A FLEXURAL ISOSTATIC MODEL OF LITHOSPHERE SHORTENING AND FORELAND BASIN FORMATION - APPLICATION TO THE EASTERN CORDILLERA AND SUBANDEAN BELT OF NW ARGENTINA

A numerical model of lithosphere shortening has been developed to comb ine flexural isostatic and structural balancing. This allows us to qua ntitatively reconcile lithosphere shortening, thrust sheet emplacement , and foreland basin formation. Lithosphere shortening is accommodated by thrusting in the upper crust and by distributed deformation in the lower crust and the lithospheric mantle. This leads to loading by thr ust sheet emplacement, crustal thickening, and thermal perturbation. A dditional loading and unloading are generated by sedimentation and ero sion, respectively. The model is kinematically controlled, each fault having its position, geometry, and movement prescribed. The model is a pplied to the Subandean late Oligocene to Recent foreland fold and thr ust belt of NW Argentina at 22 degrees 15'S. The Subandean belt of NW Argentina contains three major stratigraphic units, the Lower, Middle, and Upper Terciario Subandino Formations. By modelling these formatio ns we attempt to determine whether the Terciario Subandino Group can b e explained as a foredeep fill in response to thrust sheet loading, an d, if so, what is the required effective elastic thickness. A successf ul model must predict not only foreland stratigraphy but also exhumati on, topography, and crustal thickness. We see a change in the waveleng th of sedimentary deposition in the Subandean basin through time that can be accounted for by a change in flexural rigidity. Accommodation s pace for the Lower Terciario Subandino Formation can be generated usin g an elastic thickness of 70 km. For the Middle/Upper Terciario an ela stic thickness of 15 km produces the correct basin geometries. Shorten ing estimates of 110 km have proved sufficient to generate the observe d foreland stratigraphy, Bouguer anomaly, and topography across the Su bandean belt and Eastern Cordillera. However, an additional 140 km of shortening is required to generate the observed Bouguer anomaly and to pography across the western Andes. This discrepancy could be due to ea rlier shortening in the western Andes or to crustal thickening by magm a addition.