ALDI-ADRIA

Continent-continent collision is driven by subduction of the heavy mantle portion of the continental lithosphere. Upper crustal layers are scraped off the subducting plate and are accumulated piling up an orogenic wedge. When the resistance against subduction is exceeding the downward-directed force, mantle sectors of the subducting lithosphere can detach (slab break-off) and the orogenic wedge is suddenly uprising. Another process proposed is slab delamination. It occurs when major portions of subducted lithosphere detach over a wide area or are thermally eroded by melting at its base. In arcuate orogens, lateral boundaries (STEP faults) of retreating mantle slabs can cause similar effects. These three processes cause differential vertical motion, magmatism and are driven by distinct motion directions of the subducting/indenting plate and orogenic wedge. Late-stage intraplate convergence and indentation results in vertical motion, too. The surface signatures of these and other late-stage intra-lithospheric processes are not well understood, and the proposed project is aimed at studying specific large-scale spatial and age distribution patterns of vertical surface motion in the orogenic wedge and in the associated peripheral foreland basin. In conjunction with studies on the kinematics and the distribution of volcanism in the orogenic wedge specific intra-lithospheric processes operative in orogens during the late stage will be analyzed and distinguished. Each process is predicted to trigger distinct spatial and temporal patterns of uplift, volcanism and kinematics in the orogenic wedge and subsidence in the associated foreland basin. The selected working area is the Friuli orocline at the transition of the Southern Alps to the Dinarides on the front of the ca. northward moving Adriatic microplate. A variety of deep-lithospheric processes including subduction reversal and slab-break-off have been proposed to be responsible for the Oligocene-Recent geodynamic evolution of that specific area, and these hypotheses could be assessed by the proposed project. The large-scale uplift/exhumation pattern derived from apatite fission-track thermochronology and (U-Th-Sm)/He dating coupled with geomorphic analysis of the mountains and subsidence analysis in the Adriatic Sea and its Venetian-Istrian foreland and kinematic studies will form the base for the interpretation of deep-lithospheric tectonic processes. The analysis of the new data in conjunction with existing data should allow the distinction between various superimposed late-stage orogenic processes. Results could be also be applied to other orogens.

Summary for public relations work The project was aimed to decipher the relative importance of various late-stage orogenic processes after continent-continent collision and how deep lithospheric processes can get deduced from geological surface observations. Continent-continent plate collision is driven by subduction of the heavy mantle portion of the continental lithosphere. Upper crustal layers are scraped off the subducting plate and are accumulated piling up an orogenic wedge. When the resistance against subduction is exceeding the downward-directed force, mantle sectors of the subducting lithosphere can detach (slab break-off) and the orogenic wedge is suddenly uprising. Another process proposed is slab delamination. It occurs when major portions of subducted lithosphere detach over a wide area or are thermally eroded by melting at its base. In arcuate orogens, lateral boundaries (STEP faults) of retreating mantle slabs can cause similar effects. These three processes cause differential vertical motion, magmatism and are driven by distinct motion directions of the subducting/indenting plate and orogenic wedge. Late-stage intraplate convergence and indentation results in vertical motion, too. We studied the surface signatures of these and other late-stage intra-lithospheric processes by studying specific large-scale spatial and age distribution patterns of vertical surface motion in the orogenic wedge and in the associated peripheral foreland basin. We were able to decipher the kinematics and the distribution of magmatism in the orogenic wedge specific for intra-lithospheric processes operative in two examples, (1) in the Friuli orocline orogen at the transition between the ENE-trending Southern Alps and the NW-trending Dinarides (AlDi-Adria area) and, in a complementary structure, (2) for the Jurassic-Cretaceous evolution of the Qinling orogen in China. For the AlDi-Adria region, we note the following major results. No long-lasting subduction of continental Adria underneath the Sothern Alps and Dinarides at the Friuli orocline can be deduced because of following reasons: (1) There is no major foreland basin, which could be expected when the load by a subducted mantle lithosphere is present. (2) More or less undeformed foreland basement is exposed at the Monti Euganei and Stable Istria directly on the thrust front excluding significant flexure of the foreland lithosphere. (3) Volcanism is minor and is mainly related to Oligocene processes along the Periadriatic fault. An eastward progression of magmatism between 32 and 18 Ma argues for a late-stage of eastward migrating slab-break-off and slab-tearing. (4) Structure and kinematic indicators indicate a changing motion direction of the Adriatic microplate between 30 Ma and recent times and argue for some significant back-thrusting, building of an orogenic wedge in the eastern part, which is decoupled by from intra-orogenic shortening in a zone of dextral wrenching along the Möll Valley-Hochstuhl-Idrija wrench corridor. This NW-trending boundary coincides with the southwestern limit of the recently postulated crustal Pannonian fragment. Along the eastern part, a double-vergent orogenic wedge has been build up since late Miocene and the drainage pattern is entirely guided by the Dinaridic orogen-parallel shortening. In contrast, the exhumation pattern of the Qinling orogen in China argues for the presence of a STEP fault along-strike of the orogen.