The relationship between orogenic building and subsequent extensional collapse: a case study in the northern Rhodope Mountains of Bulgaria

Abstract

Mechanics of extensional exhumation in core-complexes is one of the most often discussed topic in structural studies. Previous research has shown that exhumation along extensional detachments often reactivates inherited nappe contacts, due to their inherent rheological weak character at crustal scale. These often reactivations result in a poly-phase deformation history of nappe stack contacts or suture zones that is often difficult to discriminate in structural studies and absolute age dating of deformation timing and mechanics. One of the best examples of nappe stack contacts re-activation during core-complex formation took place during the evolution of the Rhodope Mountains, where a significant number of Cretaceous - Eocene thrust contacts have been inverted by extensional detachments during Eocene - Miocene times. In particular, the northern margin of this structure in the Thrace region of Bulgaria was difficult to structurally understand, because the Cretaceous nappe contacts were subsequently re-activated and tilted by the overall exhumation of the core-complex, which created apparent normal faulting geometries. We have conducted a combined field and micro- structural study in the northern Rhodope Mountains in the areas located SW of the city of Plovdiv, where four major tectono-metamorphic units crop out near the limit between the core-complex and its overall hanging-wall. The results of our study demonstrate a multi-stage tectonic evolution dominantly characterized by reactivation of nappe contacts and inherited orogenic geometries. The first moment of deformation observed in our study is the top-NW thrusting of the Thracian unit and the formation of a nappe stack in its footwall. This was followed by the top-S (in the studied area) extensional formation of the main Rhodope core-complex along a detachment that re-activated the nappe contact between the Thracian and Chepinska-Madan units, and possibly also the one between the Asenica and Chepinska units. The subsequent exhumation of the Rhodope core-complex has tilted this detachment and the original nappe stack geometry from an original S-dipping to N-dipping position and has also created the presently observed tectonic omission of the Asenica unit west of our studied area. The resulted change in geometry by the exhumation of the core-complex has blocked the main detachment and has, therefore, favoured the creation of a subsequent third main extensional event, by the Oligocene creation of a top-NE extensional detachment that separated the meta-plutonic intrusion of the Asenica unit from its meta-sedimentary cover. The change in extensional direction from N-S to NE-SW is likely related to the clockwise rotational mechanics of the Rhodope core-complex during its late stage Oligocene-Miocene evolution.