Gravity modelling of the lithosphere in the Eastern Alpine-Western Carpathian-Pannonian Basin region

Abstract

Gravity models illustrate changes in the degree of continental convergence in the Eastern Alpine-Western Carpathian region, and modifications to the lithosphere due to the plate convergence and subsequent Pannonian Basin extension. Analysis of the continental collision zone incorporates a kinematic model of ocean basin closure, whereby gravity anomalies and topography are viewed as part of a continuum of continental crustal shortening, erosion and isostatic rebound. Thick crust and high topography in the Eastern Alps, along with a broad Bouguer anomaly of −140 mGal amplitude, are consistent with about 175 km of crustal shortening, followed by 10 km of isostatic rebound. Eastward, crustal thicknesses and gravity anomaly widths and amplitudes are less, so that only about 50 km of continental crustal shortening and 4 km of rebound occurred in the Western Carpathians. Preservation of thick flysch deposits and small isostatic rebound are attributable to the high-density, shallow mantle of the intact continent-ocean transition zone. Seismic delay time studies have suggested that, relative to the average thickness of the region, the lithosphere thickens by about 70 km beneath the Eastern Alps and thins by about 60 km under the Pannonian Basin. In both regions, gravity anomalies cannot be explained fully without considering this large relief on the lithosphere/asthenosphere boundary. The Eastern Alpine crustal root, which extends 15 km below the average depth for the region, overcompensates the topography and results in gravity anomalies that are 40 mGal lower than those observed; the extra 70 km of lithosphere provides excess mass that achieves isostatic equilibrium and accounts for the 40 mGal difference. Observed gravity anomalies and local isostasy are also consistent with thin crust and thin lithosphere beneath the Pannonian Basin, whereby the 60 km of extra asthenosphere provides a large part of the compensation for the elevated mantle. Regional cross sections suggest that shallowing of the lithosphere/asthenosphere boundary, associated with Pannonian Basin extension, has propagated northward beyond the Carpathians, to within the European Platform. Crustal thinning, however, appears to be confined to exotic terranes of the Carpathian interior, so that crustal structure in the Eastern Alps and Outer Carpathians is a remnant of the earlier collision orogen.