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Stephen S. Gao - One of the best experts on this subject based on the ideXlab platform.
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mantle structure beneath the incipient okavango Rift Zone in southern africa
Geosphere, 2017Co-Authors: Kelly H Liu, Dapeng Zhao, Jianshe Lei, Zhouchuan Huang, Cory A Reed, Moikwathai Moidaki, Stephen S. GaoAbstract:Numerous investigations of the mature segments of the East African Rift system (EARS) have significantly improved our understanding of the structure and processes associated with well-developed continental Rifts. In contrast, knowledge of Rifting processes at their early stage is still significantly limited. Here we present results from a teleseismic P-wave tomography investigation of the incipient Okavango Rift Zone (ORZ), which is located at the southwestern terminus of the EARS. P-wave relative travel-time residuals recorded by 17 recently deployed portable seismic stations were manually picked and inverted for three-dimensional upper-mantle and mantle transition-Zone tomographic images beneath the ORZ and its adjacent areas. High-velocity anomalies probably representing cratonic lithosphere are visible under the Congo and Kalahari cratons, extending to depths of ∼250–350 km. The tectonic boundary of the Congo craton is observed along the western edge of the ORZ. A localized low-velocity anomaly of about –1% in magnitude is revealed in the upper asthenosphere beneath the ORZ, which is interpreted to represent decompression melting induced by lithospheric thinning. The results support the notion that the initiation and early-stage development of the ORZ are mostly due to lithospheric stretching resulted from the relative motion between the Archean Congo and Kalahari cratons along preexisting ancient orogenic Zones.
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Deep structure and origin of the Baikal Rift Zone
Earth and Planetary Science Letters, 2006Co-Authors: Dapeng Zhao, Jianshe Lei, Tomofumi Inoue, Akira Yamada, Stephen S. GaoAbstract:P-wave velocity images are determined under the Baikal Rift Zone in Siberia by using teleseismic tomography. Our results show prominent low-velocity anomalies in the upper mantle under the Baikal Rift Zone and high-velocity anomalies in the lithosphere under the Siberian craton. The low-velocity anomalies are interpreted as a mantle upwelling (plume) which has played an important role in the initiation and evolution of the Baikal Rift Zone. The Rift formation may be also controlled by other factors such as older (preRift) linear lithosphere structures favorably positioned relative to the upwelling and favorable orientation of the far-field forces caused by the India–Asia collision.
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Asymmetric upwarp of the asthenosphere beneath the Baikal Rift Zone, Siberia
Journal of Geophysical Research, 1994Co-Authors: Stephen S. Gao, Paul M. Davis, H. Liu, P. D. Slack, Yuliy A. Zorin, N. A. Logatchev, M. Kogan, P. D. Burkholder, Robert P. MeyerAbstract:In the summer of 1991 we installed 27 seismic stations about lake Baikal, Siberia, aimed at obtaining accurately timed digital seismic data to investigate the deep structure and geodynamics of the Baikal Rift Zone and adjacent regions. Sixty-six teleseismic events with high signal-to-noise ratio were recorded. Travel time and Q analysis of teleseisms characterize an upwarp of the lithosphere-asthenosphere boundary under Baikal. Theoretical arrival times were calculated by using the International Association of Seismology and Physics of the Earth's interior 1991 Earth model, and travel time residuals were found by subtracting computed arrival times from observed ones. A three-dimensional downward projection inversion method is used to invert the P wave velocity structure with constraints from deep seismic sounding data. Our results suggest that (1) the lithosphere-asthenosphere transition upwarps beneath the Rift Zone, (2) the upwarp has an asymmetric shape, (3) the velocity contrast is −4.9% in the asthenosphere, (4) the density contrast is −0.6%, and (5) the P wave attenuation contrast t* is 0.1 s.
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Seismic anisotropy and mantle flow beneath the Baikal Rift Zone
Nature, 1994Co-Authors: Stephen S. Gao, Paul M. Davis, P. D. Slack, Yu A. Zorin, V. M. Kozhevnikov, V. V. Mordvinova, Hantao Liu, Robert P. MeyerAbstract:SEISMIC studies have shown that continental Rifts such as Lake Baikal and the Great Rift Valley of East Africa are like mid-ocean Rifts in that they lie above broad regions of asthenospheric upvvarp of much greater extent than the surface expression of Rifting1–4. The direction of mantle flow in such regions can be investigated using the seismic anisotropy created by flow-induced orientation of mantle olivine crystals5–8. Seismic studies of the Mid-Atlantic Ridge have revealed upwelling mantle flow beneath the ridge and flow normal to the ridge axis on either side8–10. Here we present results from an array of seismic stations across the Baikal Rift Zone in southern Siberia. The splitting in arrival times of SKS seismic waves indicates that the upper mantle beneath the Rift Zone is anisotropic, with the fast direction (which reflects the direction of mantle flow) being horizontal and normal to the Rift axis. This suggests that the broad upwarp associated with this continental Rift is caused by similar mantle flow to that at mid- ocean Rifts. This may help to elucidate the processes involved in continental Rifting.
Dapeng Zhao - One of the best experts on this subject based on the ideXlab platform.
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Deep structure beneath the southwestern flank of the Baikal Rift Zone and adjacent areas
Physics of the Earth and Planetary Interiors, 2021Co-Authors: Zhouchuan Huang, Dapeng ZhaoAbstract:Abstract The origin of the Baikal Rift Zone is under debate between active and passive Rifting hypotheses owning to the enigmatic deep structure beneath the region. Revealing the formation mechanism of the Baikal Rift Zone may help to elucidate the process of continental Rifting. Applying P-wave teleseismic tomography to an improved data set, we discover low-velocity anomalies under the southwestern flank of the Baikal Rift Zone, which are interpreted as hot mantle upwellings from under the Siberian craton, indicating hot materials transferring from beneath the craton to beneath the Rift. Our images also reveal low-velocity anomalies under the Hangai Dome, probably suggesting the process of delamination, another consequent effect connected to the Siberian upwellings. We propose that the mantle upwellings beneath the Baikal Rift Zone and surrounding areas play a vital role in the formation of deep structures and geological settings in this area.
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mantle structure beneath the incipient okavango Rift Zone in southern africa
Geosphere, 2017Co-Authors: Kelly H Liu, Dapeng Zhao, Jianshe Lei, Zhouchuan Huang, Cory A Reed, Moikwathai Moidaki, Stephen S. GaoAbstract:Numerous investigations of the mature segments of the East African Rift system (EARS) have significantly improved our understanding of the structure and processes associated with well-developed continental Rifts. In contrast, knowledge of Rifting processes at their early stage is still significantly limited. Here we present results from a teleseismic P-wave tomography investigation of the incipient Okavango Rift Zone (ORZ), which is located at the southwestern terminus of the EARS. P-wave relative travel-time residuals recorded by 17 recently deployed portable seismic stations were manually picked and inverted for three-dimensional upper-mantle and mantle transition-Zone tomographic images beneath the ORZ and its adjacent areas. High-velocity anomalies probably representing cratonic lithosphere are visible under the Congo and Kalahari cratons, extending to depths of ∼250–350 km. The tectonic boundary of the Congo craton is observed along the western edge of the ORZ. A localized low-velocity anomaly of about –1% in magnitude is revealed in the upper asthenosphere beneath the ORZ, which is interpreted to represent decompression melting induced by lithospheric thinning. The results support the notion that the initiation and early-stage development of the ORZ are mostly due to lithospheric stretching resulted from the relative motion between the Archean Congo and Kalahari cratons along preexisting ancient orogenic Zones.
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Deep structure and origin of the Baikal Rift Zone
Earth and Planetary Science Letters, 2006Co-Authors: Dapeng Zhao, Jianshe Lei, Tomofumi Inoue, Akira Yamada, Stephen S. GaoAbstract:P-wave velocity images are determined under the Baikal Rift Zone in Siberia by using teleseismic tomography. Our results show prominent low-velocity anomalies in the upper mantle under the Baikal Rift Zone and high-velocity anomalies in the lithosphere under the Siberian craton. The low-velocity anomalies are interpreted as a mantle upwelling (plume) which has played an important role in the initiation and evolution of the Baikal Rift Zone. The Rift formation may be also controlled by other factors such as older (preRift) linear lithosphere structures favorably positioned relative to the upwelling and favorable orientation of the far-field forces caused by the India–Asia collision.
Yu A. Zorin - One of the best experts on this subject based on the ideXlab platform.
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The Baikal Rift Zone: the effect of mantle plumes on older structure
Tectonophysics, 2003Co-Authors: Yu A. Zorin, V. M. Kozhevnikov, E. Kh. Turutanov, V. V. Mordvinova, T.b Yanovskaya, A. V. TreussovAbstract:Abstract The main chain of SW–NE-striking Cenozoic half-grabens of the Baikal Rift Zone (BRZ) follows the frontal parts of Early Paleozoic thrusts, which have northwestern and northern vergency. Most of the large Rift half-grabens are bounded by normal faults at the northwestern and northern sides. We suggest that the Rift basins were formed as a result of transformation of ancient thrusts into normal listric faults during Cenozoic extension. Seismic velocities in the uppermost mantle beneath the whole Rift Zone are less than those in the mantle beneath the platform. This suggests thinning of the lithosphere under the Rift Zone by asthenosphere upwarp. The geometry of this upwarp and the southeastward spread of its material control the crustal extension in the Rift Zone. This NW–SE extension cannot be blocked by SW–NE compression generated by pressure from the Indian lithospheric block against Central Asia. The geochemical and isotopic data from Late Cenozoic volcanics suggest that the hot material in the asthenospheric upwarp is probably provided by mantle plumes. To distinguish and locate these plumes, we use regional isostatic gravity anomalies, calculated under the assumption that topography is only partially compensated by Moho depth variations. Variations of the lithosphere–asthenosphere discontinuity depth play a significant role in isostatic compensation. We construct three-dimensional gravity models of the plume tails. The results of this analysis of the gravity field are in agreement with the seismic data: the group velocities of long-period Rayleigh waves are reduced in the areas where most of the recognized plumes are located, and azimuthal seismic anisotropy shows that these plumes influence the flow directions in the mantle above their tails. The Baikal Rift formation, like the Kenya, Rio Grande, and Rhine continental Rifts [Achauer, U., Granet, M., 1997. Complexity of continental Rifts as revealed by seismic tomography and gravity modeling. In: Jacob, A.W.B., Delvaux, D., Khan, M.A. (Eds.), Lithosphere Structure, Evolution and Sedimentation in Continental Rifts. Proceedings of the IGCP 400 Meeting, Dublin, March 20–22, 1997. Institute of Advanced Studies, Dublin, pp. 161–171], is controlled by the three following factors: (i) mantle plumes, (ii) older (preRift) linear lithosphere structures favorably positioned relative to the plumes, and (iii) favorable orientation of the far-field forces.
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Seismic anisotropy and mantle flow beneath the Baikal Rift Zone
Nature, 1994Co-Authors: Stephen S. Gao, Paul M. Davis, P. D. Slack, Yu A. Zorin, V. M. Kozhevnikov, V. V. Mordvinova, Hantao Liu, Robert P. MeyerAbstract:SEISMIC studies have shown that continental Rifts such as Lake Baikal and the Great Rift Valley of East Africa are like mid-ocean Rifts in that they lie above broad regions of asthenospheric upvvarp of much greater extent than the surface expression of Rifting1–4. The direction of mantle flow in such regions can be investigated using the seismic anisotropy created by flow-induced orientation of mantle olivine crystals5–8. Seismic studies of the Mid-Atlantic Ridge have revealed upwelling mantle flow beneath the ridge and flow normal to the ridge axis on either side8–10. Here we present results from an array of seismic stations across the Baikal Rift Zone in southern Siberia. The splitting in arrival times of SKS seismic waves indicates that the upper mantle beneath the Rift Zone is anisotropic, with the fast direction (which reflects the direction of mantle flow) being horizontal and normal to the Rift axis. This suggests that the broad upwarp associated with this continental Rift is caused by similar mantle flow to that at mid- ocean Rifts. This may help to elucidate the processes involved in continental Rifting.
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Baikal Rift Zone: Structure and geodynamics
Tectonophysics, 1992Co-Authors: N. A. Logatchev, Yu A. ZorinAbstract:Abstract The Cenozoic Baikal Rift Zone is superimposed on the Caledonian Baikal fold belt, representing the suture between the Precambrian Siberian craton and several microcontinents. The Baikal Rift Zone consists of a system of disconnected fault-bounded basins and extensional and wrench faults that straddles a major arch, having a topographic relief of 2–3 km. Rifting activity commenced during the Oligocene and is still active as evident from high seismicity of the Baikal Rift Zone. Across the Baikal Rift Zone, upper crustal extension is considerably smaller than would be expected from its crustal thickness which decreases by 8–11 km across the Baikal basins as compared to the adjacent unextended areas. The discrepancy between upper and lower crustal attenuation could be attributed to intrusion of a major diapir. Indeed, geophysical data indicate that the Sayan-Baikal dome coincides with such a diapir, the top of which is located at the crust/mantle boundary. This intrusion is held responsible for Oligocene and Quaternary doming of the Rift Zone. Development of the intra-continental Baikal Rift system is thought to be related to asthenospheric diapirism (active Rifting); however, intra-plate stresses in conjunction with the Himalayan collision may have played some role.
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Crustal extension in the Baikal Rift Zone
Tectonophysics, 1991Co-Authors: Yu A. Zorin, Lindrith CordellAbstract:Analysis of the gravity field along four profiles crossing the Baikal Rift Zone permits an estimate of the amount of anomalous mass produced by 1. (1) graben-fill sediments, 2. (2) Moho uplift and intrusion of mantle sills and dikes, 3. (3) an asthenospheric bulge. Crustal extension is evaluated based on the idea of mass and volume balance of material introduced into and removed from the initial volume of the crust. Extension in the Baikal Rift increases southwestward from 0.9 km in the Chara depression to 19.3 km in the South Baikal depression. These values generally agree with the position of the Euler pole determined from seismic data (fault plane solutions). Average rotation velocity for the lithospheric plates separated by the Rift Zone is estimated to be 5.93 × 10-4 rad/m.y. over about 30 m.y.
Falk Amelung - One of the best experts on this subject based on the ideXlab platform.
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pressurized magma reservoir within the east Rift Zone of kīlauea volcano hawai i evidence for relaxed stress changes from the 1975 kalapana earthquake
Geophysical Research Letters, 2015Co-Authors: Scott Baker, Falk AmelungAbstract:We use 2000–2012 InSAR data from multiple satellites to investigate magma storage in Kīlauea's east Rift Zone (ERZ). The study period includes a surge in magma supply rate and intrusion-eruptions in 2007 and 2011. The Kupaianaha area inflated by ~5 cm prior to the 2007 intrusion and the Nāpau Crater area by ~10 cm following the 2011 intrusion. For the Nāpau Crater area, elastic modeling suggests an inflation source at 5 ± 2 km depth or more below sea level. The reservoir is located in the deeper section of the Rift Zone for which secular magma intrusion was inferred for the period following the 1975 Mw7.7 decollement earthquake. Reservoir pressurization suggests that in this section of the ERZ, extensional stress changes due to the earthquake have largely been compensated for and that this section is approaching its pre-1975 state. Reservoir pressurization also puts the molten core model into question for this section of Kīlauea's Rift Zone.
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seismic evidence for a crustal magma reservoir beneath the upper east Rift Zone of kilauea volcano hawaii
Geology, 2014Co-Authors: Guoqing Lin, Falk Amelung, Yan Lavallee, Paul G. OkuboAbstract:An anomalous body with low Vp (compressional wave velocity), low Vs (shear wave velocity), and high Vp/Vs anomalies is observed at 8–11 km depth beneath the upper east Rift Zone of Kilauea volcano in Hawaii by simultaneous inversion of seismic velocity structure and earthquake locations. We interpret this body to be a crustal magma reservoir beneath the volcanic pile, similar to those widely recognized beneath mid-ocean ridge volcanoes. Combined seismic velocity and petrophysical models suggest the presence of 10% melt in a cumulate magma mush. This reservoir could have supplied the magma that intruded into the deep section of the east Rift Zone and caused its rapid expansion following the 1975 M7.2 Kalapana earthquake.
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the 12 september 1999 upper east Rift Zone dike intrusion at kilauea volcano hawaii
Journal of Geophysical Research, 2002Co-Authors: Peter F Cervelli, Asta Miklius, Falk Amelung, Paul Segall, Harold Garbeil, Charles Meertens, Susan Owen, M LisowskiAbstract:[1] Deformation associated with an earthquake swarm on 12 September 1999 in the Upper East Rift Zone of Kilauea Volcano was recorded by continuous GPS receivers and by borehole tiltmeters. Analyses of campaign GPS, leveling data, and interferometric synthetic aperture radar (InSAR) data from the ERS-2 satellite also reveal significant deformation from the swarm. We interpret the swarm as resulting from a dike intrusion and model the deformation field using a constant pressure dike source. Nonlinear inversion was used to find the model that best fits the data. The optimal dike is located beneath and slightly to the west of Mauna Ulu, dips steeply toward the south, and strikes nearly east-west. It is approximately 3 by 2 km across and was driven by a pressure of ∼15 MPa. The total volume of the dike was 3.3 × 106 m3. Tilt data indicate a west to east propagation direction. Lack of premonitory inflation of Kilauea's summit suggests a passive intrusion; that is, the immediate cause of the intrusion was probably tensile failure in the shallow crust of the Upper East Rift Zone brought about by persistent deep Rifting and by continued seaward sliding of Kilauea's south flank.
Paul G. Okubo - One of the best experts on this subject based on the ideXlab platform.
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seismic evidence for a crustal magma reservoir beneath the upper east Rift Zone of kilauea volcano hawaii
Geology, 2014Co-Authors: Guoqing Lin, Falk Amelung, Yan Lavallee, Paul G. OkuboAbstract:An anomalous body with low Vp (compressional wave velocity), low Vs (shear wave velocity), and high Vp/Vs anomalies is observed at 8–11 km depth beneath the upper east Rift Zone of Kilauea volcano in Hawaii by simultaneous inversion of seismic velocity structure and earthquake locations. We interpret this body to be a crustal magma reservoir beneath the volcanic pile, similar to those widely recognized beneath mid-ocean ridge volcanoes. Combined seismic velocity and petrophysical models suggest the presence of 10% melt in a cumulate magma mush. This reservoir could have supplied the magma that intruded into the deep section of the east Rift Zone and caused its rapid expansion following the 1975 M7.2 Kalapana earthquake.
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Kilauea East Rift Zone Magmatism: an Episode 54 Perspective
Journal of Petrology, 2003Co-Authors: Carl R. Thornber, Frank A. Trusdell, Christina Heliker, David R. Sherrod, James P. Kauahikaua, Asta Miklius, Paul G. Okubo, James R. Budahn, W. Ian Ridley, Gregory P. MeekerAbstract:On January 29±30, 1997, prolonged steady-state effusion of lava from Pu'u'O'o was briefly disrupted by shallow extension beneath Napau Crater, 1±4 km upRift of the active Kilauea vent. A 23-h-long eruption (episode 54) ensued from fissures that were overlapping or en echelon with eruptive fissures formed during episode 1 in 1983 and those of earlier Rift Zone eruptions in 1963 and 1968. Combined geophysical and petrologic data for the 1994±1999 eruptive interval, including episode 54, reveal a variety of shallow magmatic conditions that persist in association with prolonged Rift Zone eruption. Near-vent lava samples document a significant range in composition, temperature and crystallinity of pre-eruptive magma. As supported by phenocryst±liquid relations and Kilauea mineral thermometers established herein, the Rift Zone extension that led to episode 54 resulted in mixture of near-cotectic magma with discrete magma bodies cooled to 1100 C. Mixing models indicate that magmas isolated beneath Napau Crater since 1963 and 1968 constituted 32±65% of the hybrid mixtures erupted during episode 54. Geophysical measurements support passive displacement of open-system magma along the active east Rift conduit into closed-system Rift-reservoirs along a shallow Zone of extension. Geophysical and petrologic data for early episode 55 document the gradual flushing of episode 54 related magma during magmatic recharge of the edifice.
