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Antigorite occurs at seismogenic depth along plate boundary shear zones, particularly in subduction and oceanic transform settings, and has been suggested to control a low‐strength bulk rheology. To constrain dominant deformation mechanisms, we perform hydrothermal ring‐shear experiments on antigorite and antigorite‐quartz mixtures at temperatures between 20 and 500°C at 150 MPa effective normal stress. Pure antigorite is strain hardening, with frictional coefficient (μ) > 0.5, and developed cataclastic microstructures. In contrast, antigorite‐quartz mixtures (10% quartz) are strain weakening with μ decreasing with temperature from 0.36 at 200°C to 0.22 at 500°C. Antigorite‐quartz mixtures developed foliation similar to natural serpentinite shear zones. Although antigorite‐quartz reactions may form mechanically weak talc, we only find small, localized amounts of talc in our deformed samples, and room temperature friction is higher than expected for talc. Instead, we propose that the observed weakening at temperatures ≥200°C primarily results from silica dissolution leading to a lowered pore‐fluid pH that increases antigorite solubility and dissolution rate and thus the rate of dissolution‐precipitation creep. We suggest that under our experimental conditions, efficient dissolution‐precipitation creep coupled to grain boundary sliding results in a mechanically weak frictional‐viscous rheology. Antigorite with this rheology is much weaker than antigorite deforming frictionally, and strength is sensitive to effective normal stress and strain rate. The activation of dissolution‐precipitation in antigorite may allow steady or transient creep at low driving stress where antigorite solubility and dissolution rate are high relative to strain rate, for example, in faults juxtaposing serpentinite with quartz‐bearing rocks. Plain Language Summary: Some major, plate‐boundary fault zones are very weak, and in some places this is thought to be because they contain the mineral serpentine. Antigorite is a common form of serpentine at depth, but in many rock deformation experiments, antigorite is found to be relatively strong. In natural fault zones, however, antigorite can be highly deformed at seemingly low stress. The motivation for this study was to explore whether antigorite can be weak also in the laboratory. Because laboratory experiments occur relatively fast, the mechanism of deformation may be different from in nature. To address this, we mixed antigorite with quartz, which changes the pH and promotes dissolution of antigorite. We found that when deforming antigorite mixed with quartz, the experimental fault was very weak, because the dissolution of antigorite removed sticking points ‐ a process that is also documented in natural faults. The experiments also produced minor talc, from the reaction of antigorite with quartz, but too little to affect strength. We suggest that it is the activation of dissolution and precipitation of antigorite which allows deformation of major serpentine‐bearing faults at low stress at temperatures of several hundred degrees Celsius. Key Points: At 20°C and 500°C antigorite is strain‐hardening, develops cataclastic microstructures and has a frictional coefficient of ∼0.5Samples of antigorite mixed with quartz are strain‐weakening, and the friction coefficient decreases from 0.36 at 200°C to 0.22 at 500°CThe weakness of quartz‐antigorite is associated with activation of dissolution‐precipitation creep [ABSTRACT FROM AUTHOR]

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]