LITHOSPHERIC MODELING OF THE ORDOVICIAN FORELAND BASIN IN THE PUNA OFNORTHWESTERN ARGENTINA - ON THE INFLUENCE OF ARC LOADING ON FORELAND BASIN

In a qualitative sedimentological model of the Ordovician basin in the Puna of northwestern Argentina, the temporal and spatial distribution of predominantly marine volcanoclastic facies was interpreted to indi cate a basin evolution from extensionally related subsidence to that d riven by crustal down flexure, Sedimentological data were taken to ref lect a Middle Ordovician eastward thrusting of an extinct Early Ordovi cian are over part of its back-are basin in the course of the collisio n of the allochthonous Arequipa Massif terrane with the South American margin. To test the original qualitative sedimentological interpretat ion of the Puna basin, an infinite beam elastic plate loading model wa s applied using the constraints of the field data. Contrary to the ori ginal interpretation, results show that the Puna basin can be modeled as resulting entirely from the effects of lithospheric loading caused by the progressive contruction of a static load, i.e., a volcanic are. Two loading events, one in the Early Ordovician, and one in the Llanv irnian, with tectonic load thicknesses of 6000 and 2000 m, respectivel y, best explain the observed basin geometry, The onlap patterns on the distal margin on the peripheral bulge are reproduced well using a fle xural rigidity of 10(22) N m representing an equivalent elastic thickn ess of 14 km of the loaded plate, Higher values for the flexural rigid ity are considered unlikely, as they result in an enhanced peripheral bulge which uplifts marine near-shore deposits on the distal margin to unrealistic subaereal positions. The model results fit a foreland bas in development where crustal thickening was caused by the construction of a static volcanic are on a previously passive margin, Modeling the movement of thrust loads over parts of the basin according to traditi onal models of foreland basin formation in this case leads to model re sults which are inconsistent with field data, even when moderate thrus t rates are considered.