From mid-Jurassic extension to obduction-related mélange formation: sedimentary records from a displaced segment of the western (Adriatic) margin of the Neotethys (NE Hungary)

The two most interesting moments in the life-cycle of oceanic margins are the end of rifting and break-up, thus the first moment of becoming a passive margin, and the transformation into an active margin due to the closure of the ocean. This latter crucial event is the target of our investigation regarding the Adriatic passive margin facing the Neotethys Ocean during the Middle Triassic – Middle-(Late?) Jurassic interval. The reconstruction of this complex history is difficult at straight subduction zones and even more challenging at curved ones or at the termination/transfer zones of oceanic systems. This is the case with suture zones in the northernmost part of the Dinarides and its displaced segment, the Bükk nappe system. Up to now, there are two main competing models concerning the way of obduction and lower plate imbrication in the Dinarides – Albanides – Hellenides. One suggests that the Western Vardar Ophiolite nappe juxtaposed the Adriatic margin during the Late Jurassic to earliest Cretaceous (Berriasian) with W- to NW-vergency, which predates the nappe-stacking of the Adriatic passive margin. The other suggests synchronous ophiolite obduction and passive margin imbrication in the Bajocian – Bathonian interval, then propagation of the deformation both in the upper plate (internal thrusting within the ophiolite nappe), and in the lower plate during the late Bathonian – Oxfordian.
Micropaleontological, sedimentological and structural investigation of more than thirty boreholes resulted in the detailed characterisation and 3D depositional model of the Recsk Succession (Fig. 2). The sedimentation in its Bajocian - early Callovian extensional half-grabens was characterised by pelagic limestones then dark shales with sandstone intercalations (Fig. 1). Mass-flow deposits both from the footwalls of graben-bounding normal faults and from the Adriatic-Dinaric Carbonate Platform were frequent (Fig. 1, 2). The lack of contractional structures or ophiolite-derived material in the gravity mass flow exclude their attribution to any mélange units related directly or indirectly to the Neotethyan subduction front. In the overlying Tarna Olistostrome sedimentation lasted at least till the Tithonian, indicated by new nannofossil findings. This is the oldest possible age for the overthrusting of the ophiolite nappe over this segment of the Adriatic passive margin. This suggests that the change from extension to shortening occurred between the Callovain and the Tithonian at the investigated northernmost termination of the Neotethys system.