EARLY MIDDLE MIOCENE FLEXURAL UPLIFTING AND FOREBULGE MIGRATION IN THE CENTRAL ANDEAN FORELAND BASIN OF ARGENTINA

The Vinchina basin is located at northwestern of La Rioja province, Northwestern Argentina. The basin fill is conform by a coarsenning upward continental megasequence up to 10.000 meters of thickness, integrated by the Vinchina and Toro Negro formations. In the Vinchina Formation were recognized two unconformities (Marenssi et al., 2000), these are reinterpretate and related to two forebulges (Crampton and Allen, 1995), assuming a previously recognized extensional event during the Early Middle Miocene in the region (Dávila y Astini, 2003) that probably induced a decrese in the lithosphere rigidity.

The dominance of fluvial systems and alluvial fan deposits in the Vinchina basin megasequence characterizes an overfilled phase in the evolution of the foreland system, when flexural tectonism is the dominant control on the stratigraphy (Catuneanu, 2004). After the formation of the “forebulge unconformity” (basal uncomformitie), other unconformities may also form in the forebulge area, especially during the overfilled stage (Catuneanu, pers. comm.). The unconformities in the Vinchina Formation are good candidates for a migrating forebulge in an overfilled phase in response to a retrogradation of the foreland system induced by visco-elastic relaxation of the lithosphere (Catuneanu, 2004), that could be favored by the extensional event. Orogenic loading induces axial drainage along the subsiding foredeep that apears underfill (Limarino et al., 2001) because of the uplifted orogen and forebulge that flank it and act as source areas.

The megasequence of the Vinchina basin is interpretated as a flexural foredeep megasequence typically coarsenning upward. Changes in accomodation space and lithofacies assemblages are used to constrain the forebulge uplift, that leads to foredeep subsidence which became deeper and narrower. In this model (first – order foreland cycle, stage 2, of Catuneanu, 2004), the rates of accomodation in the foredeep increase proportionally with the gradual increase in the rates of visco-elastic relaxation.