Geometry of alpine overdeepenings assessed with gravimetry and 3-D modelling
Authors: Bandou Dimitri , Schläfli Patrick , Schwenk Michael , Douillet Guilhem A. , Kissling Edi , Marti Urs , Schlunegger Fritz 
Published: March 2021

&lt;p&gt;Interpretations of the processes leading to the formation of overdeepened valleys, where the bedrock lies well below sea level today, are contested as the overdeepenings have been filled by sediments or host lakes making observations difficult. Here, we combine gravimetric, GNSS (Global Navigation Satellite System) and borehole data within a 3D forward modelling framework (Gravi3D) to assess the 3-D subsurface geometry of such overdeepenings in the Swiss plateau, to the North of the Alps. Gravi3D has two components (PRISMA and BGPoly), which allow to obtain analytically the gravity effect of prisms and polygons (Nagy (1966) and Talwani &amp; Ewing (1960)). PRISMA allows first to estimate the spatial extent of an overdeepening and the density contrast between the overdeepening fill and the bedrock. In contrast, BGPoly is designed to disclose the details of a complex 3-D geometry of an overdeepening fill through an approximation of its shape with polygons. Gravi3D will be open access and is designed for a larger scientific community.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;br&gt;Here, we focus on overdeepenings beneath two valleys, the Aare valley and the G&amp;#252;rbe valley to the South of Bern. In this region, the occurrence of overdeepenings has already been disclosed through drilling, but the details about the geometry have not been elaborated yet. The study region is characterized by three mountain ranges oriented North-South and comprises Burdigalian Upper Marine Molasse bedrock. The G&amp;#252;rbe and Aare valleys in-between are c. 300 m-deep and c. 1 km-wide, where overdeepenings with a &gt;100 m-thick Quaternary fill have already been identified by drilling. The gravity data collected along an 8 km-long profile with stations spaced between 100 and 300 m yield a Bouguer anomaly that ranges from c. -99 to -106 mGal. We relate this anomaly to the regional trend (c. 2 mGal over 8 km) and to the effect of the overdeepenings&amp;#8217; sedimentary fillings (2 &amp;#8211; 4 mGal/km), disclosing a sharp anomaly pattern over the inferred tunnel valleys. The removal of the signal related to the regional trend results in a residual anomaly of c. 1 mGal for the bedrock ridge in-between the valleys (Belpberg mountain), and of -2.65 and -3.56 mGal for the G&amp;#252;rbe and Aare valley overdeepenings, respectively. We observe a steeper gravity gradient for the Eastern flank of both overdeepenings. The use of Nettleton method to model the residual gravity anomaly across Belpberg yields a density of 2.5 g/cm&lt;sup&gt;3&lt;/sup&gt; for the Molasse bedrock. In addition, the estimation of the largest gravity response through the overdeepening fill, calculated with Prisma yields a density value of c. 2.0 &amp;#8211; 2.2 g/cm3 for the Quaternary sediments. As a further information, Prisma predicts a maximum thickness of 140 m for the Quaternary suite beneath the G&amp;#252;rbe valley and at least 200 m beneath the Aare valley. This yields a minimum slope of approximately 18&amp;#176; for the G&amp;#252;rbe overdeepening.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;REFERENCES&lt;/p&gt;&lt;p&gt;Nagy, D.: The gravitational attraction of a right rectangular prism. Geophysics 31, 362&amp;#8211;371, 1966.&lt;/p&gt;&lt;p&gt;Talwani, M., Ewing, M.: Rapid computation of gravitational attraction of three&amp;#8208;dimensional bodies of arbitrary shape. Geophysics 25, 203&amp;#8211;225, 1960.&lt;/p&gt;