Andrew J. Stevenson

Subalkaline andesite from Valu Fa Ridge, a back-arc spreading center in southern Lau Basin: petrogenesis, comparative chemistry, and tectonic implications

Tholeiitic andesite was dredged from two sites on Valu Fa Ridge (VFR), a back-arc spreading center in Lau Basin. Valu Fa Ridge, at least 200 km long, is located 40–50 km west of the active Tofua Volcanic Arc (TVA) axis and lies about 150 km above the subducted oceanic plate. One or more magma chambers, traced discontinuously for about 100 km along the ridge axis, lie 3–4 km beneath the ridge. The mostly aphyric and glassy lavas had high volatile contents, as shown by the abundance and large sizes of vesicles. An extensive fractionation history is inferred from the high SiO2 contents and FeO∗MgO ratios. Chemical data show that the VFR lavas have both volcanic arc and back-arc basin affinities. The volcanic arc characteristics are: (1) relatively high abundances of most alkali and alkaline earth elements; (2) low abundances of high field strength elements Nb and Ta; (3) high U/Th ratios; (4) similar radiogenic isotope ratios in VFR and TVA lavas, in particular the enrichment of 87Sr86Sr relative to 206Pb204Pb; (5) high 238U230Th, 230Th232Th, and 226Ra230Th activity ratios; and (6) high ratios of Rb/Cs, Ba/Nb, and Ba/La. Other chemical characteristics suggest that the VFR lavas are related to MORB-type back-arc basin lavas. For example, VFR lavas have (1) lower 87Sr86Sr ratios and higher 143Nd144Nd ratios than most lavas from the TVA, except samples from Ata Island, and are similar to many Lau Basin lavas; (2) lower Sr/REE, Rb/Zr, and Ba/Zr ratios than in arc lavas; and (3) higher Ti, Fe, and V, and higher Ti/V ratios than arc lavas generally and TVA lavas specifically. Most characteristics of VFR lavas can be explained by mixing depleted mantle with either small amounts of sediment and fluids from the subducting slab and/or an older fragment of volcanic arc lithosphere. The eruption of subalkaline andesite with some arc affinities along a back-arc spreading ridge is not unique. Collision of the Louisville and Tonga ridges probably activated back-arc extension that ultimately led to the creation and growth of Valu Fa Ridge. Some ophiolitic fragments in circum-Pacific and circum-Tethyan allochthonous terranes, presently interpreted to have originated in volcanic arcs, may instead be fragments of lithosphere that formed during early stages of seafloor spreading in a back-arc basin.

Seafloor geology of the Monterey Bay area continental shelf

Abstract Acoustic swath-mapping of the greater Monterey Bay area continental shelf from Point Ano Nuevo to Point Sur reveals complex patterns of rock outcrops on the shelf, and coarse-sand bodies that occur in distinct depressions on the inner and mid-shelves. Most of the rock outcrops are erosional cuestas of dipping Tertiary rocks that make up the bedrock of the surrounding lands. A mid-shelf mud belt of Holocene sediment buries the Tertiary rocks in a continuous, 6-km-wide zone on the northern Monterey Bay shelf. Rock exposures occur on the inner shelf, near tectonically uplifting highlands, and on the outer shelf, beyond the reach of the mud depositing on the mid-shelf since the Holocene sea-level rise. The sediment-starved shelf off the Monterey Peninsula and south to Point Sur has a very thin cover of Holocene sediment, and bedrock outcrops occur across the whole shelf, with Salinian granite outcrops surrounding the Monterey Peninsula. Coarse-sand deposits occur both bounded within low-relief rippled scour depressions, and in broad sheets in areas like the Sur Platform where fine sediment sources are limited. The greatest concentrations of coarse-sand deposits occur on the southern Monterey Bay shelf and the Sur shelf.

Terrane accretion, production, and continental growth: A perspective based on the origin and tectonic fate of the Aleutian–Bering Sea region

Orogenesis in the Aleutian–Bering Sea region would create an expansive new area of Pacific-rim mountain belts. The region itself formed about 55 Ma as a consequence of the suturing of a single exotic fragment of oceanic crust—Aleutia—to the Pacifics Alaskan-Siberian margin. A massive overlap assemblage of the igneous crust of the Aleutian Arc and the thick sedimentary masses of the Aleutian Basin have since accumulated above the captured basement terrane of Aleutia. Future closure of the Aleutian–Bering Sea region, either northward toward the continent or southward toward the Aleutian Arc, would structurally mold new continental crust to the North American plate. The resulting “Beringian orogen” would be constructed of a collage of suspect terranes. Although some terranes would include exotic crustal rocks formed as far as 5000 km away, most terranes would be kindred or cotetonic blocks composed of the overlap assemblage and of relatively local (100–1000 km) derivation. The Aleutian–Bering Sea perspective bolsters the common supposition that, although disrupted and smeared by transcurrent faulting, examples of kindred assemblages should exist, and perhaps commonly, in ocean-rim mountain belts.

Evidence for cenozoic crustal extension in the Bering Sea region

Geophysical and regional geologic data provide evidence that parts of the oceanic crust in the abyssal basins of the Bering Sea have been created or altered by crustal extension and back-arc spreading. These processes have occurred during and since early Eocene time when the Aleutian Ridge developed and isolated oceanic crust within parts of the Bering Sea. The crust in the Aleutian Basin, previously noted as presumably Early Cretaceous in age (M1–M13 anomalies), is still uncertain. Some crust may be younger. Vitus arch, a buried 100- to 200-km-wide extensionally deformed zone with linear basement structures and geophysical anomalies, crosses the entire west central Aleutian Basin. We suggest that the arch and the inferred fracture zones in the Aleutian Basin are early Cenozoic structures related to the early entrapment history of the Bering Sea. These structures lie on trend with known early Cenozoic structures near the Bowers-Shirshov-Aleutian ridge junction and on the Beringian continental margin (with possible continuation into Alaska); the structures may have coeval and cogenetic(?) histories for early Cenozoic and possibly younger times. Cenozoic deformation within parts of the Bering Sea region is principally extensional, although the total amount of extension is not known. As examples, the Komandorsky basin formed by back-arc seafloor spreading, the Aleutian Ridge has been extensively sheared, and extensional block faulting is common. Sedimentary basins of the Bering shelf have formed by extension associated with wrench faulting. The Cenozoic deformation throughout the Bering Sea region probably results from the interaction of major lithospheric plates and associated regional strike-slip faults. We present models for the Bering Sea over the past 55 m.y. that show oceanic plate entrapment, back-arc faulting and spreading along Vitus arch, breakup of the oceanic crust in the Aleutian Basin at fracture zones, and back-arc spreading in Bowers Basin.

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

Abstract 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 sectors 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 trenchs 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.

Nearshore morphology and late Quaternary geologic framework of the northern Monterey Bay Marine Sanctuary, California

A combination of side-scanning sonar and high-resolution seismic reflection data image seafloor bedrock exposures and erosional features across the nearshore shelf. Sediment-filled troughs incise the inner shelf rock exposures and tie directly to modern coastal streams. The resulting bedrock geometry can be related to its resistance to erosion. Comparison of the depth of the transgressive erosional surface to recently developed sea level curves suggests a period of slow sea level rise during the early stages of post-interglacial marine transgression. The slow rise of sea level suggests an erosional episode that limited the preservation of buried paleo-channels beyond 70 m water depth. Seafloor features suggest that localized faulting in the area may have influenced the morphology of bedrock exposures and the coastline.

Tectonic and geologic implications of the Zodiac fan, Aleutian Abyssal Plain, northeast Pacific

The Zodiac fan, a large body of upper Eocene through at least lower Oligocene turbidities, underlies the Aleutian Abyssal Plain and lies just south of the Aleutian Trench and the Alaskan Peninsula in the northeastern north Pacific. The fan deposits cover an area of more than 1 × 10 6 km 2 and contain at least 280,000 km 3 of terrigenous sediment. The most striking feature of the fan is its well-developed channel distributary system, which persists nearly to the plain limits. Levee overbank deposits associated with the channels are the dominant sedimentary style, a process believed to predominate on fans receiving primarily fine-grained detritus. Four major channels have been identified on the fan with the following relative age relation (> = older than): Sagittarius > Aquarius > Taurus, and Sagittarius >Seamap. Recently obtained seismic information implies that Seamap Channel may be older than Aquarius Channel. If Seamap is the youngest channel, a depositional interval of 8 m.y. (40−32 m.y. B.P.) is indicated for the fan as a whole. If Taurus is the youngest, as we believe, the interval of deposition is greater than 8 m.y. but is probably less than 16 m.y. (40−24 m.y. B.P.). Nanno-flora, pollen, and spores obtained from Deep Sea Drilling Site 183, near the northern margin of the fan, indicate that the fan formed in a nontropical (northern) environment, and that the source terrane had a climate similar to or slightly cooler than that recorded from coeval onshore deposits in the eastern Gulf of Alaska. On the basis of the interval of deposition, the volume of the fan, and sediment yield from climatically similar modern drainages, a minimum drainage of 500,000 km 2 is believed necessary to have supplied the sediment to form the fan, assuming no sediment was deposited before it reached the fan. Tertiary plate-motion models requiring large amounts of relative convergence along the Aleutian Trench are judged unworkable, as such reconstructions require the Zodiac fan to have formed 1,500 to 3,000 km from the nearest continental landmass and separated from it by topographic barriers, requiring that the drainage be inflated in size manyfold to overcome anticipated sediment losses during such a lengthy transport. As the minimum drainage (500,000 km 2 ) is already equal to one-half of the State of Alaska, any major expansion is judged unreasonable. This requires that relative convergence at the Aleutian Trench be limited to less than ∼500 km from 40 m.y. B.P. to present. A possible method by which this limitation can be satisfied is to allow a significant portion of southern Alaska to move in concert with the Pacific plate since the upper Eocene.

Speculations on the petroleum geology of the accretionary body: an example from the central Aleutians

Abstract In the 300 km wide Adak-Amlia sector of the central Aleutian Trench ≈ 36 000 km 3 of offscraped trench fill makes up the wedge-shaped mass of the Aleutian accretionary body. Within this wedge, seismic reflection profiles reveal an abundance of potential hydrocarbon-trapping structures. These structures include antiforms, thrust and normal faults, and stratigraphic pinchouts. Maximum closure on these features is 2 km. In addition, the silt and possibly sand size sediment within the offscraped turbidite deposits, and the porous diatomaceous pelagic deposits interbedded with and at the base of the wedge, may define suitable reservoirs for the entrapment of hydrocarbons. Potential seals for these reservoirs include diagenetically-altered and -produced siliceous and carbonate sediment. The organic carbon input into the central Aleutian Trench, based on carbon analyses of DSDP Legs 18 and 19 core samples, suggests that the average organic carbon content within the accretionary body is approximately 0.3–0.6%. Heat flow across the Aleutian Terrace indicates that at present the oil generation window lies at a depth of 3–6.5 km. At depths of 8 km (which corresponds to the maximum depth the offscraped sediment has been seismically resolved beneath the lower trench slope), the probable high (170–180°C) temperatures prohibit all but gas generation. The dewatering of trench sediment and subducted oceanic crust should produce an abundance of fluids circulating within the accretionary body. These fluids and gases can conduct hydrocarbons to any of the abundant trapping geometries or be lost from the system through sea floor seepage. In the Aleutian accretionary body all the conditions necessary for the formation of oil and gas deposits exist. The size and ultimate preservation of these deposits, however, are dependent on the deformational history of the prism both during accretion and after the accretion process has been superceded by subsequent tectonic regimes.

Major evolutionary phases of a forearc basin of the Aleutian terrace: Relation to North Pacific tectonic events and the formation of the Aleutian subduction complex

Combined geologic and seismic reflection data from the Atka Basin region of the Aleutian forearc show that the upper 2-3 km of slightly deformed sediment filling the basin are probably of late Miocene to Holocene age. The depositional axis of the basin shifted arcward over time because of the progressive and differential rise of the basins outer ridge. Units filling the basin unconformably overlie and, along the edges of the basin, onlap beds of Oligocene age and older(?). The basal units of the basin fill are characterized by little variation in thickness, somewhat irregular internal reflectors, fault offsets, and possible wedge outs against units of Eocene(?) age. A fault with at least 500 m of vertical displacement cuts the outer high of the forearc basin and displaces beds of the basin-filling series relative to those trenchward of the trenchslope break. The Atka Basin appears to have formed in response to a combination of (1) initiation of trench-floor-filling turbidite deposition, in part derived from glacial marine sedimentation from mainland Alaska; (2) an increased rate and normal component in Pacific plate subduction beneath the central Aleutian arc beginning in early Pliocene time; and (3) formation of a broad and thick accretionary wedge that progressively uplifted the outer high of the Aleutian terrace.

Late Cretaceous pelagic sediments, volcanic ash and biotas from near the Louisville Hotspot, Pacific Plate, paleolatitude ∼42°S

Abstract Dredging on the deep inner slope of the Tonga Trench, immediately north of the intersection between the Louisville Ridge hotspot chain and the trench, recovered some Late Cretaceous (Maestrichtian) slightly tuffaceous pelagic sediments. They are inferred to have been scraped off a recently subducted Late Cretaceous guyot of the Louisville chain. In the vicinity of the Louisville hotspot (present location 50°26′S, 139°09′W; Late Cretaceous location ∼42°S, longitude unknown) Late Cretaceous rich diatom, radiolarian, silicoflagellate, foraminiferal and coccolith biotas, accumulated on the flanks of the guyot and are described in this paper. Rich sponge faunas are not described. ?Inoceramus prisms are present. Volcanic ash is of within-plate alkalic character. Isotope ratios in bulk carbonate δ18O − 2.63 to + 0.85, δ13C + 2.98 to 3.83) are normal for Pacific Maestrichtian sediments. The local CCD may have been shallower than the regional CCD, because of high organic productivity. In some samples Late Cretaceous materials have been mixed with Neogene materials. Mixing may have taken place on the flanks of the guyot during transit across the western Pacific, or on the trench slope during or after subduction and offscraping about 0.5 Ma.