Volcanism associated with extension at a consuming margin, W Pontides, N Turkey


KESKİN M., USTAÖMER T., YENİYOL M.

European Union of Geosciences 1999 (EUG10) in Strasbourg, Journal of Conference Abstracts, France, 1 - 04 March 1999, vol.4, no.1, pp.841

  • Publication Type: Conference Paper / Summary Text
  • Volume: 4
  • Country: France
  • Page Numbers: pp.841

Abstract

 

A thick sequence of north-dipping, Cretaceous volcaniclastic sediments and minor lava flows, termed the Kavaklar Group, crop out to the N of Istanbul, along the south coast of the Black Sea.  A Palaeozoic sequence, known as the Palaeozoic of Istanbul, is exposed to the S of the volconogenic sequence.  The contact between the two is a major thrust, along which the Palaeozoic sequence tectonically overlies the Kavaklar Group. The Kavaklar Group is represented by a flysch sequence at the base.  There are occasional lava and volcanic debris flows within this part of the succession.  Soft sediment deformation structures (i.e. slumps and slide blocks) are abundant within the flysch.  Above comes a thick sequence of volcaniclastic sediments, ca 1000 m.  The volcaniclastic sediments include volcanic breccias, lahar breccias, hyaloclastites, volcaniclastic boulderstones and sandstones.  The clasts range in composition from basaltic andesites to rhyolites and in texture from aphyric to plagioclase- and pyroxene- phyric lavas. The lava clasts range in size from a millimetre to a few metres.  Thickness of the individual beds range from a few centimetres to ca. 30 m.

Dykes and sills of basic to intermediate composition intrude into the volcanicalstic sediments.  Associated with the dykes are occasional columnar jointed lava flows. Geochemical data of the lava flows and individual clasts imply a volcanic arc-type eruptive tectonic setting. This is evident from the LIL and LREE enrichment relative to HFSE.

Our working hypothesis is that the Kavaklar Group represents extensional magmatism, related to the opening of the Black Sea.  We think that initial crustal extension resulted in an irregular topography with structural lows and highs, with associated marginal volcanism.  Terrigenous turbidites and volcanic debris flows were accumulated into structural lows while the ridges were eroded.  Thick pile of volcanics was accumulated during the initial stages of magmatism.  Mass wasting of the volcanic provenance took place, associated with continuous extension.  Products were deposited in proximal basins. We believe that the arc signature is inherited from an earlier subduction event that metasomatised the sub-continental lithosphere.  Arc signature can be seen in any tectonic setting, provided that an earlier subduction event existed.  Basin and Range Province of the W USA, the Aegean Graben System are good examples of extensional setting, containing arc component.  Such arc signatures are also found in some of the recent collisonal settings (i.e. NE Anatolia).