The early onset of magmatic rift faulting in the Edward-George Rift, Uganda

Abstract

Fault systems along slow-spreading mid-ocean ridges have a characteristic morphology including clustered, closely spaced, short faults, which emerges during the preceding phase of magma-assisted continental rifting and breakup. Current observations of faulting in magmatic rifts are, however, limited to mature magma-rich rifts where the early phases of rifting are obscured by later sedimentary and volcanic deposits. Thus, it is unclear how and when fault networks first evolve in magmatic rifts. Here, we investigate the Edward-George Rift, Uganda, where crustal thinning is minimal and surface volcanism is less than 200 ka. Using high resolution topography, we map and compile a database of 152 active faults, and systematically measure scarp heights along each fault. A region of high density (up to 2 km/km2) short (79% <5 km) intrabasin faults, with strikes rotated ∼45° from the regional tectonic stress field, is co-located with a mid-crustal magma body. Scarp height-length profiles demonstrate that 81% of faults experience laterally-restricted propagation and appear to propagate away from magmatic bodies. Magmatism influences fault growth by reducing the differential and effective stresses, and/or changing the material properties of the crust surrounding the faults. Comparisons with other fault systems suggests that fault density increases as rifting evolves, but that high-density fault networks dominated by short faults are formed rapidly at the onset of magmatic rifting and persist until sea-floor spreading.