Henry S. Fleming

Magnetic and bathymetric evidence for the “Yermak hot spot” northwest of Svalbard in the Arctic Basin

Detailed aeromagnetic data and several new bathymetric profiles collected in the Eurasia Basin off northern Greenland and Svalbard provide evidence for a “Yermak hot spot” which has affected the basement morphology of an ∼200-km-long segment of the Nansen spreading axis since rifting began here about 60 m.y. B.P. Paired aseismic basement ridges (the Morris Jesup Rise and Yermak Plateau) associated with high-amplitude (500 to over 2000 nT, where 1 nT= 1 gamma) complex, sublinear magnetic anomalies were formed at the spreading axis beginning in the lower Tertiary and ending abruptly at anomaly 12–13 time. During maximum hot spot activity (anomaly 13–18 time) the Yermak/Morris Jesup Plateau was apparently a single, Iceland-like volcanic massif emergent above sea level. The flat-topped plateaus subsided to their present ∼1–2 km depth along the standard oceanic crust subsidence curve. From anomaly 12 to 5 time (∼35to10 m.y. B.P.) the Yermak hot spot appears to have been very subdued or dormant in its influence on the ocean crust. Beginning about anomaly 5 time, relatively higher amplitudes began to develop along the same section of spreading axis that had earlier generated the Yermak and Morris Jesup rises. This “Yermak H-zone” has expanded northeastward at about 1 cm/yr, presumably due to the flow of plume-derived mantle material below the spreading axis. The zone is presently over 200 km long (anomaly 1) and the central anomaly locally exceeds 1000 nT in amplitude. At its southwestern end the H-zone appears sharply terminated at the Yermak fracture zone, perhaps because the subaxial flow is dammed there. Although no aseismic ridge is presently being generated by the hot spot, the rift valley floor and rift mountains are relatively shallower (∼4 km and ∼2 km depths) than along the Nansen Ridge to the northeast, where the valley floor locally descends below 5 km depths. Along this “normal” segment of the Nansen Ridge, dramatic along-strike amplitude variations (∼50–500 nT; 50–100 km wavelength) correlate with rift valley depth (3.7–5.3 km), possibly a manifestation of varying basalt productivity associated with the slow spreading (0.5–0.7 cm/yr). The Yermak H-zone has not been sampled, but analogy with similar regions along the Galapagos and Juan de Fuca spreading axes suggest the high amplitudes are caused by exceptionally fractionated, FeTi-enriched basalts of high remanence. Other possible manifestations of the Yermak hot spot include the early Tertiary Kap Washington Group volcanics in northeast Peary Land, Miocene basalts and Paleocene ash horizons on Svalbard, and the generally higher elevation of continental land masses in the area.

Preliminary Model for Extrusion and Rifting at the Axis of the Mid-Atlantic Ridge, 36°48′ North

The inner rift valley of the Mid-Atlantic Ridge at 36°48′ N. is 1.5 to 3 km wide and 100 to 400 m deep. It is symmetrical in profile with a discontinuous medial ridge 100 to 240 m high and 800 to 1,300 m wide along its axis. The medial ridge is replaced every 1 to 3 km with a central trough 200 to 600 m wide. The medial ridge is apparently built by eruptions of pillow basalt recurring at intervals of roughly 14,000 years at a given point. Between eruptions (and possibly during them), the ridge splits and divides along its axis and subsides, which produces the central trough. As the trough widens and deepens, it eventually taps magma in a shallow reservoir, initiating a new eruption that rebuilds the medial ridge. Outward spreading of the inward-dipping shingled halves of the former medial ridge produces a layer of pillowed basalts about 400 m thick (oceanic layer 2A), in which resides the bulk of the remanant magnetization of the ocean floor. This layer overlies a layer of intrusive rock (layer 2B) composed of a dike complex that feeds eruptions building the medial ridge as well as the outward moving, solidified shells of a shallow magma chamber.

The Greenland—Norwegian Sea and Iceland Environment: Geology and Geophysics

The Greenland—Norwegian Sea (Fig. 1) connects the northeast Atlantic and Arctic Oceans. Neither the plate tectonic evolution nor the paleooceanography of the Greenland—Norwegian Sea can be discussed effectively independently of the Eurasia Basin to the north, the northeast Atlantic to the south, or the Labrador Sea and Baffin Bay to the west. In oceanographic or sedimentological terms the northeast Atlantic and Greenland—Norwegian Sea area, hereafter referred to as the GNSA, have been the battleground between polar and subtropical water masses over the last 3 million years (Kellogg, 1975; Ruddiman and McIntyre, 1976; Schrader et al., 1976). Since polar water penetrated southward almost to the Azores during glacial extremes, a volume dealing with the Arctic must also stray that far south. North Atlantic Deep Water, formed in the Greenland Sea, flows over the Faeroe—Iceland—Greenland transverse ridge. The subsidence and breaching of this previous land bridge was a major event in the paleooceanography of the North Atlantic (Vogt, 1972a; Nilsen, 1978, Thiede 1979, 1980).

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.

Aeromagnetic study of the Mid-Atlantic Ridge near the Oceanographer Fracture Zone

An aeromagnetic study was conducted over the Oceanographer Fracture Zone on the Mid-Atlantic Ridge between 33° to 37°N and 31° to 39°W. A sea-floor–spreading interpretation of the magnetic anomalies reveals that the ridge crest is formed of short, en echelon segments 40 to 60 km long. These segments are offset by transform fractures. An average spreading rate of about 1.1 cm/yr active over the last 10 m.y. can be fitted to the ridge crest anomalies 2′ through 5. However, positive identification of the outer flank anomalies is not possible. The ridge crest anomalies younger than 7 m.y. old (anomaly 4) show a general trend of N30°E, but anomalies between 9.3 and 17.5 m.y. old (anomaly 5 to 5′) have trends of about N8°E. The oldest flank anomalies (anomaly 6) trend about N35°E. Application of the anomaly trend superposition technique to account for the offset anomaly and fracture-zone pattern has allowed a new calculation of rotation pole parameters for the North American–African plate systems. For anomaly 2′ (2.7 m.y. ago), the finite rotation pole is located south of Iceland at 58.8°N, 17.4°W, with an angular rotation of 1.26°. For anomaly 5 and the older flank anomalies 5′ and 6, the finite rotation poles are located near Svalbard at 78.6°N, 34.5°E; 80°N, 29.9°E; and 80°N, 46.1°E, with angular rotations of 2.67, 3.84, and 4.64 degrees, respectively. The major change in the pole location between anomalies 2′ and 5 about 7 m.y. ago appears to have been accompanied by the creation of a new transform fracture pattern with old fractures terminating and new ones being formed. Comparison of the two general pole locations deduced here with poles determined by others for the earlier opening history of the North American–African plate system shows that all finite poles lie in either of these locations. This suggests that a bi-stable dynamic equilibrium condition has prevailed throughout the opening history, with the rotation poles being located south of Iceland during the earliest period (200 to 80 m.y. ago) and the latest period (∼7 m.y. ago to the present) of opening. During the intervening period, the poles were located near Svalbard.

Morphology and Structure of Maury Channel, Northeast Atlantic Ocean

Maury Channel is a 3,500-km-long, erosional/depositional feature. Originating on the southern slope of the Faeroe-Iceland Ridge at about 64° N., 13° W., the channel follows the deepest axis of Rockall Basin until about 53° N., where it begins a meandering course through several northeast Atlantic fracture zones. The channel finally empties into the northern Iberian Basin. Turbidity currents and overflow boulses of Norwegian Sea deep water are thought to be responsible for the formation of the channel. Strong bottom currents are responsible for keeping the channel “open” south of 53° N. Seismic reflection profiles reveal a characteristic “signature,” indicating deposition of dense turbidite material wherever the channel is encountered.

The Mid-Atlantic Ridge at 33°N: the Hayes Fracture Zone

Abstract A geophysical study was conducted over the Mid-Atlantic Ridge between 32–39°N and 30–40°W. A particularly deep fracture was observed which offset the ridge crest 110 km in the vicinity of 33°N. A pole of relative motion between the North American and African plates was deduced from this fracture zone as being at 63.1°N, 17°W.

The Bahia seamounts, Brazil Basin

Abstract Recent geophysical studies in the Brazil Basin by the US Naval Research Laboratory (NRL) and the Brazilian Department of Hydrography and Navigation (DHN) have resulted in the discovery of a major NW/SE-trending seamount group remarkably similar in appearance to but much more extensive than the New England seamounts in the North Atlantic Ocean. The Bahia seamounts, however, consist of three sub-chains, whereas the New England seamounts comprise a single chain. The total areal extent discovered thus far covers a polygon of approximately 125,000 km 2 . Airborne geomagnetic investigations and anomalous “highs” seen in SEASAT-A data have assisted in locating many of the individual peaks. Other features — e.g. , the Pernambuco Seachannel and the western terminus of the Bode Verde Fracture Zone — were also located by bathymetry and single-channel seismic reflection profiling. The latter feature has been verified by airborne geomagnetic measurements.