Metamorphic Inheritance, Lower‐Crustal Earthquakes, and Continental Rifting.

The Malawi Rift is localized within Precambrian amphibolite‐granulite facies metamorphic belts, bounded by up to 150 km long border faults, and generates earthquakes throughout ∼40 km thick crust. Rift‐related faults are inferred to exploit pre‐existing weaknesses that allow rifting of otherwise dry and strong crust. It is unclear what these weaknesses are, and how localization into weak zones can be reconciled with strength required for lower crustal seismicity. We present results of mineral equilibria modeling, which indicate that Proterozoic metamorphism generated dry crust dominated by a quartz‐feldspar assemblage that is metastable at current conditions. For rift propagation to be possible at current cool thermal gradients and in mechanically strong, dry quartzofeldspathic rocks, mid‐ to lower‐crustal strain must be localized into relatively weak, inherited shear zones that deform primarily by aseismic, viscous creep. These shear zones are embedded within high‐strength crust, and interaction between creeping shear zones and enveloped or surrounding rocks may locally increase stress and trigger frictional, seismic slip at mid‐ to lower‐crustal depths. Over time, this interaction may produce a fracture network that allows infiltration of fluids. We therefore suggest that during rifting of previously deformed and metamorphosed crust, major faults are most likely to grow from below, with their location and orientation prescribed by underlying inherited viscous shear zones. In this case, fluids may infiltrate and locally weaken metastable lower crust, including allowing time‐dependent fluid‐driven seismicity and local partial melting, but length‐scales of this weakening is limited by the scale of the permeability network. Plain Language Summary: In Malawi, East Africa, the Earth's crust is slowly splitting apart. This "rifting" has two unusual characteristics: (a) the crust is thick and strong and therefore difficult to split apart, and (b) there are earthquakes at greater depth and temperature than typical for the continents. We propose that the reason rifting occurs with these two characteristics, in this location, is that the crust is inherited from past periods of mountain building. Based on thermodynamic principles, specifically what minerals are stable under what conditions, we model the composition of Malawi crust in the geological past. We then compare those results to current conditions. We find that the likely inherited composition of Malawi crust is unstable under current conditions, but needs an addition of water to undergo reactions to new, stable minerals. We also note that deformation is only possible in weak zones inherited from past deformation, and these zones will deform by high‐temperature flow of rock, not generating earthquakes. However, flow of local weak zones may induce fracturing in surrounding harder rocks, generating local earthquakes and a fracture system that water can flow along. Over time, this water infiltration can weaken the crust and ultimately allow continental rifting as seen further north in Africa. Key Points: The Malawi crust is mostly dry, metastable and frictionally strong, and rifting requires viscous flow in inherited lower crustal shear zonesMajor faults are likely to grow from below, with their location and orientation prescribed by underlying inherited viscous shear zonesViscous flow in heterogeneous crust may trigger lower crustal seismicity generating permeability that allows infiltration of external fluids [ABSTRACT FROM AUTHOR]