Tectonic geomorphological investigations of antiforms using differential geometry: Permam anticline, northern Iraq

Abstract

Modern differential geometric methods are applied to compute and analyze curvature quantities of the Permam anticline, which is part of the Zagros fold and thrust belt in northern Iraq, northeast of the city of Erbil. Because this particular anticline is composed of, among others, weathering-resistant limestones, the surface topography strongly resembles the antiformal fold shape. This makes it an ideal area where numerical curvature analysis, applied to Shuttle Radar Topography Mission digital elevation models (DEMs), allows drawing not only geomorphological, but also tectonic, conclusions. The curvature analysis is based on the computation of the Gaussian and mean curvatures and is used to classify the folded surface into eight geologically relevant shapes (antiform, synform, plane, dome, basin, and three types of saddles). The performed curvature analysis investigates in detail the effects of two adaptable parameters: the cutoff wavelength of the low-pass filter that is applied to the DEM before curvature calculation and the curvature threshold that is applied to the principal curvature values before the calculation of the Gaussian and mean curvatures. The analysis demonstrates that these two parameters strongly influence each other, and that they together determine the information content and interpretability of the results. By choosing appropriate parameter combinations, geomorphological-oriented studies and tectonic-oriented studies are viable using the same DEM. How tectonic-oriented studies can be used to determine fracture patterns or densities in folded layers is discussed. Further suggestions are made to combine curvature analysis of DEMs with similar studies applied to seismically mapped surfaces in three-dimensional seismic data sets where erosion is absent.