| The structure of basement-cored uplift mountains is fundamentally controlled by the geometry of the basement and the basement unconformity. In the Sierra de Hualfín, an uplift located in the Sierras Pampeanas of NW Argentina, the well-exposed basement unconformity displays along-strike structural variations. The Sierra de Hualfín is composed of a core of Ordovician granitoid overlain by and thrust over Tertiary sedimentary rocks. The range is uplifted by an east-directed thrust fault, creating a steep range-front on the east and a gently dipping backlimb on the west. In the central region, however, basement and cover rocks in the backlimb are folded. Cross sections across the uplift and a three-dimensional computer model show that the variations along strike are due to differences in the geometry of the underlying fault or faults that deform the basement unconformity.; Analysis of the folded basement unconformity on the west side of Sierra de Hualfín shows that brittle processes of faulting, cataclasis and fracture foliation help the basement attain a folded form. Pervasive and continuous joint fractures, and an unconformity-parallel fracture foliation at the surface, set up a condition for “flow” of granitic materials. Additionally, in order for basement to fold, the tip of the underlying fault must be located within the basement before it begins propagating towards the surface.; Using the knowledge gained from the detailed structural studies, two regional cross sections were constructed in the region of Sierra de Hualfín, showing spatial and geometric relations between the adjacent uplifts, and the thrust systems of the eastern Puna Plateau. The cross sections show detachment levels at 16 km and ∼27 km for uplifts in the northern Sierras Pampeanas. Thrust faults of the northern Sierras Pampeanas, and of the eastern edge of the Puna can be interpreted as part of the same east-directed thrust system, with intervening west-directed backthrust development. The combination of the modeling of Sierra de Hualfín and the regional cross sections with detachments at 16 to 27 km depth suggests that basement-cored uplifts can be formed by gently-dipping faults detaching in the middle crust and near the base of the crust. |
