Revisiting the Late Permian – Jurassic paleogeography of the Eastern Alps and Western Carpathians and tectonic implications

The ALCAPA (Alps-Carpathians-Pannonia) megaunit, encompassing most of the Eastern Alps (Austroalpine units) and the Western Carpathians (Inner and Central part), has traditionally been interpreted to represent the Neo-Tethys-facing Permo-Triassic passive margin of Pangaea. Multiple tectonic units within the ALCAPA and wider Dinaridic-Balkanic domain have been regarded as continental sutures formed during the closure of the Neo-Tethyan Ocean. A long-standing debate concerns whether the Neo-Tethys comprised a single or multiple oceanic basins. In the ALCAPA region, the Neo-Tethys is commonly called the Meliata-Hallstatt Ocean and considered to represent an oceanic branch located south (in present-day coordinates) of the salt-rich Austroalpine-Carpathian passive margins. This interpretation assumes a simple and linear arrangement of Permo-Triassic facies, from shallow water platforms in the north to pelagic and bathyal deposits in the south. This facies scheme has been generally used as the basis to explain the distribution of Permo-Triassic units in the present-day fold-and-thrust belts.

Recent advances in the understanding of salt tectonics and passive margin geodynamics, however, calls this linear paleogeographic model into question. In this contribution we present new observations from the Inner Western Carpathians and a reevaluation of the Austroalpine domain and argue that the Permo-Triassic palegeography of ALCAPA was far more complex than previously assumed. We propose that the Permo-Triassic rift system formed an anastomosing network of rift branches, some of which remained aborted while others progressed to continental breakup and mantle exhumation. This evolution produced a complex mosaic of microcontinental blocks separated by rift basins and domains of exhumed subcontinental mantle, in stark contrast to the conventional linear continent-to-ocean model. This revised paleogeographic framework has significant implications for deciphering the distribution of Permo-Triassic facies in the pre-orogenic setting. Understanding the role of continental hyperextension in the ALCAPA also satisfactorily explains the frequent (and apparently incongruous) contacts of shallow crustal units on sub-continental mantle (e.g., evaporites or platform carbonates on serpentinized mantle).

We argue that Jurassic subduction nucleated preferentially along domains of Triassic exhumed mantle, and that recognizing the complex paleogeographic architecture substantially simplifies the tectonic interpretation of the subsequent Mesozoic evolution of the region. These observations further call for a redefinition of what is considered the Neo-Tethys Ocean within the ALCAPA domain.