Rifting#
Rift zones are regions where the lithosphere is pulled apart from extensional tectonic forces. This leads to formation of subsided rift basins with elevated rift flanks.
Active rifts: Active rifts are mostly driven by mantle upwelling, and are therefore have surface volcanism. Due to the increased temperatures, the lithosphere is weakened and can be more easily deformed. Examples include the East African Rift and the Red Sea Rift.
Passive rifts: Passive rifts are mostly driven by far-field tectonic forces, and are therefore have comparitively less volcanism. Examples include the Rio Grande Rift.
Cartoon illustrating stretching of lithosphere under tectonic forces (passive rifts) and magmatic forces (active rifts). Image: Corti (2009)
Forces governing rifting#
Rifts are influenced by various forces summarized in the review by Brune et al. (2023) (see figure below). These forces include:
Slab pull: the subducting slab stretches as the slab sinks in the mantle, which can create rift zones in the attached plate.
Slab rollback: the overriding plate stretches due to the back-arc extension caused by the slab rollback.
Mantle basal drag: horizontal mantle flow between the convection cells generates extension in the overlying lithosphere.
Vertical mantle upwelling: vertical tractions due to mantle upwelling extends the overlying lithosphere as positive dynamic topography is generated.
Gravitational potential energy: lateral variations in the crustal and lithospheric mantle thickness creates gradients in the lithospheric buoyancy forces, which induces extension in the regions with lower gravitational potential energy.
Driving and resisting forces related to rifting. Image: modified from Brune et al. (2023)
Impact of physical processes on rift evolution#
Some of the physical process that promote and resist rifting include:
Factors that promote rifting:
Thermal weakening: regions with localized thermal anomalies (e.g., mantle upwelling) or with thinned lithosphere, have high heat flow, that reduces the strength of the lithosphere.
Magmatic weakening: magmatic intrusions (e.g., dikes) or inherited weak zones can significantly reduce the fault strength.
Surface processes: erosion can promote rifting by reducing the normal stress on faults, which can facilitate fault slip.
Fault weakening: faults may degrade after a chemical alteration (e.g, fluid interaction), reducing its strength.
Factors that resist rifting:
Thermal cooling: dissipation of heat through the lithosphere over time increases its strength
Fault healing : over time, faults can heal depending on the temperature, pressure, and mineralogy of the contact surface.
Subsidence: in regions of negative dynamic topography, the lithosphere is under compression, which can resist rifting.
Impact of processes on rift weakening and strengthening. Image: modified from Brune et al. (2023)
References#
Corti, Giacomo. Continental rift evolution: from rift initiation to incipient break-up in the Main Ethiopian Rift, East Africa. Earth-science reviews 96.1-2 (2009): 1-53.
Brune, S., Kolawole, F., Olive, J. A., Stamps, D. S., Buck, W. R., Buiter, S. J., … & Shillington, D. J. (2023). Geodynamics of continental rift initiation and evolution. Nature Reviews Earth & Environment, 4(4), 235-253.
