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

Abstract

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.

Active Fault Scarps in Southern Malawi and Their Implications for the Distribution of Strain in Incipient Continental Rifts

“…An extension azimuth of 072° is recorded geodetically at Zomba (Figure ; Stamps et al, ) in contrast to the NW‐SE extension in the Zomba Graben. This variation in extension direction is consistent with previous observations of deviation between local and regional extension directions in Malawi (Delvaux & Barth, ; Hodge et al, ; Williams et al, ).…”

“…This contradicts previous rift‐wide estimates of extension (Delvaux & Barth, ; Figure ). However, these estimates likely reflect that normal faulting events in the Malawi Rift are approximately purely dip‐slip despite regional changes in fault strike (Williams et al, ), as also indicated by the 2009 Karonga earthquakes (Biggs et al, ), the 1989 Salima earthquake (Jackson & Blenkinsop, ), and a M w = 5.6 earthquake in March 2018 (red focal mechanism in Figure b). An extension azimuth of 072° is recorded geodetically at Zomba (Figure ; Stamps et al, ) in contrast to the NW‐SE extension in the Zomba Graben.…”

“…The onshore rift south of Lake Malawi consists of three linked half‐grabens (Figure , Williams et al, ). The border faults have escarpment heights of <1,000 m and so are less prominent than those within the lake (Laó‐Dávila et al, ; Figures c–e).…”

“…Slickensides along the Chingale Step fault (Figures b–d; plunging 52° towards 301°) and kinematic indicators on faults elsewhere in the Zomba Graben (Bloomfield, ; Chorowicz & Sorlien, ) indicate an approximately NW‐SE extension direction (Williams et al, ). This extension is orthogonal to the surface traces (015 ± 11°) of the five faults, indicating a generally dip‐slip displacement (Williams et al, ). This contradicts previous rift‐wide estimates of extension (Delvaux & Barth, ; Figure ).…”

“…In contrast, the surface expression of a rift—whether faults form symmetric grabens or asymmetric half‐grabens and the length, orientation, and segmentation of major faults—is controlled by crustal rheology (Hodge et al, ; Huismans & Beaumont, ). Within the EARS, there is evidence for both narrow and wide rift modes ( sensu Buck, ; Ebinger & Scholz, ), symmetric and asymmetric grabens (Ebinger et al, ; Laó‐Dávila et al, 2015), and a range of relationships between fault strike, metamorphic foliation, and regional stresses (Ring, ; Wheeler & Karson, 1989; Dawson et al, ; Hodge et al, ; Williams et al, ). The lack of constraints on strain distribution, geochronology, and geophysical properties in East Africa makes it challenging to ascertain the relative roles of shallow crustal rheology, heat flow, crustal thickness, and finite strain in shaping the geometry of incipient rift basins.…”