~.-" ~, . ~. ~ ~ ~'i "~ ....i ~..k- ~ .'¢,' ~i~i ( ~.'~:' .~ ELSEVIER Tectonophysics259 (1996) 245-258 TECTONOPHYSICS Lithospheric modeling of the Ordovician foreland basin in the Puna of northwestern Argentina: on the influence of arc loading on foreland basin formation Heinrich Bahlburg *, Kevin P. Furlong Department of Geosciences, 503 Deike Building, The Pennsyluania State University, University Park, PA 16802, USA Received 17 November 1994; accepted 20 September 1995 Abstract 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 indicate a basin evolution from extensionally related subsidence to that driven by crustal down flexure. Sedimentological data were taken to reflect a Middle Ordovician eastward thrusting of an extinct Early Ordovician arc over part of its back-arc basin in the course of the collision of the allochthonous Arequipa Massif terrane with the South American margin. To test the original qualitative sedimentolog- ical interpretation of the Puna basin, an infinite beam elastic plate loading model was applied using the constraints of the field data. Contrary to the original 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 arc. Two loading events, one in the Early Ordovician, and one in the Llanvirnian, with tectonic load thicknesses of 6000 and 2000 m, respectively, best explain the observed basin geometry. The onlap patterns on the distal margin on the peripheral bulge are reproduced well using a flexural rigidity of 1022 Nm representing an equivalent elastic thickness of 14 km of the loaded plate. Higher values for the flexural rigidity 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 basin development where crustal thickening was caused by the construction of a static volcanic arc on a previously passive margin. Modeling the movement of thrust loads over parts of the basin according to traditional models of foreland basin formation in this case leads to model results which are inconsistent with field data, even when moderate thrust rates are considered. 1. Introduction The development of foreland basins is generally regarded as connected to crustal loading by moving * Corresponding author. Present address: Geologisch- Pal~iontologisches Institut, Universitiit Heidelberg, Im Neuen- heimer Feld 234, 69120 Heidelberg,Germany. thrust wedges (e.g., Beaumont et al., 1982; Flemings and Jordan, 1989). The geometry of the basin and its sedimentary fill is determined by the interplay of a variety of factors, among them most prominantly the thickness and extent of the orogenic wedge, the rate of thrust advance, the flexural rigidity of the overri- den plate and the input to and distribution of sedi- ment in the basin (Flemings and Jordan, 1989; Jor- 0040-1951/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0040- 195 1(95)001 29-8