Unraveling the role of ancient orogens in present-day rifting using tectonic geomorphology in Shanxi, North China

Abstract

<p>Many rifts are influenced by pre-existing structures and heterogeneities during their evolution, a process known as structural inheritance. During a rift’s evolution, these heterogeneities may aid the nucleation of the rift, growth and segmentation of faults, aid linkage of various segments or even inhibit the formation of faults in various places. Structural inheritance is well explored in offshore rift settings due to the availability of high-quality 3D seismic, which enables good constraint on the structural evolution. However, the degree of structural inheritance in onshore active rifts is more difficult to constrain due to a lack of subsurface datasets. Yet, understanding how structural inheritance influences early rift evolution is vital to better understand seismic risk in areas of active rifting. The Shanxi Graben in the North of China is a densely populated active rift system that is believed to have formed along the trend of the Precambrian Trans North China Orogen. However, the influence of these Precambrian structures on the present-day rifting is poorly constrained. Here we show how the impact of structural inheritance on a young active rift may be investigated using tectonic geomorphological techniques - e.g., hypsometric integral, channel steepness (KsN) and drainage network analysis (chi analysis). Using the geomorphic expression of active faults, we can quantify their geomorphic response and identify faults that show higher levels of activity. Our results show that large basin bounding faults broadly follow the trends of basement fabrics but show a lower geomorphic response, while smaller faults that link the main basins show higher levels of geomorphic response but seemingly crosscut the basement fabrics. We interpret that those large faults formed first in regions with basement fabrics that were preferably orientated to the principal stress direction. Faults in the linkage zones between major basins likely formed later due to local perturbations of the stress field by the major rift faults. This means that there is no need for a changing stress field during the evolution of the Shanxi Graben, as previously proposed, but that the graben evolved under a relatively uniform stress field. Using the hypsometric integral or drainage network analysis may prove useful when applied to other areas with active rifts influenced by structural inheritance such as East Africa. Due to the lack of data in these regions, geomorphic analysis might prove useful in the study of the temporal evolution of structural inheritance in young active rifts.</p>