Reconstruction of Cretaceous rifts incorporated in the Outer West Carpathian wedge by balancing

Abstract

Two balanced cross-sections were constructed through the Outer Carpathians of Poland in order to restore the Early Cretaceous rifts. The rift fill is incorporated into the Tertiary Carpathian accretionary wedge. The data compiled for this study confirm that the Early Cretaceous rifting in the Silesian Basin area was followed by Late Cretaceous–Paleocene basin inversion, Eocene pelagic deposition, and Oligocene syn-orogenic deposition. The original width of the Silesian Basin is about 130–138 km. The overall wedge shortening ranges between 31 and 58% with 57 and 58% in the Silesian Nappe. The rifting commenced at the Jurassic/Cretaceous boundary and formed the horst and graben structures defined by NW–SE striking normal faults. Portions of the horsts become emergent at various times during rifting stages. Sedimentation rates in the grabens varied around 4.7, 2.1 and 1.3 cm/ka during three rifting stages, while sedimentation rates on their slopes varied around 0, 0–1.26 and 0 cm/ka. The altitude difference between horst tops and graben floors did not exceed 2 km. The reconstructed widths of the horsts range between 17.5 and 18.3 km. The reconstructed widths of several grabens in both cross-sections have values 14.5, 45.7, 57.7 and 79.3 km. The rifting-related extension, calculated from fault-striae data, was NE–SW directed. Sometimes, the very low ( σ 2– σ 3)/( σ 1– σ 3) stress ratio of the driving stress configuration resulted in polydirectional extension when the value decreased below 0.1.