Constraints on the deep structure and dynamic processes beneath the Alps and adjacent regions from an analysis of gravity anomalies

Abstract

S U M M A R Y Gravity anomalies, including terrain corrections, over the western Alps and the Molasse Basin are within 30 mgal of those calculated assuming local Airy isostatic equilibrium, but isostatic anomalies over regions NW and SE of the Alps exceed 50mgal. Therefore, mass excesses and deficits that must be supported by the strength of the lithosphere or by dynamically induced stress in the asthenosphere appear to be greater beneath the areas surrounding the Alps than beneath the Alpine chain itself. Attempts to account for the gravity gradient over the Molasse Basin and sub-Alpine chains and the small Bouguer anomalies measured over the elevated areas of the Vosges and Black Forest by the flexure of an elastic plate require an absurd set of parameters. The high elevations of the Vosges and the Black Forest are presumably due to high temperature in the underlying uppermost mantle, but the deviations from isostatic equilibrium, as well as the high temperature, probably result from active upwelling in the asthenosphere beneath this area. The absence of large isostatic anomalies over the Alps and the failure of an elastic model to account for the gravity anomalies over the Molasse Basin suggest that the dynamic processes that flexed the European plate down to form the Molasse Basin and that built the Alpine chain have waned. The late Cenozoic uplift of the Molasse Basin and the Alps might be a consequence of a diminution or termination of downwelling of mantle material beneath the Alps.

Erosion-driven uplift of the modern Central Alps

“…As demonstrated by numerical models (Gemmer and Houseman, 2007;Göğüs and Pysklywec, 2008), lithospheric gravitational instabilities potentially induce both rock uplift and subsidence, and may be applied to the Central Alps. Based only on gravity anomalies, Lyon-Caen & Molnar (1989) reached a similar conclusion, and found that recent rapid uplift of the Alps might be the result of the removal of a downward force that maintained the chain in a state of overcompensation. In the Central Alps, a contribution of processes operating at the interface between mantle and lower crust has to be invoked to explain the rock uplift pattern at the southern side of the belt, where denudation exceeds rock uplift.…”

“…=5-20km, Steward and Watts, 1997), or even close to Airy conditions (Lyon-Caen and Molnar, 1989). The slope of the "rock uplift vs. denudation" relationship of the Central Alps (0.88 ± 0.14, Fig.…”

“…These studies explore a whole range of geodynamic processes (or a combination of them), which potentially lead to the vertical motions. Active shortening between the Adriatic Promontory and the Eurasian Plate (Persaud and Pfiffner, 2004), often associated with mantle processes (Lyon-Caen and Molnar, 1989;Kuhlemann, 2007), melting of ice (Gudmundsson, 1994;Stocchi et al, 2005;Barletta et al 2006), or erosion (Guillaume and Guillaume, 1981;Schlunegger & Hinderer, 2001;Cederbom et al, 2004) are invoked to explain the observed rock uplift. The finding that erosion rates are so similar to rock uplift rates let Wittmann et al (2007) to discuss possible causal relationships between uplift and erosion across temporal scales.…”

A quantification of the glacial imprint on relief development in the French western Alps

“…Flexural models for the Molasse foreland basin in Switzerland (e.g., Burkhard and Sommaruga, 1998;Pfiffner et al, 2002) have come up with widely varying equivalent elastic thicknesses between 10 and 35 km. Moreover, these values may be strongly reduced within the core of the orogen, due to both crustal thickening and large bending stresses (Burov and Diament, 1995;Lyon-Caen and Molnar, 1989). In Figure 12, we show calculated spatial distributions of isostatic rebound across the massifs for equivalent elastic thicknesses of 3, 10, and 30 km.…”

Uplift of the Transantarctic Mountains and the bedrock beneath the East Antarctic ice sheet

“…This nonuniqueness can be partlycountered by applying constraints from other data, such as magnetic and seismic. However, before such constraints are applied, the nonuniqueness of predicted structures can also be reduced by considering only density structures that maintain mechanical equilibrium with some reasonable Earth structure [e.g., Lyon-Caen and Molnar, 1989]. The initial model used here is one in which isostatic equilibrium is maintained between East and West Antarctica ( …”