TECTONOSTRATIGRAPHIC MODEL FOR UNDERFILLED PERIPHERAL FORELAND BASINS- AN ALPINE PERSPECTIVE

Advances in the development of quantitative models of foreland basin s tratigraphy have outpaced the observational data used to constrain the input parameters in such models. Underfilled peripheral foreland basi ns comprise a broad threefold subdivision of depositional realms that translates into three stratigraphic units which are commonly superimpo sed during basin migration; these units are here termed the ''underfil led trinity.'' The three units of the trinity reflect (1) carbonate de position on the cratonic margin of the basin (the lower unit), (2) hem ipelagic mud sedimentation offshore from the cratonic margin of the ba sin (the middle unit), and (3) deep water turbiditic siliciclastic sed imentation toward the orogenic margin of the basin (the upper unit). T heoretical predictions of how such a complex basin fill initiates and evolves through time are not currently available; hence this study rev iews the stratigraphy of underfilled peripheral foreland basins and pr os ides a unique data set comprising rates of thrust advance and basin fill migration for the Tertiary foreland basin of the European Alps. The Paleocene to Oligocene Alpine foreland basin of France and Switzer land comprises a well-developed underfilled trinity that is preserved within the outer deformed margins of the Alpine orogen. Structural res torations of the basin indicate a decrease in the amount of basin shor tening from eastern Switzerland (68%) to eastern France (48%), to sout heastern France (35%). Structurally restored chronostratigraphic diagr ams allow rates of basin migration to be calculated from around the Al pine are Paleogeographic restorations of the Nummulitic Limestone (lon er unit) illustrate a radial pattern of coastal onlap on to the Europ ean craton. Time-averaged rates for northwestward coastal onlap of the underfilled Alpine basin across Switzerland were between 8.5 and 12.9 mm/yr. Time-equivalent westward to southwestward coastal onlap rates in France were between 4.9 and 8.0 mm/yr. The direction of migration o f the cratonic coastline of the basin was parallel to the time-equival ent thrust motions, and oblique to the Africa-Europe plate motion vect or. By comparing rates of thrust propagation into the orogenic margin of the basin to rates of coastal onlap of the cratonic mar gin of the basin, it is possible to suggest that the Alpine foreland basin of cen tral Switzerland migrated with an approximately steady state geometry for at least 210 km northwestward over the European craton. The westwa rd and southward decrease in the basin migration rate around the Alpin e arc was associated with an increase in the degree of syndepositional normal faulting on the European plate; this is thought to relate to t he opening of the Rhine-Bresse-Rhone graben system.