Metamorphic Inheritance, Lower‐Crustal Earthquakes, and Continental Rifting
Abstract 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 othe...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
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Wiley
2024-03-01
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| Series: | Geochemistry, Geophysics, Geosystems |
| Subjects: | |
| Online Access: | https://doi.org/10.1029/2023GC011305 |
| _version_ | 1797205624514150400 |
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| author | Å. Fagereng J. F. A. Diener C. J. Tulley B. Manda |
| author_facet | Å. Fagereng J. F. A. Diener C. J. Tulley B. Manda |
| author_sort | Å. Fagereng |
| collection | DOAJ |
| description | Abstract 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. |
| first_indexed | 2024-04-24T08:54:05Z |
| format | Article |
| id | doaj.art-254c3dbf444c4212b241e10e60b966a7 |
| institution | Directory Open Access Journal |
| issn | 1525-2027 |
| language | English |
| last_indexed | 2024-04-24T08:54:05Z |
| publishDate | 2024-03-01 |
| publisher | Wiley |
| record_format | Article |
| series | Geochemistry, Geophysics, Geosystems |
| spelling | doaj.art-254c3dbf444c4212b241e10e60b966a72024-04-16T08:35:30ZengWileyGeochemistry, Geophysics, Geosystems1525-20272024-03-01253n/an/a10.1029/2023GC011305Metamorphic Inheritance, Lower‐Crustal Earthquakes, and Continental RiftingÅ. Fagereng0J. F. A. Diener1C. J. Tulley2B. Manda3School of Earth and Environmental Sciences Cardiff University Cardiff UKDepartment of Geological Sciences University of Cape Town Rondebosch South AfricaSchool of Earth and Environmental Sciences Cardiff University Cardiff UKChancellor's College University of Malawi Zomba MalawiAbstract 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.https://doi.org/10.1029/2023GC011305continental riftingEast African Rift Systemmetamorphic petrologylower crustal earthquakeslithospheric rheology |
| spellingShingle | Å. Fagereng J. F. A. Diener C. J. Tulley B. Manda Metamorphic Inheritance, Lower‐Crustal Earthquakes, and Continental Rifting Geochemistry, Geophysics, Geosystems continental rifting East African Rift System metamorphic petrology lower crustal earthquakes lithospheric rheology |
| title | Metamorphic Inheritance, Lower‐Crustal Earthquakes, and Continental Rifting |
| title_full | Metamorphic Inheritance, Lower‐Crustal Earthquakes, and Continental Rifting |
| title_fullStr | Metamorphic Inheritance, Lower‐Crustal Earthquakes, and Continental Rifting |
| title_full_unstemmed | Metamorphic Inheritance, Lower‐Crustal Earthquakes, and Continental Rifting |
| title_short | Metamorphic Inheritance, Lower‐Crustal Earthquakes, and Continental Rifting |
| title_sort | metamorphic inheritance lower crustal earthquakes and continental rifting |
| topic | continental rifting East African Rift System metamorphic petrology lower crustal earthquakes lithospheric rheology |
| url | https://doi.org/10.1029/2023GC011305 |
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