ALBERT

Description: The occurrence of exposed high-metamorphic rocks (granulites) in combination with various gravity anomalies aligned along the direction of Variscan strike characterize a special terrain (Saxothuringikum) which has been sandwiched between two major tectonic units during the Variscan orogeny. Near surface geological studies show evidence that the Saxothuringian zone represents extended crust. Therefore the model of a "metamorphic core complex" is often used to explain the exhumation of the "Saxonian granulites". The thickness of the crust, the geometry of the Moho, and the composition of middle and lower crust that underlie such" metamorphic core complexes" have remained largely unconstrained. Because these physical parameters are critical for understanding the extensional processes acting at depth, we have carried out a seismic refraction experiment in order to resolve the deeper structure of an exposed "granulite-complex". From May 6th to May 13th 1995 a seismic refraction - wide angle reflection experiment was carried out as part of the DFG-priority program: "Orogenic processes – their quantification and simulation at the example of the Variscides". Two lines, A and B, were completed in two deployments (see map in GRANU95_report.pdf). In total 12 shots were fired and over 4500 seismograms were collected using 130 instruments. Only two different types of instruments (Reftek and PDAS) have been used for recording the explosions. All instruments were equipped with a 3-component 1Hz seismometer. The 90 km long NW-SE line (deployment A, 74 instruments) from Leipzig to the Erzgebirge through the Saxonian Granulites was carried out on the 8th and 9th May 1995. Additionally 56 stations were placed symmetrically to shotpoint D along line B (perpendicular to line A). Shots were fired on locations A1, A3, A4, A2 (see map in GRANU95_report.pdf). The station spacing for this deployment was around 1.3 km. The 260 km long SW-NE line (deployment B, 93 instruments) from Dresden to Bamberg, also crossing the Saxonian Granulites was completed from 11th to 13th May 1995. Every second instrument from deployment A was moved to complete line B. Shots were fired on locations B, C, D, E, F, G, H and I (see map in GRANU95_report.pdf) and recorded along line B and line A (perpendicular to line B) at a receiver spacing of about 2.6 km.