The three-dimensional structure of the oberpfalz- an alternative interpretation of the dEKORP-KTB data

A grid of deep seismic reflection profiles recorded in the Oberpfalz, West Germany for DEKORP and the West German Deep Drilling Project (KTB) provides an opportunity to study the three-dimensional structure of a small part of the Variscan fold belt. The grid straddles the Erbendorf Line (the surface expression of the suture between the Moldanubian and the Saxuthuringian zones of the Variscides), and the edge of the Bohemian Massif, marked by the Franconian Line. The principal features that can be recognised on the data are a series of gently SE-dipping reflections that may be thrusts, one of which seems to correspond to a high-velocity zone that is one of the specific targets of the deep borehole. This thrust has been traced as a characteristic reflection and mapped out over five of the profiles, including the intersection that is the proposed drill site. It appears to be disrupted by steep NW-SE trending faults that are interpreted as dextral strike-slip faults, expressed, in effect, as positive and negative flower structures. The pattern of transpression and transtension shown by these flowers is consistent with dextral shear parallel to the Bayerischer Pfahl, a dextral shear zone more than 150 km long.