Tools to analyse misleading kinematic interpretations of faults offsetting inclined or folded surfaces: Applications to Asturian Basin (NW Iberian Peninsula) examples

Abstract

When working with geological maps and cross-sections, and no kinematic criteria on fault motion are available, we usually classify the type of fault (dip-slip, strike-slip, oblique) according to the observed separation of planar markers, usually bedding. Although in many cases the separation agrees with fault slip, and therefore, the deduced type of fault is correct, these deductions can be completely wrong in certain circumstances. Amongst other parameters, the observed separation depends on the angular relationship between the cut-off line of the planar marker on the fault plane and the slip vector. In this paper, we elaborate on this relationship and present a tool to facilitate classifications of faults whose motion does not involve rotation of the fault blocks, and cut and offset previously tilted or folded surfaces. This tool consists of graphs that predict how the observed separation will be in cross sections and maps. In addition, the influence of different parameters on the magnitude of the separation in a geological section across a fault is discussed. The validity of the presented tool is demonstrated through its application to two field examples of strike-slip faults offsetting inclined layers and a fold train in the Asturian Basin. Our graphs can be used to improve geological interpretations, in a predictive way when constructing geological maps and cross-sections, and to decipher the existence of layers tilted or folded before fault development.