Structural Evolution of the German Alpine Molasse Basin
The Alpine Molasse Basin formed subsequent to the Mesozoic to Tertiary continent-continent collision and formation of the Alpine fold-and-thrust belt. The accommodation space generated by the down-bending European plate was filled with sediments of Oligocene to Upper Neogene age. Tectonically the Molasse Basin has been separated into an undisturbed (autochthonous) and disturbed (allochthonous) unit, in some areas with a transition zone - the par-autochthonous.
Several oil and gas fields were discovered in the basin in the last decades. Recent exploration focused on the German part, west of Munich. A number of new 2D and 3D seismic data were acquired. Surface geology and interpretation on seismic lines were used to perform section balancing on seismic 2D lines. Restoration of the major thrusts reflect that at least two stages of thrusting sequences occurred: (1) in-sequence thrusting generated foreland-/north vergent duplex thrusts with formation of triangles and (2) out-of-sequence thrusting, as well with north-vergent thrusts.
The presence of triangles is visible in the par-autochthonous unit but can also be inferred for the allochthonous units. Restoration of major thrusts showed that duplex structures are present before thrusting and were dissected by the later out-of-sequence thrusting. Several duplexes were modelled in order to obtain the structures which are required prior to out-of-sequence thrusting. Out-of-sequence thrusting is obvious on seismic. Structures like fault-bend-folding which are present in the foot-wall of a thrust are not visible in the hanging-wall. Regarding the structural evolution the following picture can be drawn: First detachments occurred along the base of the Molasse sediments. Duplex thrusts and flat-ramp-flat structures generated several triangle zones. Thrusting happened in-sequence from south to north. Only early Molasse sediments (up to early Lower Freshwater Molasse) are incorporated into thrusting. The following phase is marked by thrusts reaching shallower levels. Restoration showed that these thrusts occurred successively out-of-sequence from north to south.
The driving force for the out-of-sequence thrusting is assumed to be a movement of the critical wedge from north to south. Establishing of a stable surface drainage system led to increased erosion in the southern part of the basin. Flattening of the critical wedge resulted in relaxation in the overburden and thus thrusting was able to develop again in this area.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009