Subducted, detached, and torn slabs during early orogeny: evidence from deep earthquakes under the Greater Caucasus

The Arabia-Eurasian collision is the second-largest active collisional orogen on Earth and provides a rare opportunity to investigate the role that pre-suturing tectonism plays in defining the deformational response of the upper plate to continental collision and suturing. The Greater Caucasus Mountains, which define the northern margin of the Arabia-Eurasia collision zone between the Black and Caspian seas, result from late Cenozoic closure of the Greater Caucasus basin, a Mesozoic back-arc basin that opened during northward subduction of Neotethys beneath the Lesser Caucasus island arc. However, both the extent to which the basin closed by subduction and its original width remain controversial. Many previous estimates suggest that the basin was very narrow and that no subduction occurred during the formation of the Greater Caucasus. However, newly compiled earthquake locations show that the central and eastern Greater Caucasus are underlain by a northeast-dipping subducted slab. We assembled the new catalog of earthquake locations by combining earthquake records from local networks in Georgia, Russia, and Azerbaijan with previously published data. Our dataset includes records from 3820 events with magnitudes M>2. Visualization of the final catalogue in an immersive visualization environment at the UC Davis Keck Center for Active Visualization in the Earth Sciences (KeckCAVES) provides a clear image of a northeast-dipping zone of seismicity beneath the Greater Caucasus, which we interpret as the remnant of a subducted slab. Beneath the central Greater Caucasus (45°E), the zone of seismicity extends to a depth of at least 158 km and dips to the northeast at ~40°. In contrast, beneath the western GC (i.e., west of 45°E) there is a pronounced lack of events below 25 km, which we infer to reflect slab breakoff. We also observe a gap in intermediate-depth seismicity (30 to 60 km) at the western end of the subducted slab beneath the central Greater Caucasus, which we interpret as an eastward-propagating tear. This tear coincides with a region of minimum horizontal GPS convergence rates between the Lesser and Greater Caucasus, as expected in a region of active slab breakoff. Evidence of subduction beneath the Greater Caucasus suggests that the relict back-arc basin was significantly wider than previously estimated. This further suggests that closure of this basin may have been an important mechanism for accommodating plate convergence in this nascent orogen and that the Greater Caucasus may represent a form of cryptic suture. Such relict-basin closure may also be an important, but overlooked process in the early stages of other orogens where it has proven difficult to reconcile total crustal shortening with estimates of total plate convergence.