The usefulness of applying morphometric analyses in intrabasinal faults: the Galera Fault (central Betic Cordillera, S Spain)

Here we prove the usefulness of applying morphometric analyses, typically used in basin-border faults, to evaluate the geomorphic expression of an intrabasinal structure. The target fault of our study is the Galera Fault, a SW-NE, ca. 30 km-long fault located in the Guadix-Baza Basin (central Betic Cordillera, southern Spain). This fault is characterized by low displacement rates, with a major left-lateral (0.5±0.3 mm/yr) and minor (0.02-0.05 mm/yr) vertical slip components. Moreover, the Galera Fault cuts across the Plio-Quaternary basin infilling, so poorly-lithified sedimentary rocks crop out in both fault blocks.

Since the Guadix-Baza Basin was captured in the Middle Pleistocene, it has been dominated by extensive erosion, which has shaped a very young landscape influenced by the activity of the Galera Fault. To evaluate the imprint of the fault on the landscape, we carried out an analysis of the topography and the drainage network from high-resolution digital elevation models (DEMs). In addition, we apply different geomorphic indices, such as the profile relief ratio (PRR), the normalized channel steepness index (ksn), the asymmetry factor (AF), and the valley floor width-to height ratio (Vf).

Our study evidence that the combination of low slip rates and the high erodibility of the juxtaposed rocks favors a rapid landscape response to fault displacement that erases many landscape effects related to active tectonics. This masking is more effective on features generated by strike-slip displacement, leaving only subtle evidence, such as local stream deflections and upstream widening of catchments. In contrast, geomorphic effects related to vertical displacement are better preserved, including the control of the geometry of the main rivers and morphological differences in the drainage network between the two fault blocks. On the upthrown block, streams are generally shorter, steeper and valley incision is more accentuated. These differences between fault blocks are reflected in the development of an impressive badland landscape that is restricted to the upthrown block.

Slow intrabasinal faults can be difficult to detect in studies involving structural mapping, seismic hazard assessment, or exploration of resources, especially when they offset highly erodible deposits and do not present a marked uplift. However, here we demonstrate that the geomorphic anomalies that these structures can leave on the landscape can be identified by applying a proper morphometric analysis.