GRAVITY MODELING OF THE LITHOSPHERE IN THE EASTERN ALPINE WESTERN CARPATHIAN PANNONIAN BASIN REGION

Gravity models illustrate changes in the degree of continental converg ence in the Eastern Alpine-Western Carpathian region, and modification s to the lithosphere due to the plate convergence and subsequent Panno nian Basin extension. Analysis of the continental collision zone incor porates a kinematic model of ocean basin closure, whereby gravity anom alies and topography are viewed as part of a continuum of continental crustal shortening, erosion and isostatic rebound. Thick crust and hig h topography in the Eastern Alps, along with a broad Bouguer anomaly o f - 140 mGal amplitude, are consistent with about 175 km of crustal sh ortening, followed by 10 km of isostatic rebound. Eastward, crustal th icknesses 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 dep osits and small isostatic rebound arc attributable to the high-density , shallow mantle of the intact continent-ocean transition zone. Seismi c delay time studies have suggested that, relative to the average thic kness of the region, the lithosphere thickens by about 70 km beneath t he Eastern Alps and thins by about 60 km under the Pannonian Basin. In both regions, gravity anomalies cannot be explained fully without con sidering 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 g ravity anomalies that arc 40 mGal lower than those observed; the extra 70 km of lithosphere provides excess mass that achieves isostatic equ ilibrium and accounts for the 40 mGal difference. Observed gravity ano malies and local isostasy arc also consistent with thin crust and thin lithosphere beneath the Pannonian Basin, whereby the 60 km of extra a sthenosphere provides a large part of the compensation for the elevate d mantle. Regional cross sections suggest that shallowing of the litho sphere/asthenosphere boundary, associated with Pannonian Basin extensi on, has propagated northward beyond the Carpathians, to within the Eur opean Platform. Crustal thinning, however, appears to be confined to e xotic terranes of the Carpathian interior, so that crustal structure i n the Eastern Alps and Outer Carpathians is a remnant of the earlier c ollision orogen.