Coeval extensional shearing and lateral underflow during Late Cretaceous core complex development in the Niğde Massif, Central Anatolia, Turkey

Abstract : The Nig¢de Massif, at the southern tip of the Central Anatolian Crystalline Complex, consists of two structural units. Foliations in the lower unit define a dome cored by migmatites. The contact between the two units bears all the elements of a ductile to brittle extensional detachment. Hence the Nig¢de Massif represents an extensional metamorphic core complex. Top-to-NE/ENE shearing at higher levels of the lower unit relates to displacement along the detachment. Deeper levels of the lower unit display hightemperature top-to-SSW ductile shearing. The two shearing deformations show a difference in the mean trend of stretching lineations of up to 58. New 40Ar/39Ar ages combined with previously published data enable us to infer that the two shears were contemporaneous. In our favored interpretation, oblique shearing in the core of the dome reflects lateral underflow, i.e., horizontal flowing of the lower crust in a direction highly oblique to the direction of extension. As a result of the interaction between lateral underflow and downdip shearing along the overlying detachment, distinct structural domains are expected to exist within the migmatitic part of the core complex, with observed counterparts in the Nig¢de dome. Lateral underflow may reflect ''inward'' flow on the scale of the core complex. Regional-scale channel flow is an alternative that would better account for the record of non-coaxial deformation in the core of the dome. More generally, we suspect that the development of lateral underflow in a metamorphic core complex more likely reflects regional channel flow, rather than local inward flow.