ON THE EDGE OF A PLATE: THE NORTHERN ADRIATIC MICROPLATE AND ITS INTERACTION WITH THE EASTERN ALPS AND DINARIDES

We study the large-scale expression of the Oligocene to Recent motion of the Adriatic microplate towards the Eastern Alps and Dinarides. Late Miocene (?) to Recent subduction of the Adriatic microplate beneath the Friuli orocline of the Eastern Alps and NW Dinarides has been proposed. Rotation of the Adriatic microplate indicates decoupling from the internal Alpine-Dinaridic mountain ranges. Initially, the Periadriatic fault, and then the Fella-Sava-Zumberek fault system – a prime candidate for guiding rotation since the Pliocene - decoupled the plates. Fault and striae data indicate mostly NW-SE to NNW-SSE strike-slip compression. This is consistent with ongoing seismicity, which shows spatiotemporal segmentation into three areas (W, central, SE) with the present-day main activity in the central N Friuli - W Julian Alps. Based on new structural and thermochronological data in conjunction with a large body of existing data, we propose that some sectors of the internal Southalpine unit and NW Dinarides just reached very low- to low-grade metamorphic conditions during likely Eocene times followed by differential uplift and erosion of several km. In the internal Southalpine and Dinarides, existing apatite fission-track ages are partly between 7 and 30 Ma (Southalpine unit) and ca. 42–33 Ma (Dinaric belt) (Stefani et al. 2008, J. Sediment. Res. 77, 867–887 and refs) and mostly show post-depositional thermal overprint at minimum 110°C, decreasing towards the foreland. As a principal result of structural, thermochronological and sedimentological observations we find: (1) a Mid-Late Miocene stage of surface uplift, intra-orogenic subsidence of sedimentary basins reflecting intra-orogenic crustal-scale folding, and (2) a Late Pliocene-Pleistocene stage of convergence, which led to overall surface uplift in the orogen and pronounced subsidence in the foreland basin. The two stages of convergence resulted from different orogen-foreland basin coupling which was weak in the earlier stage and strong in the later stage, as reflected by subsidence in the associated foreland basin.