Norman Z. Cherkis

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.

The Bahia Seamounts: test of a hotspot model and a preliminary South American Late Cretaceous to Tertiary Apparent Polar Wander Path

Abstract Detailed bathymetric and magnetic data collected over the Bahia Seamounts in the western South Atlantic (Brazil Basin) are analyzed. Six paleomagnetic poles are determined, one from three seamounts sharing a ridge, and five from isolated seamounts. Two poles fall near a continental Late Cretaceous South American pole and are assumed to be of Late Cretaceous age. Another seamount has a preliminary 40 Ar 39 Ar age of 62 ± 4 Ma. We check this age and estimate the ages of other seamounts by rotating their poles into North American coordinates and comparing them with North American paleomagnetic reference poles. These ages and the characteristic magnetic polarities of all Bahia Seamounts are used to test hotspot models of formation of the seamount group. The northern and central Bahia Seamount chains could have formed from a single hotspot. The southern Bahia chain may require a second hotspot. The two postulated hotspots were probably concurrent and separated by 150 to 200 km. The paleomagnetic poles trace out a smooth curve from the Late Cretaceous poles to a pole that is indistinguishable from the spin axis, which we estimate as having an Early Eocene age. The pole ages remain preliminary until better age dates are found and rocks are recovered from more seamounts.

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.

Vesteris Seamount: An enigma in the Greenland Basin

Vesteris Seamount is a solitary submarine volcano located at 73°30′ N, 9°10′W in the Greenland Basin. Steeply rising from a base depth of 3100 m to a minimum depth of ~ 130 m and striking 030°/210°, the feature lies ~ 300 km east of the east Greenland margin on an otherwise nearly flat and featureless seafloor. The main body of the seamount appears to have been formed episodically, the last of which culminated about 110 000 years ago. Subsequent, lower intensity volcanic activity continued sporadically until about 25 000 years ago, as evidenced by ash layers found in cores near the base of the feature. The smoothed surfaces at the summit make it likely that the seamount actually broached the surface during the Weichselian glacial period, between 8000 and 13 000 years ago. Two multibeam bathymetric investigations aboardPFS Polarstern during ARKTIS II/4 (1984) and ARKTIS VII/1 (1990), combined with geologic sampling, single-channel seismic profiling and underwater television coverage, have resulted in a new interpretation of both the morphology and origins of the seamount. Data collected aboardPolarstern from ARKTIS II/4 (1984) have been previously reported by Hempelet al. (1991), however, when combined with the ARKTIS VII/1 (1990) data set, a more detailed interpretation of the morphology and structure was feasible. This included the elongated shape of the feature and showed the existence of several small volcanic cones on the seamount flanks.

Geological control of shallow gas and pockmarks in the Norwegian channel; high resolution shallow subbottom profiling of small scale features

High resolution bathymetric and fine-scale parametric subbottom profiling along a line to the SW of Stavanger, Norway near the NE flank of the Norwegian Channel, show pockmarks clustered over neotectonic shallow fold structures in Quaternary sediments. Detailed profiles of the pockmarks indicate that they are collapsed gas seeps, rather than being collapse structures that followed doming and breaching with a more dramatic gas burst. The gentle folding and weak structures along the margin of a Mesozoic through Cenozoic sedimentary basin are probably due to differential uplift generating light compressional strain.

Magnetic and Tectonic Fabric of the North Fiji Basin and Lau Basin

Detailed airborne and shipboard magnetic studies conducted over the eastern marginal basins of the southwest Pacific between New Zealand and the Solomon Islands suggest that seafloor spreading in the North Fiji Basin, which began in the latest Miocene, continues today. The North Fiji Basin is marked by at least two triple junctions, located in the central (16°50’S, 173°45’E) and northeastern (14°S, 179°30’E) parts of the basin. Both were apparently formed during the past five million years in response to continuing adjustments in the strike of the active spreading centers in the North Fiji Basin. Two additional contemporary spreading centers appear to have formed within the last one million years to the north and immediately to the west of Viti Levu. Although magnetic anomalies from 1 to 3 (0–5 Ma) are seen to form a clearly defined lineation pattern over the North Fiji Basin, earthquake foci suggest that only portions of the present North Fiji/Lau Basin spreading center system are currently active.

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.