Alpine Metamorphism and Granitoid Magmatism in the Strandja Zone: New Data from the Sakar Unit, SE Bulgaria

Authors: IANKO GERDJIKOV

Abstract: The Strandja Zone is a pre-Cenomanian orogen consisting of three tectonic units: the Veleka, Strandja and Sakar units. The first two units display very low-grade Alpine metamorphism, whereas in the Sakar unit the Triassic sediments are metamorphosed to the amphibolite facies. For years this high-grade metamorphism was enigmatic and most of the metamorphic rocks were interpreted as a reworked Precambrian crust of the Rhodope. This contribution begins with a review of the recent data about the structure and evolution of the Sakar unit. Probably the most important recent finding was the demonstration of the lack of a structural break between the Triassic and older rocks in the Sakar unit. This paper present results of the structural analysis of a key area in the southeastern part of the Sakar unit. The oldest unit is a volcano-sedimentary association intruded by granitoids of probable Late Palaeozoic age. These rocks contain NE-SW-striking foliation and well-defined stretching lineation plunging to the SE or SSE, formed synchronously with the medium-grade metamorphism of pre-Cenomanian age. The synmetamorphic fabrics suggest top-to-the-NW shear. Younger, but also typically foliated granite bodies, termed the Varnik granites, cut the other units. No single large magmatic body could be defined because of the intimate interfingering with older rocks. The Varnik granites are clearly distinguished from the older granites by their higher temperature fabric, cross-cutting relations and the post-tectonic character of some of the associated veins and dykes. The solid-state fabric also records NW-vergent shear deformation. The emplacement of the Varnik granites is interpreted as syntectonic with respect to the pre-Cenomanian metamorphism. The regional-scale increase in metamorphic grade within the Sakar unit is most easily explained by a combined effect of several pluton-driven thermal pulses.

Keywords: Strandja zone, Sakar unit, Alpine tectonics, syntectonic granitoids, structural geology

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