Gravity interpretation of a unified 2-D acoustic image of the central Alpine collision zone

Abstract

Four individual deep seismic reflection profiles across the eastern and southern Swiss Alps have been combined along the trend of the Bouguer gravity anomalies onto the European Geotraverse. The profiles were migrated ray theoretically with the migration velocity model derived from a smoothed reinterpretation of the seismic wide-angle profiles running parallel to the strike of the Alpine arc. The contours of this velocity model were ray theoretically depth migrated together with the combined digitized line drawings of the seismic reflection profiles. The resulting acoustic image depicts the subduction of parts of the lower crust of the European plate beneath the Adriatic promontory of the African plate at a low angle of 15". Based on the interpretation of the seismic data, orogenic crustal thickening is attributed to the stacking of crystalline nappes onto the upper crust of the European plate and wedging of the European and Adriatic middle and lower crusts the latter being ill-constrained by the seismic evidence alone. The south-vergent thrusting of the Southern Alps can be accounted for by the observed downbending of the Adriatic Moho and the lower crust in conjunction with the inferred wedging at mid-crustal levels. Using the geometric constraints provided by seismic data, gravity modelling of the Alpine lithosphere/asthenosphere system relative to stable central European platform clearly favours a gently inclined subduction zone reaching down to at least 200 km depth. Such a gently dipping subduction zone is at odds with prominent models on Alpine geodynamics which favour a near-vertical orientation of the subducting lithosphere ('Verschluckung'). In agreement with the wedging hypothesis indirectly inferred from the seismic data the short wavelength part of the Alpine gravity anomaly requires a middle crust of anomalously high density in the axial zone of the orogen. Both seismic and gravimetric evidence therefore suggest that late-orogenic lithospheric shortening and crustal thickening was governed by the mechanical decoupling of the upper, middle and lower parts of the crust. The amount of subducted lower crustal material cannot be constrained by gravity modelling since its gravity effect cannot be separated from the effect of the subducting slab.

A Lithospheric Cross-Section Through the Swiss Alps-I. Thermokinematic Modelling of the Neoalpine Orogeny

“…The resultant structure at 0 Ma-after 35 Ma evolutionis displayed in Fig. 6b, in comparison with the density structure (Holliger & Kissling 1992). The result shows good overall agreement between the observed and calculated pattern of the crust-mantle and lithosphere-asthenosphere boundaries.…”

“…Within the crust, the structural development is visualized by plotting the deformed boundaries at initial depths of 0 km, 10 km and 20 km. At mid-crustal depths the 'melange zone' of Holliger & Kissling ( 1992), representing material of lower-crustal density at that depth, is largely reproduced by the model. In the upper crust, modelled and observed structures are not comparable, since fault and thrust tectonics at or near the surface are not included in the model.…”

“…However, these models describe only the uplift history and not the whole mountainbuilding process, which is a collage of crustal shortening, thickening and uplift, as well as subduction of mantle lithosphere. In a first attempt these processes were considered by (b) Gravity interpretation along the EGT through the Swiss Alps (after Holliger & Kissling 1992). CE refers to the generalized central European structure, for which we assumed a crustal thickness of 30 km and a lithospheric thickness of 70 10 km (Panza et al 1980;Suhadolc et d. 1990), indicated by the dashed line.…”

Crustal density structure across the Central Indian Shear Zone from gravity data

“…It is well known that neither the average velocities can be unambiguously translated into density values nor there exists a one-to-one correlation between highly reflective seismic layer and density layer boundaries (e.g., Holliger and Kissling, 1992). Holding the configuration of the constrained crustal geometry based on the most trustworthy seismic layer boundaries, the average density of the individual layers was inverted within the logical limits of velocity to match the calculated with the observed gravity anomalies.…”

Determination of sedimentary cover and structural trends in the Central Sinai area using gravity and magnetic data analysis

“…Assuming a relatively constant crustal density, large positive Bouguer anomalies corresponding to thin crust and negative Bouguer anomalies corresponding to thick crust (Wanga et al, 2010). Gravity modelling with several bodies of variable geometry and density is more valuable for excluding rather than proposing structural models (Holliger and Kissling, 1992). In the present analysis gravity and magnetic data are used to determine the depth to the basement surface and delineates the subsurface structural trends affecting the study area.…”