Combined automatic fault mapping and geodesy to investigate the spatial and temporal evolution of tectonic strain across time scales: an application to the Afar rift (East Africa)

Continental rift systems are characterised by spatial and temporal changes in the style (distributed vs. focused), location and mechanisms (magmatic vs. tectonic) of plate spreading. Understanding the long-term evolution of continental rift systems thus requires investigation of magmatic and tectonic processes across the spatial and temporal scales. However, this understanding is limited by relatively short temporal coverages of geophysical techniques and by spatially discontinuous geological datasets. Detailed maps of rift structures (i.e., tectonic faults), combined with independent geophysical and geological observations are key for a thorough view on the long-term evolution of strain during rifting.

In this study, we developed a novel method for the automatic extraction of faults and the calculation of time-averaged strains using Digital Elevation Models. We extended the Python-based Fault Analysis Toolbox (Fatbox) developed by Wrona et al. (2022) by implementing new filters, and building up a novel workflow for analysing fault-related deformation, such as the horizontal extension and the second invariant of strain. In Fatbox, the extraction of linear elements, such as faults, is performed through edge detection algorithms that can be applied on several type of data (e.g., seismic profiles, analogue and numerical models, and DEMs). Faults are then distinguished from noise using a normalized scale-dependent linearity filter that considers the area covered by linear elements. Dense displacement measurements are finally obtained at the scale of individual fault-scarp portions and converted to maps of strain or horizontal extension. A comparison with manually mapped datasets indicate that our method successfully resolves 93.4% of the total strain.

We applied this method to investigate a ~330 x 275 km-wide area in the Afar rift (East Africa), the locus of the spreading of Nubian, Arabian and Somalian plates. Rifting in Afar began approximately 31 Myrs ago after the impingement of a mantle plume, the eruption of flood basalts (Stratoid Series), and is currently accommodated along three main rift branches. The Stratoid series has covered fault scarps, which resets fault scarps and thereby provides an essential time marker for our strain analysis.

We combined our data with literature rock dating and geodetic measurements to reconstruct the evolution of the rift during the last 4.5 Ma and its relationship with tectonic and magmatic activity. We showed that the margins of the central Afar rift have been abandoned, and rifting processes have migrated toward todays axis where increased strain rates are likely due to magmatic emplacement. A northwest-directed increase of strain suggests a progressive migration of the rifting process in the same direction, responding to the Danakil block rotation. Conversely, the southern portion of Afar shows two systems of cross-cutting faults that respond to different co-acting tensional forces induced by the separations of the Arabian and Somalian plates from Nubia (Maestrelli et al., 2024).

References

Wrona, et al. (2022) Fatbox - Fault Analysis Toolbox, https://doi.org/10.5880/GFZ.2.5.2022.002

Maestrelli, et al. (2024). Reconciling plate motion and faulting at a rift-rift-rift triple junction, Geology, 1–5, https://doi.org//10.1130/G51909.1