70. STRUCTURAL STUDY OF BASALTIC ROCKS SHOWING BRITTLE DEFORMATION (DEEP SEA DRILLING PROJECT LEGS 51, 52, AND 53, SITES 417 AND 418)

A sample of a non-oriented core with known polarity, when subjected to microtectonic analysis, may throw light on the tectonic event — compressional or tensional — that has affected the rocks and may indicate whether the microstructures appeared during one or several phases of deformation. This analysis is based upon the presence of relative movement discontinuities separating "blocks" that have not undergone penetrative deformation, upon their behavior and upon the spatial compatibility between the geometry of the discontinuities and the observable displacement (Tjia, 1964, 1967; Badgley, 1965; Arthaud and Choukroune, 1972; Durney and Ramsay, 1973; Engelder, 1974). The structures employed in the analysis were: striated microfaults, joints (without visible displacement), open cracks, and Riedel shears. The displacement on fault planes is determined by the following criteria: The striation on the fault plane indicates the direction of displacement. (Measurement is made of the angle between these striations and the horizontal line of the plane considered, i.e., the pitch.) The relative positions of the mylonitized zones and the open zones filled by secondary minerals, those of microfaults and gashes, and the orientation of clear markers (such as plagioclase phenocrysts occurring as striating elements) indicate the direction of the relative movement. These criteria are summarized in Figure 2. The representation of the planar or linear structural elements is by stereographic projection on a Wulff net (lower hemisphere), north being arbitrarily defined by the horizontal line of one of the observed fault planes (Figure 3).