Journal of the Virtual Explorer

The characteristics of the boundaries of the mafic dykes suggest that the pre-existing fracture system into which the mafic dyke swarms intruded was formed in an extension regional stress field. First, chilled margins on both sides of the dyke and baked margins of the host rocks on the contacting boundary with the dykes exist widely. Second, the abundant xenoliths and xenocrysts derived from the host rocks have been found on the edges of the dykes. Third, some tensional breccias are found in the contact zone between the dykes and the host rocks, which are composed of basic matrix and granitic breccias and have apparent edges and corners. At last, most of the dykes have irregular boundaries.

Dyke swarms are regarded as valid markers for extension event. Crustal extension in the EB was calculated based on the width statistics of dyke swarms on 7 survey lines. The formula

was employed in the calculation, where λ is the extension ratio; Σd_i is the total width of dykes on the survey line and L is the length of survey line cross the observed dyke swarms (Hou et al., 2006b). Local extension ratios were calculated and are presented in (Table 1). An average crustal extension ratio of 0.43% was obtained by the calculation of the extension ratio for the EB, which was contributed by the mafic dyke swarms. The small magnitude of overall extension suggests that the mafic dyke swarms were emplaced as limited elastic fractures in the EB, and indicates the EB had become a brittle plate prior to the emplacement of the mafic dyke swarms.