Publikationsdatum:
2022-07-18
Beschreibung:
Research cruise No. 4 of R.V. "Meteor" investigated the crest and upper flanks of the southern Reykjanes Ridge, in the region bounded by 52° 15' N, 56° 45' N, 33° W, and 37° W (fig. 1). Tracks spaced about 20 nm were laid out on a northsouth and east-west grid (fig. 3). The final smoothed track plot achieved acceptable relative accuracy, despite the poor navigational control. The median rift valley, not found on the Reykjanes Ridge north of 57.5° N, begins to be a well defined feature between 53.5° N and 54.5° N (figs. 2 and 6). In the survey area, free-air anomalies are entirely positive with a mean of + 60mgals. The anomaly pattern (fig. 7) mirrors sea-floor topography. A low ( 〈 + 40 mgals), associated with the rift valley, trends north along 35° 15' W from 53° 20' N to 54° 20' N. Less pronounced ( 〈 + 60 mgals) it can be followed as far north as 56.5° N. Some more transverse positive (〉 + 80 mgals) and negative ( 〈 + 50 mgals) free-air anomalies trend east between 54° N and 56° N and a parallel low ( 〈 ± 0 mgals) lies just to the south, coinciding with the Charlie Fracture Zone at 52° 45' N. In contrast to our results, TALWANI et al. (1971) found relatively positive zones of free-air anomalies over the ridge axis, and parallel to it, at a morphologically smoother part of the ridge, north of 59° N. The dependence between water depths and free-air anomalies yields an average of 2.3 g/cm3 without, and 2.6 g/cm-3 with topographic correction. This value as well as the positive free-air anomalies alone evidences a small amount of isostatic compensation. Bouguer anomalies have been calculated with three-dimensional topographic correction for standard density of 2.67 g/cm-3 (fig. 8) and for "true" density determined from rock samples of 2.90 g/cm-3 (fig. 9). In both cases, a slope of regional negative Bouguer values, centered on the ridge axis, points to a material in greater depths which is lighter than the normal mantle material ("root"). Similar gravimetric and seismic results, respectively, were obtained by M. TALWANI et al. (1965) south, and K. ARIC (1970 and 1972) north of the survey area. A thick layer of low density material under the axis (fig. 13) may be partially molten ultrabasics ascending into the rift axis as part of the sea-floor spreading process. The rift axis is characterized by magnetic anomalies of + 1000 to 1500 gamma amplitudes (figs. 10 and 11 ). A lineation pattern, symmetrical about the rift axis, is approximated by the model profiles computed from the standard HEIRTZLER reversal chronology and 1.12 cm/year spreading half-rate in the 095° T direction of relative motion between the Europe and Americas plates (fig. 14). The same rate, within measurement error, was found by HEIRTZLER et al. (1968) on the northern pare of the Reykjanes Ridge. The decay of anomaly amplitude away from the spreading axis suggests a 50 % reduction in magnetization intensity, or a similar reduction in thickness of the magnetized layer, in the first 5 my of crustal age. The former explanation is more probable; a similar decay is found elsewhere (VOGT et al. 1970). A significant finding is that the spreading axis as well as the anomaly pattern at least back to 5 mybp is not straight like the northern Reykjanes Ridge. In contrast to the physiographic interpretation of HEEZEN who shows transverse fractures perpendicular to and offsetting the rift valley, the axis is broken into oblique sections of the order 100 km long. Spreading rates normal to these sections range from 0.97 to 1.10 cm/year. The present irregular shape of the axis was formed in the late Tertiary, some time prior to 5 mybp but after 30 mybp. A similar irregular trend, discovered from detailed surveys between 48° and 50° N, was created about 20 mybp (JOHNSON & VOGT 1972).
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