scholarly journals Thermo-Rheological Properties of the Ethiopian Lithosphere and Evidence for Transient Fluid Induced Lower Crustal Seismicity Beneath the Ethiopian Rift

2021 ◽  
Vol 9 ◽  
Author(s):  
Ameha A. Muluneh ◽  
Derek Keir ◽  
Giacomo Corti
Keyword(s):  
Strain Rate ◽  
Pore Pressure ◽  
Lower Crust ◽  
Ethiopian Rift ◽  
Plate Boundaries ◽  
Term Strength ◽  
Main Ethiopian Rift ◽  
Crustal Earthquakes ◽  
Lower Crustal

Lower crustal earthquakes at plate boundaries and intraplate settings occur at depth where deformation is normally expected to occur in a ductile manner. Here we use the available earthquake catalogs and compute theoretical predictions for a range of conditions for the occurrence of lower crustal earthquakes beneath the Main Ethiopian Rift (MER) and adjacent north-western (NW) plateau. Yield strength envelops are constructed using information on geothermal gradient, strain rate, and composition constrained by geophysical observations. Our models suggest that away from the MER beneath the NW plateau the depth distribution of earthquakes in the lower crust is best explained by strong mafic lower crustal rheology and hydrostatic fluid pore pressure conditions. In the same region the effective elastic thickness is similar to seismogenic thickness showing that the lower crust has long-term strength and hence can physically support brittle deformation. On the contrary, in the central MER the seismogenic thickness is much larger than the effective elastic layer thickness implying that the lower crust has no long-term strength. Here our models show that both hydrostatic and near-lithostatic fluid pore pressures fail to explain the observed seismicity and instead a combination of near-lithostatic pore fluid pressure and transient high strain rate due to the movement of fluids provide a plausible mechanism for the occurrence of seismicity in the lower crust. Our interpretations are supported by occurrence of swarms of deep earthquakes beneath the MER, as opposed to more continuous background deep seismicity away from the rift. Using time-depth progression of earthquakes, we estimate permeability values of 5.9 × 10−15 m2 and 1.8 × 10−14 m2 at lower crustal depth. The range of permeability implies that seismicity can be induced by pore-pressure diffusion, likely from fluids sourced from the mantle that reactivate preexisting faults in the lower crust. Our thermo-rheological models explain the first order differences in lower crustal earthquakes both directly beneath and outboard of the rift valley.

scholarly journals Dynamic earthquake rupture in the lower crust

2019 ◽  
Vol 5 (7) ◽  
pp. eaaw0913 ◽  
Author(s):  
Arianne Petley-Ragan ◽  
Yehuda Ben-Zion ◽  
Håkon Austrheim ◽  
Benoit Ildefonse ◽  
François Renard ◽  
...  
Keyword(s):  
Lower Crust ◽  
Failure Process ◽  
Wall Rock ◽  
Ductile Deformation ◽  
Earthquake Rupture ◽  
Seismic Slip ◽  
Crustal Earthquakes ◽  
Sequence Of Events ◽  
Deformation Processes ◽  
Lower Crustal

Earthquakes in the continental crust commonly occur in the upper 15 to 20 km. Recent studies demonstrate that earthquakes also occur in the lower crust of collision zones and play a key role in metamorphic processes that modify its physical properties. However, details of the failure process and sequence of events that lead to seismic slip in the lower crust remain uncertain. Here, we present observations of a fault zone from the Bergen Arcs, western Norway, which constrain the deformation processes of lower crustal earthquakes. We show that seismic slip and associated melting are preceded by fracturing, asymmetric fragmentation, and comminution of the wall rock caused by a dynamically propagating rupture. The succession of deformation processes reported here emphasize brittle failure mechanisms in a portion of the crust that until recently was assumed to be characterized by ductile deformation.

scholarly journals Origin of Pumice in Sediments from the Middle Okinawa Trough: Constraints from Whole-Rock Geochemical Compositions and Sr-Nd-Pb Isotopes

2019 ◽  
Vol 7 (12) ◽  
pp. 462
Author(s):  
Xue Fang ◽  
Zhigang Zeng ◽  
Siyi Hu ◽  
Xiaohui Li ◽  
Zuxing Chen ◽  
...  
Keyword(s):  
Volcanic Activity ◽  
Lower Crust ◽  
Late Quaternary ◽  
Okinawa Trough ◽  
Volcanic Rocks ◽  
The Okinawa Trough ◽  
Primitive Magma ◽  
The Indian Ocean ◽  
Lower Crustal

Frequent volcanic activity has occurred in the Okinawa Trough (OT) during the late Quaternary, which attracted much attention to the origin of volcanic rocks. Pumice collected from the seafloor has been extensively investigated, whereas few studies paid attention to the pumice in the sediment. The geochemical compositions of pumice preserved in sediments generally provide insight into past volcanic activity and regional magmatism. Here, we present major and trace element compositions and Sr-Nd-Pb isotope data, together with the established age framework for pumice samples recovered from sediment core S9 in the middle OT (MOT) to investigate their possible formation. Compositionally, the S9 pumice samples are dacite and are characterized by relatively higher Sr (87Sr/86Sr = 0.70480–0.70502) and Pb (206Pb/204Pb = 18.321-18.436, 207Pb/204Pb = 15.622–15.624, and 208Pb/204Pb = 38.52–38.63) and lower Nd (143Nd/144Nd = 0.51272–0.51274) isotope compositions than basalts from the MOT. The geochemical compositions of pumice clasts from different layers of core S9 display no temporal variation trends and vary within narrow ranges. On the basis of the geochemical characteristics of S9 pumice samples, we infer that the parent magma of these samples might generate from hybrid magma through an extensive fractional crystallization process. The Indian Ocean MORB-type mantle was first metasomatized by the subducted Philippine Sea sediments to form the primitive magma; then, followed by assimilation of a small amount of lower crustal component occurred in the lower crust. The long-term magmatism and relatively consistent isotopic compositions indicate that a magma chamber might have existed in the lower crust of the MOT between 11.22 and 12.96 cal. ka BP.

scholarly journals Lower‐Crustal Seismicity on the Eastern Border Faults of the Main Ethiopian Rift

2020 ◽  
Vol 125 (8) ◽  
Author(s):  
Sacha Lapins ◽  
J. Michael Kendall ◽  
Atalay Ayele ◽  
Matthew Wilks ◽  
Andy Nowacki ◽  
...  
Keyword(s):  
Ethiopian Rift ◽  
Main Ethiopian Rift ◽  
Eastern Border ◽  
Lower Crustal

scholarly journals Catching the Main Ethiopian Rift Evolving Towards Plate Divergence

2021 ◽  
Author(s):  
Eugenio Nicotra ◽  
Marco Viccaro ◽  
Paola Donato ◽  
Valerio Acocella ◽  
Rosanna Rosa
Keyword(s):  
Ethiopian Rift ◽  
Plate Boundaries ◽  
Magma Chambers ◽  
Evolutionary Processes ◽  
Main Ethiopian Rift ◽  
Continental Breakup ◽  
Rapid Propagation ◽  
Scoria Cones ◽  
Ti Content ◽  
Eruptive Fissures

Abstract Magmatism accompanies rifting along divergent plate boundaries, although its role before continental breakup remains poorly understood. For example, the magma-assisted Northern Main Ethiopian Rift (NMER) lacks current volcanism and clear tectono-magmatic relationships with its contiguous rift portions. Here we define its magmatic behaviour, identifying the most recent eruptive fissures (EF) whose aphyric basalts have a higher Ti content than those of older monogenetic scoria cones (MSC), which are porphyritic and plagioclase-dominated. Despite the similar parental melt, EF and MSC magmas underwent different evolutionary processes. While MSC magmas were stored at intermediate crustal levels, EF magmas rose directly from the Moho without contamination, even below older polygenetic volcanoes, suggesting rapid propagation of transcrustal dikes across solidified magma chambers. Whether this recent condition in the NMER is stable or transient, it highlights a transition from central polygenetic to linear fissure volcanism, indicative of increased tensile conditions and volcanism directly fed from the Moho, suggesting transition towards mature rifting.

scholarly journals Lower crustal earthquakes near the Ethiopian rift induced by magmatic processes

2009 ◽  
Vol 10 (6) ◽  
pp. n/a-n/a ◽  
Author(s):  
Derek Keir ◽  
Ian D. Bastow ◽  
Kathryn A. Whaler ◽  
Eve Daly ◽  
David G. Cornwell ◽  
...  
Keyword(s):  
Ethiopian Rift ◽  
Crustal Earthquakes ◽  
Magmatic Processes ◽  
Lower Crustal

scholarly journals Catching the Main Ethiopian Rift evolving towards plate divergence

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Eugenio Nicotra ◽  
Marco Viccaro ◽  
Paola Donato ◽  
Valerio Acocella ◽  
Rosanna De Rosa
Keyword(s):  
Evolutionary History ◽  
Ethiopian Rift ◽  
Plate Boundaries ◽  
Magma Chambers ◽  
Main Ethiopian Rift ◽  
Continental Breakup ◽  
Rapid Propagation ◽  
Scoria Cones ◽  
Ti Content ◽  
Eruptive Fissures

AbstractMagmatism accompanies rifting along divergent plate boundaries, although its role before continental breakup remains poorly understood. For example, the magma-assisted Northern Main Ethiopian Rift (NMER) lacks current volcanism and clear tectono-magmatic relationships with its contiguous rift portions. Here we define its magmatic behaviour, identifying the most recent eruptive fissures (EF) whose aphyric basalts have a higher Ti content than those of older monogenetic scoria cones (MSC), which are porphyritic and plagioclase-dominated. Despite these differences, calculations highlight a similar parental melt for EF and MSC products, suggesting only a different evolutionary history after melt generation. While MSC magmas underwent a further step of storage at intermediate crustal levels, EF magmas rose directly from the base of the crust without contamination, even below older polygenetic volcanoes, suggesting rapid propagation of transcrustal dikes across solidified magma chambers. Whether this recent condition in the NMER is stable or transient, it indicates a transition from central polygenetic to linear fissure volcanism, indicative of increased tensile conditions and volcanism directly fed from the base of the crust, suggesting transition towards mature rifting.

scholarly journals Low lower crustal velocity across Ethiopia: Is the Main Ethiopian Rift a narrow rift in a hot craton?

2009 ◽  
Vol 10 (5) ◽  
pp. n/a-n/a ◽  
Author(s):  
Katie M. Keranen ◽  
Simon L. Klemperer ◽  
Jordi Julia ◽  
Jesse F. Lawrence ◽  
Andy A. Nyblade
Keyword(s):  
Ethiopian Rift ◽  
Main Ethiopian Rift ◽  
Crustal Velocity ◽  
Lower Crustal

scholarly journals Traction and strain-rate at the base of the lithosphere: an insight into cratonic survival

2019 ◽  
Vol 217 (2) ◽  
pp. 1024-1033 ◽  
Author(s):  
Jyotirmoy Paul ◽  
Attreyee Ghosh ◽  
Clinton P Conrad
Keyword(s):  
Strain Rate ◽  
High Viscosity ◽  
Plate Boundaries ◽  
Term Survival ◽  
Lithosphere Thickness ◽  
Lithosphere Deformation ◽  
Stress And Strain Rate ◽  
Weak Zones ◽  
Insight Into

SUMMARY Cratons are the oldest parts of the lithosphere, some of them surviving since Archean. Their long-term survival has sometimes been attributed to high viscosity and low density. In our study, we use a numerical model to examine how shear tractions exerted by mantle convection work to deform cratons by convective shearing. We find that although tractions at the base of the lithosphere increase with increasing lithosphere thickness, the associated strain-rates decrease. This inverse relationship between stress and strain-rate results from lateral viscosity variations along with the model’s free-slip condition imposed at the Earth’s surface, which enables strain to accumulate along weak zones at plate boundaries. Additionally, we show that resistance to lithosphere deformation by means of convective shearing, which we express as an apparent viscosity, scales with the square of lithosphere thickness. This suggests that the enhanced thickness of the cratons protects them from convective shear and allows them to survive as the least deformed areas of the lithosphere. Indeed, we show that the combination of a smaller asthenospheric viscosity drop and a larger cratonic viscosity, together with the excess thickness of cratons compared to the surrounding lithosphere, can explain their survival since Archean time.

scholarly journals Long-lived Paleoproterozoic eclogitic lower crust

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Sebastian Buntin ◽  
Irina M. Artemieva ◽  
Alireza Malehmir ◽  
Hans Thybo ◽  
Michal Malinowski ◽  
...  
Keyword(s):  
High Velocity ◽  
Lower Crust ◽  
Crustal Layer ◽  
Crustal Rocks ◽  
Long Term Stability ◽  
Eclogite Facies ◽  
Conventional Models ◽  
Lower Crustal

AbstractThe nature of the lower crust and the crust-mantle transition is fundamental to Earth sciences. Transformation of lower crustal rocks into eclogite facies is usually expected to result in lower crustal delamination. Here we provide compelling evidence for long-lasting presence of lower crustal eclogite below the seismic Moho. Our new wide-angle seismic data from the Paleoproterozoic Fennoscandian Shield identify a 6–8 km thick body with extremely high velocity (Vp ~ 8.5–8.6 km/s) and high density (>3.4 g/cm3) immediately beneath equally thinned high-velocity (Vp ~ 7.3–7.4 km/s) lowermost crust, which extends over >350 km distance. We relate this observed structure to partial (50–70%) transformation of part of the mafic lowermost crustal layer into eclogite facies during Paleoproterozoic orogeny without later delamination. Our findings challenge conventional models for the role of lower crustal eclogitization and delamination in lithosphere evolution and for the long-term stability of cratonic crust.

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