Spatiotemporal Growth of Seismic-Scale Syn-Flexural Normal Faults in the German Molasse Basin

Abstract

Flexure in pro-foreland basins results from the interplay between (sub)surface loading, foreland plate strength, inherited crustal architecture, and the degree of plate coupling. It is expected that lateral variations in these controlling mechanisms will result in along-strike variations in the flexural profile of the foreland basin. This will directly influence the position and width of the forebulge, thereby altering the associated extensional stress field in space and time around which syn-flexural normal faults accommodate deformation. As such, spatiotemporal variations in the growth of the syn-flexural normal faults in foreland basins may provide valuable information regarding the evolution of an orogen-foreland basin system. However, the relation between syn-flexural normal fault growth and the mechanisms controlling foreland basin flexure remains underexplored. Here, we quantify lateral and vertical throw distributions for growth strata of syn-flexural normal faults in the German Molasse Basin. This allowed us to develop a 4D fault growth model. Our results indicate that the flexure in the German Molasse was associated with both the nucleation of new faults and selective reactivation of pre-flexural faults, with the latter depending on fault burial depth at the onset of flexure. Furthermore, our results suggest that localisation of the extensional strain and deformation at the top of the European plate during flexure controlled the nucleation site of the syn-flexural normal faults in the German Molasse. Additionally, the spatiotemporal variation in the onset of syn-flexural normal fault activity suggests a northward migration rate of 7.8 mm/year of the orogen-foreland basin system. This is consistent with previous estimates based on other independent methods. Lastly, a west-to-east increase in cumulative syn-flexural offsets down-dip the normal faults in the German Molasse Basin may have been controlled by orogen-parallel lithospheric strength variations in the downgoing European plate.