How Do Variably Striking Faults Reactivate During Rifting? Insights From Southern Malawi

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 20; no. 7; pp. 3588 - 3607
• Main Authors: Williams, Jack N., Fagereng, Åke, Wedmore, Luke N. J., Biggs, Juliet, Mphepo, Felix, Dulanya, Zuze, Mdala, Hassan, Blenkinsop, Thomas
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.07.2019
• Subjects: continental rift; Direction; Earth; Earth crust; East African Rift; Fault lines; fault reactivation; Fault zones; Faults; Fractures; Geological time; Joints (timber); normal faults; Obliquity; Orientation; Rifting; stress inversions; tectonic stress; Trends; Malawi
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2019GC008219

Crustal extension is commonly thought to be accommodated by faults that strike orthogonal and obliquely to the regional trend of the minimum compressive stress (σ3). Activation of oblique faults can, however, be conceptually problematic as under Andersonian faulting, it requires preexisting crustal weaknesses, high fluid pressures, and/or stress rotations. Furthermore, measurements of incremental fault displacements, which are typically used to identify oblique faulting, do not necessarily reflect regional stresses. Here, we assess oblique faulting by calculating the stress ratio (σ3/σ1, where σ1 is the maximum compressive stress), slip tendency, and effective coefficient of friction (μs′) required to reactivate variably striking normal faults under different trends of σ3. We apply this analysis to NW and NNE striking active faults at the southern end of the Malawi Rift, where NE‐SW, ENE‐WSW, E‐W, and SE‐NW σ3 trends have previously been proposed. A uniform σ3 trend is inferred for this region as recent joints sets do not rotate along the rift. With a NE‐SW trending σ3, NW‐striking faults are well oriented, however, NNE‐striking faults require μs′ < 0.6 to reactivate. This is inconsistent with a lack of frictionally weak phyllosilicates detected in the fault zone rocks. With an ENE‐WSW to E‐W trending σ3, all faults can reactivate at μs′ > 0.55. These σ3 trends are also comparable to a focal mechanism stress inversion, regional joint orientations, and previously reported geodetically derived extension directions. We therefore conclude that unlike typical models of oblique rifting, the southern Malawi Rift consists of faults that all strike slightly oblique to σ3. Plain Language Summary Stretching of the upper brittle part of the Earth's crust should be accommodated by fractures (faults) oriented at 90° to the stretching direction. However, this idealized scenario is rarely observed because of crustal heterogeneities, or because the stretching direction rotates over geological time. Thus, faults are often nonorthogonal (i.e., oblique) to the stretching direction. Here, we use a mechanical analysis to test the obliquity of faults in southern Malawi at the southern juvenile end of the East African Rift system where the crust is actively extending at ~2 mm/year. This section is of interest as fault orientation varies along the rift, and a range of stretching directions have been proposed previously. Our mechanical analysis indicates that extension is most likely accommodated in southern Malawi by faults that are all slightly oblique to an ENE‐WSW to E‐W stretching direction. This is in contrast to previous models of oblique extension, which suggest that stretching is accommodated by some faults at 90° to the stretching direction, while others are at a very low (<40°) angle to stretching. Key Points Proposed stress states for the southern end of the Malawi Rift are tested by assessing fault reactivation potential Variably oriented faults reactivate by all striking slightly obliquely to an ENE‐WSW to E‐W trending minimum principal compressive stress Faults may locally accommodate pure normal dip slip due to the presence of a deep seated crustal weakness