Deep tow studies of the Tamayo transform fault

The Tamayo transform fault occurs at the north end of the East Pacific Rise where it enters the Gulf of California. The two deep-tow surveys reported here show that the transform fault zone changes significantly as a function of distance from the spreading center intersections. At site 1, near the intersection, one side of the fault is young and the fault zone is narrow and well-defined. Strike slip occurs in a zone approximately 1-km wide suggesting a correspondingly narrow zone of decoupling between the Pacific and North American plates. On the young side of the strike-slip zone, normal faults occur along shear zones which are 45°–50° oblique to the transform strike. They occur parallel to the short axis of the strain ellipse for transform fault strain here, i.e., perpendicular to the least compressive stress. The transform walls are formed by normal faulting as has been pointed out in previous detailed surveys. Here, however, the age contrast of 2.5 m.y. across the transform valley is apparent in the morphology of the normal fault scarps. While the scarps are steep and well-defined on the young side, the scarps on the older side have gradual 10°–30° slopes and appear to be primarily talus ramps. Apparently, the scarps have been tectonically eroded by continued strike slip activity after the initial stages of normal faulting. Thus, transform valleys should be quite asymmetric in cross-section where there is a significant age contrast and one side is less than approximately 0.5 m.y. old. Also, along older sections of the transform valley walls, normal faulting may not be at all obvious due to degradation of the scarps by tectonic erosion. This phenomenon makes the likelihood of transform faults providing windows into the oceanic crust most unlikely except in special cases.The picture of transform deformation is more complex at site 2 in the central portion of the fault where both sides of the fault are greater than 1 m.y. old. Here the transform valley is wider (25–30 km as opposed to 2–5 km). There is no clear simple zone of strike slip tectonics. In fact, the only clear evidence for deformation is the intrusion of magmatic or serpentinite diapirs through the sediments of the transform valley floor. The diapirs have deformed the turbidite layers flooring the valley and in one carefully studied case the turbidite sequence has been uplifted, perched atop the diapir. The pattern of deformation on this outcropping diapir shows radial and concentric fractures which can be modeled by a vertical intrusion circular in plan view. Magnetic studies limit the possible composition to basalt or serpentinite. A 60-km-long median ridge is also likely to be the product of intrusion along the transform fault. The survey at site 2 pointed out the importance of vertical tectonics in the transform valley floor and in particular the importance of diapiric intrusions of either basaltic or serpentinite composition.Based on initial boundary conditions and present tectonic elements in the Tamayo fault zone, a possible history of the mouth of the Gulf of California is outlined. The median ridge was emplaced starting approximately 0.8 m.y. ago by regional extension across the transform fault, the result of leaky transform faulting. The diapirs occur along a possible relay zone of extension midway along the fault which began approximately 0.15 m.y. ago. The extension in this case is parallel to the trend of the transform fault, is still occurring at present, and may evolve into a true spreading center.Contribution of the Scripps Institution of Oceanography, new series.     

Mesopelagic fishes of the Bering Sea and adjacent northern North Pacific Ocean

Fourteen midwater trawl collections to depths of 450 m to 1,400 m were taken at eleven stations in the Bering Sea and adjoining regions of the northern North Pacific by the R/V Hakuho Maru during the summer of 1975. A total of 29 kinds of fishes were identified. Mesopelagic fishes of the families Myctophidae, Gonostomatidae and Bathylagidae predominated in the catches, contributing 14 species (94%) of the fishes caught.Seventeen species of fishes were caught in the Bering Sea, and all of these are known from nearby areas. The mesopelagic fish fauna of the Bering Sea is similar to that in adjoining regions of the northern North Pacific Ocean: endemic species are rare or absent. Stenobrachius nannochir was usually the most common mesopelagic fish in our catches.Stenobrachius leucopsarus is a diel vertical migrant that is usually the dominant mesopelagic fish in modified Subarctic waters of the northeastern Pacific. The change in dominance fromS. nannochir in the western Bering Sea toS. leucopsarus in the eastern Bering Sea is related to differences in oceanographic conditions.     

Seasonal variation of zinc in the barnacle, Balanus balanoides (L.) maintained on a raft in the Menai Strait

A population of the intertidal barnacle, Balanus balanoides (L.) maintained on a raft in the Menai Strait was sampled regularly over a two-year period and pooled fractions analysed for zinc and other trace metals. The seasonal variation of zinc in three fractions (body, other soft tissues and shell) has been followed and related to physiological condition. The body accumulates zinc almost continuously whilst this trace metal accumulates in the ovary only as it develops and is lost in the gametes when these are laid. The newly formed egg-masses therefore may be the more useful sample for indicating zinc pollution.     

Spreading of Antarctic Bottom Water and its effects on the floor of the Indian Ocean inferred from bottom-water potential temperature,turbidity, and sea-floor photography

Data on bottom-water potential temperature, turbidity and current indications show that in the Southern Ocean west of the Kerguelen Plateau, Antarctic Bottom Water (AABW) of Weddell Sea origin spreads northwards from the Atlantic—Indian Basin in two directions: (1) AABW enters the Agulhas Basin through relatively deep areas in the Mid-Indian Ridge at 20–25°E and possibly at 35°E, and flows northwards into the Mozambique Basin as far as its northern limits; (2) a more easterly spreading path extends from the Atlantic—Indian Basin through the Crozet into the Madagascar, Mascarene, Somali and Arabian Basins. The passage in the western branch of the Indian Ridge for the AABW spreading from the Crozet into the Madagascar Basin appears to be at 29-26°S and 60–64°E.East of the Kerguelen Plateau in the South Indian Basin, the bottom water formed mainly along the Adélie Coast and Ross Sea travels west towards the Kerguelen Plateau and then parallel to it. This water finally flows eastwards hugging the Southeast Indian Ridge. Significant deviations from this general circulation pattern occur due to local topographic effects. Some AABW in the South Indian Basin exits through a passage at 120–125°E in the region of the Australian—Antarctic discordance in the Southeast Indian Ridge and enters the South Australian Basin and subsequently the Wharton Basin. This passage is clearly indicated by the northward extension of a cold, bottom-water tongue as shown by the temperature distribution in the region; the bottom-water effects in the passage are reflected in the high turbidity and current lineations on the sea floor.In the Southern Ocean basins, bottom-water turbidity is generally high, reflecting in part the strong bottom-water activity. The effects of AABW circulation on the sea floor—in the form of well-developed small- or large-scale current ripples and erosional/depositional features, manganese-nodule formations, and unconformities and reworking of sediments observed in cores — are also marked in these basins. Even though the AABW in the Wharton Basin is cold, its spreading effects on the sea floor are minimal in this basin in contrast to the basins west of the Mid-Indian Ridge at comparable latitudes.     

Mikroskopische und mikroanalytische Untersuchungen an metallischen Kügelchen aus Tiefseesedimenten des Zentralen Pazifik

Metallic spherules from Central Pacific Ocean sediments (“Valdivia” expedition VA 05-1, 1973) were studied by means of the ore microscope and by electron microprobe. The data suggest that most of the spherules are of meteoritic origin, i.e. ablation drops from meteorites as suggested in former studies on spherules from marine environment by Schmidt and Keil (1966), Finkelmann (1970, 1972) and others. The metallic spherules contain up to about 60% Ni, 37% Fe and 3% Co. Unusually high contents of Cr (up to 12%) were noted in some spherules.     

Deformations and Manifestations of Degassing in the Sedimentary Cover of the Equatorial Segment of the West Atlantic: Implications for Lithospheric Geodynamics

Manifestations of fluids and deformations in the sedimentary cover, which are both factors of brightening (blanking anomalies) in seismoacoustic records, in the equatorial segment of the Atlantic coincide with the sublatitudinal zones of the activated passive parts of transform faults and with zones of lower gravity anomalies and higher values of remnant magnetization, which form as a result of serpentinization. The cause-and-effect sequence of intraplate phenomena includes: the contrasting geodynamic state → horizontal movements that form macrofractures → water supply to the upper mantle → serpentinization of rocks in the upper mantle → deformations associated with vertical uplift of basement and sedimentary cover blocks, coupled with fluid generation → and fluid accumulation in the sedimentary cover, accompanied by the formation of anomalies in seismoacoustic records. Based on the seismic data, we have identified imbricate-thrust deformations, diapir structures, stamp folds, and positive and negative flower structures, indicating the presence of strike-slip faults in the passive parts of transform faults. The general spatial distribution of deformation structures shows their concentration in cold mantle zones. Correlative comparison of the structural characteristics of deformations shows the direct relationship between the heights of structures and the development of serpentinization processes. As per the age of the basement, deformations range from 27–38 to 43–53 Ma; a quite thick sedimentary cover makes it possible to reveal them based on the characteristic types of seismoacoustic records. The formation of the Antilles arc ca. 10 Ma ago affected the equatorial segment of the Atlantic; it formed kink bands where lithospheric blocks underwent displacements with counterclockwise rotations, deformations related to compression and vertical uplift of crustal fragments, and local extension that favored degassing of endogenous fluids. Sublatitudinally oriented imbricate-thrust deformations with different vergences indicate irregularity and alternating strike-slip directions as blocks between fractures were laterally influenced.     

Nonlinear dynamics of gravity flows in sloping channels

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