Final answer:
The circular structure at the center of the investigation area could likely be a salt dome, considering the geological evidence of salt tectonics, diapirs, and historical sedimentation including salt in the region.
Step-by-step explanation:
The circular structure in the center of the investigation area could represent a variety of geological features formed by different geological processes. Based on the information provided, a diapir is a domed rock formation where low-density material moves upward into overlying strata. This could explain the circular structure if the core is made of a substance like salt or magma that has risen. Salt tectonics may be at play, where structures such as salt domes form due to the movement of salt under geological stresses. The presence of igneous activity, as suggested by references to mountains with subsurface volcanism, raises the possibility of the circular structure being a volcanic feature, such as a granitic intrusion surrounded by copper veins or a volcanic conduit that has eroded to a volcanic neck. Considering the geological context of the region including the history of subduction-related volcanism and the evidence of sedimentary strata affected by salt diapirs, the circular structure could indeed be a salt dome, especially if the area has a history of sedimentation including salt and events like the continental collision between the African and European plates.