Pre-Andean and Andean shortening in the Eastern Cordillera of NW Argentina at 23°S

The Eastern Cordillera, in NW Argentina, is an imbricate thrust system involving Precambrian to Cenozoic stratigraphic units. Structural analysis shows that the Cretaceous unconformity truncates previous thrusts and related folds, but most of the Andean structures result from the reactivation of pre-Andean thrusts and folds. The total shortening for the Eastern Cordillera is about 46 km, of which 30 km are Andean in age and about 16 km correspond to the original Paleozoic imbricate.

A complete cross-section through the Eastern Cordillera and the Interandean Zone at 23°S is presented and a sequential restoration of Andean and pre-Andean structures was carried out. In this section, two basement sheets, each about 10 km thick, have been inferred below the Eastern Cordillera from displacement transfer analysis. The thrust sequence was established on the basis of quantitative analysis of displacement transfer. The first basement sheet to be emplaced reactivated the floor thrust of the previous Paleozoic imbricate system, and accounts for the cumulative shortening of Andean thrusts in the Eastern Cordillera. The second one developed in forward sequence, transferring more than 60 km to the sole thrust of the Subandean imbricate system. It is the main thrust and gave rise to the uplift and deep exhumation of the primitive Eastern Cordillera, causing a fast increase in subsidence rates in the Subandean foreland basin. Later, a second movement of the first basement sheet transferred about 8 km of displacement towards several out-of-sequence thrusts, giving rise to the intermontane Humahuaca Basin.