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Linkage between the Southern and Eastern Carpathians: structural scenarios for the Bend Zone

Abstract

The Carpathian Bend Zone (BZ) defines the area of transition between the roughly N-S trending Eastern Carpathians and the E-W trending Southern Carpathians. The BZ is located along the southward termination of the main structures observed in the outcropping nappes of the Eastern Carpathians (Tarcau, Marginal) which bend from a N-S to a roughly NE-SW trend. Towards the west, the BZ has been informally defined to end against the Intra-Moesian Fault (IMF), one of the faults of Dobogrea area which belongs to the southern extension of the Tornquist-Teisseyre Zone. The IMF is a basement-related feature defined by geophysical anomalies, with present-day seismic activity.

The BZ is characterized by the generalized outcrop of Oligocene-Early Miocene rocks and the presence of abundant diapirs and walls formed by Burdigalian and Badenian salt. The structure of the area is difficult to interpret due to the abundant presence of salt at different structural levels and its structural complexity. Structural understanding of the BZ is further hindered by the difficulty of understanding the termination of eastern Carpathian tectonic units and the apparent disconnection of the BZ from the Southern Carpathians due to the IMF.

Despite these obstacles, analysis of available data (geologic maps, published cross sections, gravity and magnetic maps) provides powerful insights into the details of the structural linkage between the Southern and Eastern Carpathians across the BZ. Map relationships make it possible to correlate the key structural elements of the Eastern Carpathians (Tarcau and Marginale nappes and Burdigalian wedge) with equivalent structures in the BZ. The main difference between both domains is that towards the south and southwest there is a significant loss of displacement on the Tarcau nappe and a significant narrowing of the Marginale nappe. Furthermore, in the BZ, there is an increased presence of salt structures, potentially related to oblique slip on the key thrusts. The combination of the southward termination of structures and the interference of salt tectonics leads to structures becoming less laterally continuous and an overall plunge towards the SW. On the other hand, the Peri-Carpathian thrust and its hangingwall (the Burdigalian wedge) are observed to be almost identical in character in both zones.

Correlation towards the west into the Southern Carpathians is also possible. As occurs towards the north, the Peri-Carpathian thrust and the Burdigalian wedge are almost perfectly continuous between the BZ and the Southern Carpathians, calling into doubt the role of the IMF during its emplacement. As for the Tarcau and Marginale nappes, these continue to lose displacement and width, to the point where they conform a very narrow band of deformation in the Southern Carpathians, associated to high-angle inversion structures (as opposed to the low-angle thrusts observed in the BZ and Eastern Carpathians). This transition across the IMF is interpreted to be related to the control of this deep fault on the location of a regional-scale, shallow lateral ramp during thrust emplacement. The result is the present-day apparent lateral offset on the shallower thrust units (Tarcau and Marginale) and the increase in topographic relief to the east.

The correlation of structural units across the BZ opens exploration opportunities in deep sub-thrust plays in which Oligo-Miocene reservoirs have undergone a similar depositional, burial and tectonic history to those reservoirs currently producing in shallow thrust units of the BZ.