DEEP-STRUCTURE AND EVOLUTION OF THE HARZ MOUNTAINS - RESULTS OF 3-DIMENSIONAL GRAVITY AND FINITE-ELEMENT MODELING

A new Bouguer anomaly map is presented for the region of the entire Ha rt Mountains based on more than 60,000 gravity values. The various gra vity anomalies are discussed and interpretation is carried out by high -resolution 3-D gravity modeling. One of the main subjects of interest in the investigation is the northern boundary fault zone of the Hart Mountains, separating the Mesozoic sediments in the north from the Pal aeozoic rocks of the Hart in the south. Dip and vertical displacement are determined for this fault zone; mean values are 3400 m and 70 degr ees, respectively. Gravity modeling shows that the Brocken and the Ram berg Granites are distinctly different. The Brocken Granite is shallow , whereas the Ramberg Granite has a maximum depth of 8.5 km and a N-S dimension of 37 km. The prominent Benneckenstein Gravity High is expla ined by two different models, one based on a granodioritic intrusion ( 2900 kg/m(3)) with a center-depth of 14 km and the other based on phyl lites (2740 kg/m(3)) on a depth of 3-4 km. Studies on the geodynamic e volution of the Hart Mountains are carried out using the finite-elemen t method. On the basis of a 3-D model, vertical displacements that can be related to horizontal forces are computed. For the period of the V ariscan Orogeny an uplift of 600 m in the Hart area is calculated, for Late Cretaceous and Tertiary 400 m are determined. The total amount o f 1000 m is about 1/3 of the vertical displacement of the northern bou ndary fault zone of the Hart Mountains shown by the gravity modeling. These results do not contradict geological ideas.