Abstract

Abstract. We use three-dimensional seismic reflection data from the southern German Molasse Basin to investigate the structural style and evolution of a geometrically decoupled fault network in close proximity to the Alpine deformation front. We recognise two fault arrays that are vertically separated by a clay-rich layer – lower normal faults and upper normal and reverse faults. A frontal thrust fault partially overprints the upper fault array. Analysis of seismic stratigraphy, syn-kinematic strata, throw distribution, and spatial relationships between faults suggest a multiphase fault evolution: (1) initiation of the lower normal faults in the Upper Jurassic carbonate platform during the early Oligocene, (2) development of the upper normal faults in the Cenozoic sediments during the late Oligocene, and (3) reverse reactivation of the upper normal faults and thrusting during the mid-Miocene. These distinct phases document the evolution of the stress field as the Alpine orogen propagated across the foreland. We postulate that interplay between the horizontal compression and vertical stresses due to the syn-sedimentary loading resulted in the intermittent normal faulting. The vertical stress gradients within the flexed foredeep defined the independent development of the upper faults above the lower faults, whereas mechanical behaviour of the clay-rich layer precluded the subsequent linkage of the fault arrays. The thrust fault must have been facilitated by the reverse reactivation of the upper normal faults, as its maximum displacement and extent correlate with the occurrence of these faults. We conclude that the evolving tectonic stresses were the primary mechanism of fault activation, whereas the mechanical stratigraphy and pre-existing structures locally governed the structural style.

Highlights

  • In the last decade, there has been an increasing interest in foreland basins because some of them contain deep aquifers that host geothermal resources (e.g. Schulz et al, 2004; Weides and Majorowicz, 2014)

  • The vertical stress gradients within the flexed foredeep defined the independent development of the upper faults above the lower faults, whereas mechanical behaviour of the clay-rich layer precluded the subsequent linkage of the fault arrays

  • Such deformation patterns show that the basin were subject to a variety of stress states that develop during the lithospheric flexuring, subsidence, and sedimentation as the orogenic front progresses forward

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Summary

Introduction

There has been an increasing interest in foreland basins because some of them contain deep aquifers that host geothermal resources (e.g. Schulz et al, 2004; Weides and Majorowicz, 2014). There has been an increasing interest in foreland basins because some of them contain deep aquifers that host geothermal resources Foreland basins have complex deformation structures that range from normal faults towards the foreland to contractional and inverted faults near the orogenic front (DeCelles and Giles, 1996; Tavani et al, 2015). Such deformation patterns show that the basin were subject to a variety of stress states that develop during the lithospheric flexuring, subsidence, and sedimentation as the orogenic front progresses forward. The resultant composite structural history can be correctly deciphered using a three-dimensional approach, as can be derived from three-dimensional seismic datasets

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