Nancy Anne Van Wagoner

Segregation vesicles commonly occur in sea-floor basalts. They generally are spherical in shape, 0.1 - 0.3 mm in diameter, and partially filled with a dark, crescent-shaped lining of aphanitic residue. This residue is a chilled residual melt which entered the vesicle sometime after crystallization had proceeded sufficiently to maintain the integrity of the vesicle but before final crystallization, and flowed to the bottom of the vesicle before solidifying. The crescent-shaped lining can, therefore, be used as an indicator of the cooling orientation of the rock.

By measuring the segregation vesicle inclinations in a pair of thin sections cut along two mutually perpendicular planes, a plane defining the horizontal orientation of the rock can be determined. Results of testing this technique on 17 sea-floor basalt samples, previously oriented by megascopic features, show that segregation vesicle and megascopic orientation agree. The technique can be used to determine the dip of basalt samples recovered in drill cores and to orient basalts in order to determine the paleomagnetic inclination acquired upon cooling. Analyses of the scatter of segregation vesicle inclinations of 34 rock samples indicate that the error associated with dip and magnetic inclination determinations generally are less than +36^o and +44^o, respectively.

Application of the technique to rocks recovered by dredge and submersible from the Mid-Atlantic Ridge around 35.6^o N during the AMAR expedition increased the number of samples available for a paleomagnetic polarity study by almost 50 percent. The results of this study show that about 85 percent of the rocks collected are normally magnetized.

Results of application of the technique for structural and magnetic polarity studies of DSDP cores 332A, 332B, 334, 395 and 395A, and 396B, which penetrated 117 to 583 m into North Atlantic oceanic basement crust, suggest that this crust is structurally heterogeneous. Although rocks dipping 4^o to 40^o occur in Holes 334, 395 and 395A, and 396B, dips in Holes 332A and 332B range from 20^o upright to 14^o overturned. As a consequence of tilting, the true thickness of rock units in Holes 332A and 332B may be 50 percent less than the vertical thickness. Tilting may also result in reorientation of the original direction of magnetization acquired by the rocks upon cooling. This suggests that, at least some of the time, the source of marine magnetic surface anomalies may not be in the upper 500 m of oceanic crust. The tilting may result from a number of deformation processes including subsidence, normal block faulting, and mass wasting.

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Supervisor:  James Hall