures in only a remarkably few areas which suggest that an underthrusting oceanic plate is steadily folding the sedimentary fills of either the Peru-Chile or Aleutian Trenches. The lack of widespread evidence for compressive deformation may mean that the folding in the vicinity of the inner wall is so intense that individual folds are acoustically unresolvable. However, because the trench fill is undisturbed except in the immediate vicinity of the inner wall (i.e., a gradual buildup of folding intensity is typically not observable), it seems questionable that the semiconsolidated deposits of the fill have sufficient strength to fold abruptly to form the steeply sloping surface of this commonly 500–1,500-m-high escarpment, which in many areas is steeper than 15–20° and locally may exceed 40°. It is also reasonable to believe that in some manner the sedimentary sequences filling the Peru-Chile and Aleutian Trenches are subducted from them (i.e., inserted below the crust rather than folded against it) without producing much acoustically “visible” evidence of deformation. This supposition means that the existing volume of terrigenous debris in the trench can be only a fraction of the volume actually supplied. However, conservative figures show that the volume of Pleistocene detritus forming the bulk of the turbidite sequence in the Peru-Chile Trench and flooding the adjacent seafloor is in good balance with the amount that could have been eroded from the nearby continents. The conclusion therefore can be drawn that no significant volume of the turbidife fill of this trench has been removed tectonically. The figures are less convincing for the Aleutian Trench, but they are permissive of such a conclusion. The sedimentary and volcanic fillings of "ancient" trenches generally are presumed to be represented in part by the deformed and intruded (ultramafic) Mesozoic rocks of the Circum-Pacific eugeosyncline. The amount of underthrusting involved in the formation of segments of this fold belt is shown by field mapping to be on the order of a few hundred kilometers. Internal tectonic churning (i.e., formation of mélange units) possibly can account for a few hundred kilometers more. However, modern rates of convergence and plate-reconstruction schemes for the Mesozoic suggest that thousands of kilometers (5,000–7,000) should have been involved. Large distances of relative plate motion also imply that enormous volumes of oceanic pelagic debris were swept into the Mesozoic trenches fringing the Pacific. If these deposits accumulated in the trenches as tectonic shavings, then approximately 30 percent of the rocks of the folded Circum-Pacific eugeosyncline should be pelagic offscrapings. Although the terrigenous masses of these belts of rocks are present in geosynclinal proportions, field mapping reveals that the volume of pelagic deposits in them is very low (e.g., less than 1 percent in the Franciscan Formation of California). If many thousands of kilometers of underthrusting took place, then the disproportionately small amount of pelagic deposits can mean that selective subduction of pelagic deposits took place. Selective subduction of pelagic deposits is consistent with a model allowing for subduction of the bulk of the deposits reaching the trench (whether pelagic or not) and for entrapment of the greater part of the terrigenous debris shed to the offshore areas in marginal basins or structural terraces upslope from the trench. Therefore, the bulk of the highly deformed terrigenous masses of the Circum-Pacific belt is not trench deposits. Our observations can mean that little underthrusting (no more than several hundred kilometers) has taken place in either modern or ancient trenches peripheral to the Pacific Basin. However, because geological and geophysical data are convincing that new oceanic crust has been added to the earth since at least early Mesozoic time, our observations can be cited as evidence that the earth is expanding. Alternatively, in view of the strong circumstantial evidence that thousands of kilometers of oceanic crust have been subducted beneath marginal trenches, we can make the following conclusions: (1) for geologically long periods of time (at least several million years), tectonic removal of deposits from trenches may take place only periodically or at an average pace that is slower than the geophysically inferred rate, which is typically 5 cm/year or more; (2) if terrigenous debris has been swept from modern trenches at the inferred rate, then continental erosion during late Cenozoic time has been at least twice as high as other data would indicate; (3) when tectonic removal does occur, subduction rather than off-scraping may be the dominant process; and (4) ancient trench deposits may not be represented by the preserved or nonsubducted terrigenous masses of the circum-Pacific.