Floating the Vrancea slab and tectonic reconstruction of the collapse of the PalaeoPannonian Basin

Abstract

<p>Here we present the first 4D tectonic reconstruction that models the Vrancea slablet and incorporates the floated slab as a constraint on the magnitude of slab rollback during collapse of the Palaeo-Pannonian Basin. Seismic tomographic images provide insight into the geometry and tectonic history of subducted slabs. High velocity anomalies can be interpreted as &#8216;cold&#8217; lithosphere penetrating &#8216;warmer&#8217; lower velocity asthenosphere, and 3D models created using the <em>SKUA-GOCAD</em> modelling software. Combined with information from the 3D distribution of earthquake hypocentres, we thereby obtain a simple approximation to slab geometry beneath the Vrancea region. The resultant DXF was imported into the <em>Pplates</em> tectonic reconstruction software, and floated back to the Earth&#8217;s surface. The method utilised assumes no significant deformation (stretching, buckling, folding, shortening) during or after subduction, so that the obtained geometry estimates the pre-subduction configuration. The resultant floated slab is then incorporated as a constraint on 2D + time tectonic reconstructions. We apply a double-saloon-door rollback model, which involves propagation of a slab tear along the mid-Hungarian lineament. Each saloon-door rolls back independently of the other and this leads to two epochs of extension. AlPaCa is &#8216;pulled&#8217; eastwards and rotated counter-clockwise as the western saloon-door rolls back. The Tisza-Dacia unit is then &#8216;pulled&#8217; eastward, and rotated, but in a clockwise sense as the eastern saloon-door rolls back. Once the subduction hinge reached the East European Platform, the slab was left hanging. Gravitational forces then drove slab-boudinage and detachment in a similar fashion as occurs today beneath the Hindu Kush. This model explains the large opposing-sense vertical-axis rotations that occurred during convergence of the AlPaCa and Tisza-Dacia terranes. The zipper fault model rotates the microplates without requiring large-scale thrusting. Interpretation of the Mid-Hungarian lineament as a zipper-fault system is also consistent with the geodynamic effects expected because of tearing in a subducting plate leading to a double-saloon-door rollback. The vertical extent of the slab is roughly 300 km, which only fills half of the basin, consistent with the double-saloon-door roll-back model interpretation.</p>