Michael S. Marlow

Peru-Chile Trench Sediments and Sea-Floor Spreading

The hypotheses of sea-floor spreading and plate tectonics require the removal of sediment from oceanic trenches either by crustal underthrusting or by folding against the base of a continental or insular margin. Accordingly, over a period of time the volume of sediment removed by way of spreading must be equal to the difference between the observable volume of undeformed terrigenous deposits in a trench and the volume contributed to it by continental erosion. To assess possible sediment loss from the central Chilean segment (23°–44° S.) of the Peru-Chile Trench, we have compared the volume of terrigenous deposits overlying the land, the continental margin, and filling the trench with that expected from continental denudation. Our data indicate that an episode of sediment removal occurred at the base of the margin and adjacent deep-sea floor in Late Cretaceous and perhaps earlymost Tertiary time and may imply spreading. Nearly 100 × 10 3 km 3 of deposits of Tertiary age, chiefly Eocene to Pliocene, have accumulated on the margin, and perhaps an additional 5 × 10 3 km 3 in the trench. This amount of offshore sediment could be supplied by fairly low rates (3 cm/10 3 yrs) of Tertiary erosion. However, many uncertainties in our denudation-sedimentation budget make it impossible to determine whether or not sediment reaching the base of the margin was removed tectonically in Tertiary time. Between 27° and 44° S., the trench contains nearly 70 × 10 s km 3 of turbidite deposits that we believe accumulated during late Cenozoic periods of glacially lowered sea level. The volume of turbidites in the trench is virtually equal to that expected from continental erosion, which is estimated to have probably been no greater than 5 cm/10 3 yr for the arid region between 27° and 31°, and 50 cm/10 3 yr for the humid and partially glaciated region from 36° to 42°. During this time of rapid erosion and trench filling, magnetic data indicate that convergence of lithospheric plates was taking place below the trench at a rate between 5 and 10 cm/yr. If turbidite deposits were swept from the trench at these rates, then continental denudation must have been exceedingly rapid: 20–40 cm/10 3 yr for the arid zone, and 110–165 cm/10 3 yr for the partially glaciated region. If more conventional estimates of erosion are valid, then either (1) late Cenozoic underthrusting has not taken place (or at a rate much slower than that implied by geophysical data), or (2) underthrusting at the prescribed rates has not involved the removal of a significant volume of sediment from the trench.

Offscraping and underthrusting of sediment at the deformation front of the Barbados Ridge: Deep Sea Drilling Project Leg 78A

On Leg 78A we drilled Sites 541 and 542 into the seaward edge of the Barbados Ridge complex, and Site 543 into the adjacent oceanic crust. The calcareous ooze, marls, and muds at Sites 541 and 542 are lithologically and paleontologically similar to the upper strata at Site 543 and are apparently offscraped from the down-going plate. A repetition of Miocene over Pliocene sediments at Site 541 documents major thrust or reverse faulting during offscraping. The hemipelagic to pelagic deposits offscraped in the Leg 78A area include no terrigenous sand beds, but they contain numerous Neogene ash layers derived from the Lesser Antilles Arc. Hence, this sequence is quite unlike the siliciclastic-dominated terranes on land that are inferred to be accretionary complexes. The structural fabric of the offscraped deposits at Sites 541 and 542 is disharmonic, probably along a decollement, with an underlying acoustically layered sequence, suggesting selective underthrusting of the latter. The acoustically layered sequence correlates seismically with pelagic strata cored at Site 543 on the incoming oceanic plate. Cores recovered from the possible decollement surface at both Sites 541 and 542 show scaly foliation and stratal disruption. Approximately lithostatic fluid pressure measured in the possible decollement zone probably facilitates the underthrusting of the pelagic sediments beneath the offscraped deposits. In the incoming section, a transition from smectitic to radiolarian mud with associated increases in density and strength probably controls the structural break between offscraped and underthrust strata. In the Leg 78A area, the underthrust pelagic section can be traced seismically at least 30 km arcward of the deformation front beneath the Barbados Ridge complex.

Plate tectonic model for the evolution of the eastern Bering Sea Basin

The eastern Bering Sea Basin, composed of the Aleutian and Bowers Basins, is flanked to the north by Mesozoic foldbelts that probably represent zones of plate subduction in Mesozoic time. Present plate subduction occurs 400 to 1,000 km farther south, at the Aleutian Trench. North-south magnetic lineations that formed at an oceanic spreading ridge, probably in Mesozoic time (117 to 132 m.y. ago), have been identified in the Aleutian Basin. The orientation and age of those anomalies can be explained by reconstructing Kula-Farallon Pacific plate motions during late Mesozoic–early Tertiary time. In Mesozoic time, subduction of the Kula plate occurred north of the Aleutian Trench near the present location of the Bering Sea continental margin. At about 70 m.y. B.P. (Late Cretaceous), the zone of subduction shifted south to the present location of the Aleutian Trench, thereby trapping a fragment of oceanic plate imprinted with north-south magnetic lineations within the eastern Bering Sea Basin. A stable basin framework has prevailed behind the Aleutian arc since early Tertiary time.

Tectonic History of the Central Aleutian Arc

The Aleutian arc is a ridge-trench geomorphic system associated with active volcanism, high seismicity, and a hypothetical transition zone from underthrusting to non-underthrusting approximately midway along its length. Two contrasting acoustic signatures can be recognized on seismic reflection profiles in the area, and these are correlated with informally designated early series and late series rock units recognized on the Aleutian Islands. The two series are separated by an unconformity on the summit and flanks of the Aleutian Ridge. The early series consists of altered and deformed sedimentary and volcanic rocks of Eocene to middle Miocene age (45 to 14 m.y.). The late series includes generally unaltered and undeformed sedimentary and volcanic units that range in age from late Miocene to present (10 to 0 m.y.). Reflection profiles show that as much as several kilometers of the late series underlies the Aleutian Trench, the Aleutian Terrace, and summit basins on the Aleutian Ridge; the series reflects sediment draping and basin infilling on an extensionally fragmenting ridge. Magnetic and refraction data and preferred gravity models demonstrate that the southern Aleutian Terrace and inner (northern) Aleutian Trench wall are underlain by low-velocity, low-density rock. A study of the sedimentary fill and inner wall of the trench revealed no structural difference between the postulated underthrust and non-underthrust segments, and it cannot be concluded on this basis that underthrusting has occurred. However, the gravity data (and magnetic and refraction data) do allow the inference that sedimentary offscrapings could form the inner trench wall and underlie the Aleutian Terrace along the postulated zone of underthrusting. Recent JOIDES data from the North Pacific suggest a limited amount (less than 1,000 km) of underthrusting since the Eocene; this interpretation is most compatible with the model of discontinuous plate motion for the North Pacific deduced earlier by Pitman and Hayes. The tectonic, magmatic, and depositional histories of the arc also agree with their model and suggest an episode of active underthrusting in the early Tertiary, a slowing or cessation throughout the middle Tertiary, and a resumption of underthrusting in the late Cenozoic. The early Tertiary underthrusting is indicated by the rapid growth of the ridge during this time. Cessation or slowing in the middle Tertiary is apparently marked by uplift and folding, widespread epizonal plutonism (but no intense volcanic activity), and formation of the unconformity separating the rocks of the early and late series. Resumption of underthrusting in the late Cenozoic is correlated with the outbreak of the present episode of volcanism and fragmentation of the ridge crest to form numerous summit basins.

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.

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).

Structure and Evolution of Bering Sea Shelf South of St. Lawrence Island

The virtually featureless Beringian shelf south of St. Lawrence Island is underlain structurally by at least 14 basins. Encompassing a total area of more than 300,000 sq km, most of the basins are either elongate structural sags, grabens, or half (asymmetric) grabens beneath the outer shelf. The regional trend of these basins is northwest, parallel with that of the continental margin. Two of the basins, St. George and Navarin, contain 7 to 10 km of Upper Cretaceous(?) and Cenozoic sedimentary strata. A major divergence in dip of beds in the upper half of the sedimentary section may reflect an abrupt shelf-wide change in the rate of sedimentation and/or subsidence, probably during the Miocene. The outer sub-shelf basement grabens and adjacent ridges (horsts) are bounded by high-angle normal faults that exhibit growth-type structure. St. Matthew basin, an elongate, southwest-trending feature of the inner shelf, lies along the offshore expression of the Kaltag fault of western Alaska. The Kaltag fault, like the Denali fault in southwestern Alaska, does not extend to the outer Bering Sea shelf but ends or turns parallel with the margin within the inner shelf. The inner shelf is underlain by a broad basement high, Nunivak arch, the seaward half of which is characterized by an arcuate belt of high-frequency and high-amplitude magnetic anomalies. This zone of intense magnetic anomalies along the shelf is probably the signature of a Mesozoic magmatic arc that extends from southwestern Alaska to eastern Siberia and consists of Jurassic to Cretaceous plutonic and volcanic rocks. We speculate that this magmatic arc resulted from oblique convergence and subduction in the Mesozoic between the Kula(?) and North American plates along the eastern Beringian margin. Folding and uplift in the area of the present outer shelf occurred contemporaneously with magmatism along the inner shelf. Plate convergence apparently ceased by the end of the Mesozoic or t e beginning of the Cenozoic. Subsequently, the foldbelt underlying the outer shelf was eroded extensively and rifted extensionally to form large, deep basins. On the average, the shelf has subsided more than 1.5 km. Subsidence and sediment burial of the eroded orogen formed the modern Beringian shelf.

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.

Eocene Age of the Adak ‘Paleozoic (?)’ Rocks, Aleutian Islands, Alaska

In 1948, several specimens identified as the plant genus Annularia, a primitive horsetail of Pennsylvanian or Permian age, were found in tuffaceous sandstone exposed near the northern end of Adak Island, Alaska. These beds form the basal part of the Andrew Lake Formation, a newly named sequence of marine sedimentary rocks that is more than 850 m thick, and, in the main, consists of northwest-dipping tuffaceous sandstone, siltstone, shale, and siliceous siltstone and shale interbedded with basaltic flows or penecontemporaneous(?) sills (or both) a few tens of meters thick. This formation rests depositionally(?) on the Finger Bay Volcanics, the massive and intensely altered andesitic and basaltic flows and pyroclastic rocks that form the bulk of Adak Island. Mollusks, foraminifers, sponge spicules, and fish scales and skeletal remains occur in the lower 350 m of the section immediately overlying the basal “Annularia”-bearing beds. Included in this fauna is the pecten Pro-peamussium (cf. P. stanfordensis Arnold), of probable Eocene age; the associated foraminiferal fauna is provincially considered to be of late Eocene (Narizian) age, and the fish scales are similar to those found in the Narizian and Refugian (Eocene and Oligocene) of California. Examination of the matrix surrounding specimens of “Annularia” revealed a substantial dinoflagellate flora—establishing that the “Annularia”- bearing beds are themselves marine units of middle or late Eocene age. The Andrew Lake Formation probably accumulated in a perched basin along the crestal region of an early Tertiary Aleutian ridge. Accordingly, there is no evidence for a Paleozoic Aleutian ridge. There is only scant evidence that the ridge existed in Mesozoic time.

Mesozoic and Cenozoic Structural Trends Under Southern Bering Sea Shelf

Mesozoic rocks exposed near the tip of the Alaska Peninsula form an antiformal structure that flanks the southern side of Bristol Bay basin and that can be traced with geophysical data about 700 km offshore to the vicinity of the Pribilof Islands. Upper Jurassic sandstone and Upper Cretaceous mudstone dredged from the top and flanks of this structure near the islands confirm that Mesozoic rocks extend from the Alaska Peninsula to the Bering sea margin. The southern part of the Bering Sea Shelf is underlain by several large structural basins: St. George, Amak, and Bristol Bay basins. These filled basins encompass an offshore area of about 31,000 sq km; St. George basin contains more than 10 km of strata. Reflection profiles show that the surface of the offshore antiformal structures is an angular unconformity overlain by Cenozoic beds. This unconformity can be traced toward the axes of the adjacent subshelf basins where, as a disconformity, it parallels underlying and overlying strata. Dredge data suggest that the unconformity and disconformity may be as old as middle to Late Cretaceous. The downdip trace of the unconformity in Bristol Bay basin is underlain by reflectors paralleling the contact, a relation suggesting that the basin nd perhaps other shelf basins may be underlain by ancient Mesozoic depocenters. The bulk of the thick sections in these basins is, however, thought to be mainly Cenozoic in age. Strata in the basins are cut by high-angle growth faults. The faults commonly offset the seafloor, which implies that basin subsidence and filling continue to the present. Shallow-water diatomaceous mudstone of Eocene and Oligocene age dredged from the continental slope near the Pribilof Islands indicates that collapse of the margin and outer shelf basins began by at least early Tertiary time. In Mesozoic time, the Bering margin between Siberia and the Alaska Peninsula (Beringian margin) may have been a zone of either oblique underthrusting or transform motion between the North American and Pacific lithosphere (Kula plate?). This motion may have rifted the edge of the North American plate, resulting in the formation of a series of elongate basins and ridges paralleling the plate edge. These hypothetical basins may have controlled the location and initial subsidence of Bristol Bay, Amak, and St. George basins. Formation of the Aleutian Island arc in late Mesozoic or earliest Tertiary time presumably terminated plate interaction along the Beringian margin. Sediment loading and subsequent subsidence of the remnant plate within the abyssal Bering Sea may have caused continuing ollapse of the Beringian margin in latest Cretaceous and earliest Tertiary time.