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For dykes in the upper crust, the strike of the dyke is parallel to the σ1-σ2 plane, and the normal to the dyke is aligned with the least compressive stress σ3. This relation enables the determination of the palaeo- stress distribution from the orientation of the dykes. In order to estimate the stress distribution in the Late Cretaceous in the Istanbul region, the orientation of the dykes in the Paleozoic sedimentary rocks have been measured. The dykes consist of andesite and basaltic andesite, and form massif, highly fractured pale yellow, beige and grey rocks. Their thickness ranges from 10-20 cm to 10-11 m and their length generally is measured in several tens of metres. Petrographically the dykes show a porphyritic texture with plagioclase, hornblende and augite phenocrystalls, 1.5-3.5 mm across, in a fine grained microlithic matrix. The measured strikes of the dykes show a wide scatter with a few prominent directions (N80°E, N40°E and N35°W). The wide scatter in the dyke direction suggests that the Istanbul dykes form a local dyke swarm above an unexposed pluton. Another possibility is that the Istanbul dykes were emplaced in a complex stress regime controlled by the opening of the oceanic West Black Sea basin and activity of the West Black Sea fault.

ORIENTATION OF THE ANDESITIC DYKES IN THE İSTANBUL REGION: AN APPROACH TO THE CRETACEOUS STRESS DISTRIBUTION