Charlie-Gibbs Fracture Zone Research Articles

Sedimentary basins of the atlantic margin of North America

Scismic exploration has identified eight distinct basin structures along the North American Atlantic continental margin forming a chain of elongate depocenters parallel to the continental slope and interrupted by transverse basement arches and impinging oceanic fracture zones. From south to north these are: South Florida—Bahamas Basin bounded on the north by Peninsular Arch and Bahama Escarpment fracture zone; Blake Plateau Basin with Cape Fear Arch and the impinging Great Abaco and Blake Spur fracture zones; Baltimore Canyon Trough bounded by the Long Island Platform and impinging Kelvin fracture zone; Georges Bank Basin with the bounding Yarmouth Arch; Scotian Shelf Basin with Scartarie and Canso Ridges and impinging Newfoundland Ridge fracture zone; Grand Banks Troughs and the intervening horst ridges; and the East Newfoundland Basin separated by Cartwright Arch and the impinging Gibbs fracture zone from the Labrador Shelf Basin. All the basins are characterized by great depths to basement filled with from 7 to 14 km of possible Triassic, Jurassic, Cretaceous and Tertiary sediments. Basement faulting controls the basins' boundaries and the faults have affected the overlying sediments. The major boundary faults of the basins undoubtedly formed during the initial rifting of the Atlantic margin in the Jurassic or perhaps Triassic. However, throughout the Mesozoic and Cenozoic these basement faults have moved in response to different orientations of stress and strain rates produced by continued spreading of the Atlantic Ocean. As a result, the basement faults of the Atlantic Margin were apparently influenced by at least three different local stress systems, spatially overlapping but temporally independent. These are the east—west extensional Atlantic Ocean stress system, the northwest—southeast extensional White Mountain stress system, and the north-south extensional Labrador Sea stress system. Some consequences of this basic tectonic setting were differential cross-strike tilts of the basin blocks with each basin moving somewhat independent of its neighbor. The resulting buildup of the basins' sedimentary geometries reflect these tectonic tilts and varying strain rates. Correlations are found between changes in orientation and rates of Atlantic sea-floor spreading with observed major sedimentary events such as progradations, planar bedding episodes, reef platform development, regressive hiatuses, and transgressions. An understanding of this marginal geosyncline could yield a model with predictability.

Mode of the strain release along the Gibbs fracture zone, Mid-Atlantic ridge

The Gibbs fracture zone (52°N, 35°W) is one of the major transform faults in the Atlantic. A large earthquake ( m b = 5.8, M s = 6.9) occurred on October 16, 1974 on this fracture zone. This earthquake was preceded by similar large earthquakes along the fracture zone in 1967 ( m b = 5.5, M s = 6.5), 1954 ( M s = 6.5), 1941 ( M s = 6.25) a 1923 ( M s = 6.5). Detailed analyses of seismic body waves and surface waves over the period range from several seconds to 200 sec are made for the 1967 and 1974 events. The major conclusions are: 1. (1)|In addition to the obvious disparity in the m b vs. M s relation, the excitation of long-period waves by these earthquakes is also anomalously large. The seismic moments determined by using the long-period (40–200 sec) G 1 and R 1 waves are 3.4 · 10 26 and 4.5 · 10 26 dyne · cm for the 1967 and 1974 events, respectively. 2. (2)|Wave form analyses of body waves suggest a fault length of 60 km for the 1967 event and 70 km for the 1974 event. The point dislocation particle velocity of about 20 cm/sec obtained for both events is about an order of magnitude smaller than for other earthquakes of comparable magnitude. The large fault length and the slow particle velocity explain the disparity between m b and M s , and M s and seismic moment. 3. (3)|The amount of dislocation (about 170 cm) together with the fault length and the repeat time (13 years on the average) suggest a seismic slip rate of about 2.6 cm/year along the Gibbs fracture zone — a rate which is comparable to that for sea-floor spreading in the northern Atlantic. This agreement suggests that the slip along the fracture zone is primarily seismic at least to a depth of 10 km, the plates moving in jerks once every 13 years on the average, rather than in continuous creep. 4. (4)|Seismic activity along the ridge crest was very high during the 5-year period before the 1974 event, while activity along the fracture zone was very high during the 3-year period prior to the 1967 event. The seismic activity became very low after the 1967 event. This pattern may be interpreted in terms of an intermittent upwelling at the ridge which subsequently triggered, either directly or indirectly, the fracture zone events. The sudden release of stress in these events may lead to an initiation of the next episode of upwelling and eventually renewed seismic activity along the ridge—fracture zone system.

The distribution of 3He in the western Atlantic ocean

Over 250 samples of Atlantic seawater have been analyzed for the dissolved helium isotopes, and distinctive pattern has emerged. Two components of excess 3He are seen: a component due to in situ decay of nuclear-era tritium, and a primordial component evolved from the solid Earth. A prominent feature at about 3-km depth can be traced from 5°N along the western boundary to the equator. The source of this feature is most probably in the Gibbs fracture zone, where we suppose that primordial 3He is released into westward-flowing bottom water. The South Atlantic profiles clearly show the effect of 3He-rich CCircumpolar Water, entrained by Antarctic Intermediate Water flowing northward. The excess 3He in the upper 1 km, when combined with tritum concentrations measured byO¨stlund, Dorsey and Rooth (1974, Earth and Planetary Science Letters, 23, 69–86) at the same locations and depths, yields ‘tritium-helium ages’, which in some cases represent the time interval between equilibration wwith the atmosphere and sampling.

A geological framework for the continental margin to the west of Ireland

AbstractOn the basis of newly released magnetic data, a major fault feature is traced southwest across the Irish continental shelf; along the northwestern margin of the Slyne‐Erris Trough, the northern termination of the Porcupine Seabight, and across the Porcupine Ridge toward the vicinity of the eastern termination of the Gibbs Fracture Zone. This major fault passes into, and is identified with, the Great Glen Fault system. Apparent sinistral movement of the order of 50–70 km along the fault displaces a feature tentatively identified as the Highland Boundary Fault. A weak magnetic feature is also identified that might represent the course of the Moine Thrust. The Slyne‐Erris Trough has been referred to as part of a ‘failed’ rift associated with the Seabight opening, but might instead be related to the wrench faulting similar to that seen elsewhere along the Great Glen Fault line. An apparent transform movement along the portion of the fault at the head of the Porcupine Seabight may relate to rifting and the generation of oceanic crust in the central part of the Seabight.Two possible courses for the westward continuation of the Hercynian Front are recognized, and a major Tertiary intrusive centre is identified and related to a zone of dyke swarms.

The Gibbs Fracture zone (reply)

The Gibbs Fracture zone (reply)

Charlie-Gibbs Fracture Zone

The Charlie-Gibbs fracture zone has been surveyed along its entire western section between the rift valley at 35°W and the Canadian continental margin at 49°W (representing around 2200 km). These data are combined with all other available data to describe and discuss the structure of the fracture zone from 27.5° to 49°W. Emphasis is put on two changes of orientation, a major one around 60 m.y. B.P. and a second one around 47 m.y. B.P. In spite of these changes the continuity of the fracture zone from the eastern margin of Labrador, where it is clearly related to a left lateral offset of the margin, to Rockall trough, west of Porcupine bank, seems well established. Last, an interpretation is proposed for the active part of the fault.

Is the Gibbs Fracture Zone a Westward Projection of the Hercynian Front into North America?

Recent geophysical measurements across the Gibbs Fracture Zone eastwards from 30° W clearly show that the fracture zone's eastern limits may be extended to east of 17° W. Alignment with the Hercynian Front in Europe and its extension in North America is offered as a possible clue to the age and origin of the fracture zone.

Rocks from the Gibbs fracture zone and the Minia seamount near 53°N in the Atlantic Ocean

Rocks dredged and drilled from both the rift mountains of the Mid-Atlantic Ridge (Minia seamount) and from the northern wall and the median ridge of the adjacent Gibbs fracture zone near 53°N include tholeiites, serpentinized and mylonitized peridoties, and gabbroic rock. The tholeiites include: (1) pyroxene-tholeiites, commonly without phenocrysts and containing less than 15% Al 2O 3; (2) plagioclase-tholeiites with small (1.1–2.2 mm length) plagioclase phenocrysts and an Al 2O 3 content varying between 14–17%; and (3) high-alumina plagioclase tholeiites with large (> 2.5 mm length) plagioclase phenocrysts and more than 17% Al 2O 3. The apparently transitional differences among the three groups support the possibility that differentiation by crystal fractionation of the high-alumina plagioclase-tholeiites gave rise to the plagioclase-tholeiites with less Al 2O 3 and smaller phenocrysts and to the pyroxene-tholeiites. A small portion of the basalts ampled show effects of low-grade metamorphism. The peridotites may represent evidence of intrusive emplacement of peridotitic material beneath the tholeiitic rocks.

The Atlantic's Grand Opening

Three years ago Sir Edward Bullard of the University of Cambridge wrote that scientists had a sketch of the outlines of a history of the oceans; what remained was a mass of detail. There is still much to be learned about the formation of the oceans, but new evidence on crustal ages, location of ocean floor fractures, and magnetic anomaly patterns has been accumulating rapidly and is being interpreted in terms of the theory of plate tectonics In the June GEOLOGICAL SOCIETY OF AMERICA BULLETIN Joseph D. Phillips and Donald Forsyth of Woods Hole Oceanographic Institution propose a detailed history of the Atlantic Ocean. Bullard and his colleagues have described the motions of the continents in terms of rotations about a pole. The Woods Hole geophysicists combined data on the age of the ocean floor at different locations with finite rotations of the continents around Bullard's poles. The classic predrift arrangement of the continents proposed by Bullard, in which the east coast of South America fits against the west coast of Africa, and Greenland fills the gap between Europe and North America, has not been successfully challenged. The birth of the Atlantic began 200 million years ago, with rifting in two places. Northwestern Africa separated from North America, and the Rockall Trough, a narrow basin between Britain and Iceland, began to open. Volcanic rocks between 190 million and 202 million years old in the eastern United States are believed to be associated with the initial rifting. For the first 70 million years, sea-floor spreading in the central Atlantic proceeded at a rate of about 1.7 centimeters per year. Between 130 million and 65 million years ago, the spreading rate in the central Atlantic decreased to about 1.4 centimeters per year, and has slowed down even more since then, to about 1.2 centimeters per year. Because the Rockall Trough was not complete until 130 million years ago, the spreading rate there must have been extremely slow, even by continental drift standards: about 0.1 centimeter per year. Next the Labrador Sea between Greenland and North America began opening, probably about 80 million years ago. Then things really got complicated. Before the Labrador Sea had opened completely, spreading began in two other places about 65 million years ago: the Reykjanes Ridge between Iceland and the Gibbs Fracture Zone and along the Mid-Atlantic Ridge north of the Azores. Spreading in the Labrador Sea was completed by 45 million years ago, but for 20 million years spreading was occurring in three different locations in the north Atlantic. The opening of the South Atlantic was much simpler, and consisted of rifting between South America and Africa beginning sometime after 150 million years ago. Spreading along this southern section of the Mid-Atlantic Ridge is known to have been steady at 2.0 centimeters per year for at least the last 70 million years. A 125-millionyear-old volcanic formation in South America may be associated with the initial rifting, the researchers believe. If so, a constant rate of 2.0 centimeters per year would just about account for the present width of the ocean. Over the course of the 200-millionyear period, the entire Atlantic plate system has gradually shifted northward. The interactions between the continents bounding the Atlantic allow some inferences about the histories of adjoining smaller bodies of water. Because the north and south Atlantic opened at different times along the same ridge, there must have been a fault or spreading ridge between North and South America to take up the differential motion. The researchers estimate that the Caribbean Sea would have opened as a consequence of separation between the Americas. The ancient Tethys Sea between Africa and Europe was closed by the convergence of these two continents; what remains is the Mediterranean. When molten rock solidifies it absorbs the prevailing magnetism of the earth's field. The magnetism preserved in rocks formed at the same time should therefore point toward the same spot-the location of the earth's magnetic pole at the time of solidificationunless the rocks have subsequently been moved. Phillips and Forsyth checked their proposed model by calculating the continental positions and movements that would account for observed magnetism of rocks from the different continents. They report the proposed reconstruction agrees quite well with paleomagnetic data, with a few exceptions. For example, Carboniferous (345 million to 280 million years ago) data for South America and Permian (280 million to 230 million years ago) data for Africa do not fit the proposed model. To try to resolve these discrepancies, they tested a slightly different model, in which Greenland was rotated more than in the other. The alternate model, in general, fits the magnetic data better, they conclude, but the improvement is not great enough to justify rejecting the original model. 0 200 million 130 million years ago ~~~~~~~~~years ago

Flow through the Charlie-Gibbs Fracture Zone, Mid-Atlantic Ridge

The flow of deep water across the axis of the Mid-Atlantic Ridge through the Charlie–Gibbs Fracture Zone near 52.5 °N was monitored for 7 days during June, 1970 with current meters moored 50 m and 650 m above the bottom in each of the two zonal channels of the fracture. Flow was west-going through the fracture. The mean rate of flow was about 3 cm/s with an oscillatory flow superimposed of semi-diurnal tidal period.

Uranium content of the oceanic upper mantle

Fission track determinations of both the whole rock contents and the distribution of uranium in individual phases were made on twenty serpentinized ultramafic rocks from the Mid-Atlantic Ridge at 45°N (Hudson Geotraverse) and 52°N (Gibbs Fracture Zone). The rocks are thought to represent uppermost oceanic upper mantle material. Whole rock uranium concentrations, varying from 0.19 to 0.70 ppm, reflect more the subsequent histories of metasomatism (serpentinization, amphibolization and rodingitization) than concentrations of the original fresh rocks. Relic minerals reveal that, unlike continental mantle equivalents, most of the uranium is homogeneously distributed in primary orthopyroxenes (1 ppm), and to a lesser extent (0.2 ppm) in primary clinopyroxenes. Primary olivine is relatively depleted in uranium (0.03 ppm), as is primary chrome spinel (0.09 ppm). Extrapolation to pre-metasomatic conditions suggests that at the time of crystallization these ultramafic rocks had concentrations of at least 0.1 to 0.5 ppm uranium, up to an order of magnitude greater than expected. These concentrations suggest that the ultramafic rocks are unlikely to be directly genetically related to the overlying basalts and gabbros containing 0.25 ppm uranium, but are probably primary ultramafic material from which there has been no previous episode of basalt extraction. These uranium concentrations suggest that the oceanic upper mantle (plate) has quite high radioactive heat production in contrast to low heat production in the continental upper mantle. The equality of oceanic and continental heat flows is explained by the data, since the total heat produced in an oceanic plate is estimated to be about equal to that of the continental crust. One can construct a model that has an isothermal, low velocity (partial melt) layer at a shallower depth under the oceans than under the continents and that has the same heat flux from below the oceanic and shield plates. Lateral convective heat transfer in the low velocity layer is not required. High radioactive heat production of the oceanic plate can explain the high heat flows measured behind trenches with downgoing slabs.

Reply [to “Discussion of paper by X. Le Pichon, S. Eittreim, and J. Ewing, ‘A sedimentary channel along Gibbs Fracture Zone’”

Reply [to “Discussion of paper by X. Le Pichon, S. Eittreim, and J. Ewing, ‘A sedimentary channel along Gibbs Fracture Zone’”

Discussion [of paper by X. Le Pichon, S. Eittreim, and J. Ewing, “A sedimentary channel along Gibbs Fracture Zone”

Cooper [1955] first suggested that Norwegian Sea overflow water from the Iceland–Scotland ridge region passed through a gap in the mid-Atlantic ridge from the European basin into the Labrador basin of the North Atlantic. He was able to distinguish it from other deep water masses by its relatively high salinity. His suggestion was confirmed by many others on the basis of data collected during the International Geophysical Year, and subsequently Worthington and Volkmann [1965] estimated the westerly volume transport through the gap to be 4.6×106 m3/sec on the basis of dynamic computations and direct current measurements.

A sedimentary channel along Gibbs Fracture Zone

The depositional asymmetry associated with a channel along the fracture zone at 52°N in the Atlantic is explained by an eastward-flowing bottom current. This current, below 3000 m, is opposite to the known westward flow of the overlying Scotland-Iceland overflow water. This eastward flow is significant in distribution of bottom sediments and may explain the temperature and salinity similarity of the European basin bottom water to North Atlantic deep water of the western basin.

The Gibbs Fracture Zone: A double fracture zone at 52�30?N in the Atlantic Ocean

A bathymetric survey of the offset in the Mid-Atlantic Ridge Crest at approximately 53°N revealed an east-west offset of 190 nautical miles and north-south offset of 75 nautical miles. The offset is filled with two valleys separated by a sill below 1900 fm. The valley strend approximately 95° east of north and are inconsistent with spreading poles calculated for the north Atlantic. Their trends have been used by earlier authors to calculate poles of rotation. It is proposed to name the offset The Gibbs Fracture Zone after the ship that made the survey.

Systematic and biological studies of the macrourid fishes (Anacanthini-Teleostii) : Marshall N.B., 1965. Deep-Sea Res., 12 (3): 299–322

Direct time-series observations of deep-sea biological activity are largely restricted to passive observations of benthic epifauna using unbaited time-lapse camera systems. However, highly mobile fauna such as scavenging fish are not generally observed at unbaited instruments. Here, we describe the successful adaptation of existing baited camera technology to incorporate an autonomous periodic bait-release system. This technology enables long-term high-frequency time-series observations of deep-sea scavenging demersal fish and crustaceans to be made for the first time. The periodic bait-release system was deployed to 3664 m in the Charlie–Gibbs Fracture Zone, Mid-Atlantic Ridge, for 38 days and incorporated six individual bait-release events. The arrival/departure pattern of Munidopsis spp. and macrourids at the camera was indicative of successive responses to individual small baits. A mean macrourid population density estimate of 8 fish km−2 was calculated from first-arrival times at successive releases. The arrival pattern and lingering behaviour of zoarcids were comparable to observations at more persistent large food falls. Seventy-four percent of observed zoarcids were <100 mm in total length, and it is suggested that the location of the deployment in the Charlie–Gibbs Fracture Zone may be of importance as a zoarcid breeding site or nursery ground. Small, possibly juvenile, Coryphaenoides armatus were also observed infrequently. The periodic bait-release design has potential for further development.