Orientation Of The Andesitic Dykes In The İstanbul Region: An Approach To The Cretaceous Stress Distribution

Year 2005, Volume: 130 Issue: 130, 17 - 27, 01.11.2005

Zahide Özgörüş Aral İ. Okay

Abstract

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 palaeostress
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. 

Keywords

Black Sea, dykes, Istanbul, Cretaceous, andesite, stress

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