Continental rifts are important sources of mantle carbon dioxide (CO
) emission into Earth's atmosphere
. Because deep carbon is stored for long periods in the lithospheric mantle
, rift CO
flux ...depends on lithospheric processes that control melt and volatile transport
. The influence of compositional and thickness differences between Archaean and Proterozoic lithosphere on deep-carbon fluxes remains untested. Here we propose that displacement of carbon-enriched Tanzanian cratonic mantle concentrates deep carbon below parts of the East African Rift System. Sources and fluxes of CO
and helium are examined over a 350-kilometre-long transect crossing the boundary between orogenic (Natron and Magadi basins) and cratonic (Balangida and Manyara basins) lithosphere from north to south. Areas of diffuse CO
degassing exhibit increasing mantle CO
flux and
He/
He ratios as the rift transitions from Archaean (cratonic) to Proterozoic (orogenic) lithosphere. Active carbonatite magmatism also occurs near the craton edge. These data indicate that advection of the root of thick Archaean lithosphere laterally to the base of the much thinner adjacent Proterozoic lithosphere creates a zone of highly concentrated deep carbon. This mode of deep-carbon extraction may increase CO
fluxes in some continental rifts, helping to control the production and location of carbonate-rich magmas.
A kinematic model for the East African Rift Stamps, D. Sarah; Calais, Eric; Saria, Elifuraha ...
Geophysical research letters,
March 2008, Letnik:
35, Številka:
5
Journal Article
Recenzirano
Odprti dostop
The kinematics of the East African Rift (EAR) is the least well‐known of all major plate boundaries. Here, we show that present‐day data (a GPS+DORIS geodetic solution and earthquake slip vectors) ...are consistent with 3.2 Myr‐average spreading rates and transform‐fault azimuths along the Southwest Indian Ridge and support a kinematic model that includes three subplates (Victoria, Rovuma, and Lwandle) between Nubia and Somalia. Continental rifting in the EAR appears to involve localized strain along narrow rift structures that isolate large lithospheric blocks.
Faulting and earthquakes occur extensively along the flanks of the East African Rift System, including an offshore branch in the western Indian Ocean, resulting in remobilization of sediment in the ...form of landslides. To date, constraints on the occurrence of submarine landslides at margin scale are lacking, leaving unanswered a link between rifting and slope instability. Here, we show the first overview of landslide deposits in the post-Eocene stratigraphy of the Tanzania margin and we present the discovery of one of the biggest landslides on Earth: the Mafia mega-slide. The emplacement of multiple landslides, including the Mafia mega-slide, during the early-mid Miocene is coeval with cratonic rifting in Tanzania, indicating that plateau uplift and rifting in East Africa triggered large and potentially tsunamigenic landslides likely through earthquake activity and enhanced sediment supply. This study is a first step to evaluate the risk associated with submarine landslides in the region.
The southern Tanganyika Rift, within the Western rift, Africa, has earthquakes to depths of 37 km, yet few constraints exist on crustal thickness, or of early stage rifting processes in apparently ...amagmatic rift sectors. The aim of the TANGA14 experiment was to constrain bulk crustal properties to test whether magmatic processes modify the lithosphere in areas of deep seismicity, and the degree of lithospheric thinning. We use 11 broadband seismometers to implement receiver function analysis using H‐κ stacking, a method sensitive to crustal thickness and VP/VS ratio, to determine bulk crustal properties. Analyses include extensive error analysis through bootstrap, variance, and phase‐weighted stacking. Results show the Archean Tanzanian Craton and Bangweulu Block are characterized by VP/VS ratios of 1.75–1.77, implying a felsic bulk composition. Crust beneath the fault‐bounded basins has high VP/VS (>1.9). Anorthosite bodies and surface sediments within the region may contribute to localized high VP/VS. However, elevated VP/VS values within fault‐bounded extensional basins where elevated heat flow, hydrothermal vent sites, and deep earthquakes are observed suggest that magma may be intruding the lower crust beneath the southern Tanganyika Rift. Crustal thicknesses on/near the relatively unextended Tanzanian craton and Bangweulu Block are 41.6–42.0 km. This contrasts with the Tanganyika Rift where crustal thicknesses are 31.6 km to 39 km from north to south. Our results provide evidence for ~20% crustal thinning localized to fault‐bounded basins. Taken together, they suggest a previously unrecognized role of magma intrusion in early stage continental rifting in the Western rift, Africa.
Key Points
We present the first detailed study of crustal structure at Lake Tanganyika using receiver functions inversion and forward modeling
Crustal thinning along the eastern Tanganyika rift basin is ~20% but may increase beneath the central basin
Elevated VP/VS ratios, evidence for gradational Moho, and deep seismicity suggest previously unrecognized role of magma intrusion
A re‐evaluation of existing onshore and offshore gravity, magnetic, seismic reflection, and well data from the Australo‐Antarctic margins suggests that magmatism and along‐strike lithospheric ...heterogeneities have influenced the localization of initial rifting. The 3‐D crustal architecture of the Australian and Antarctic margins, which formed during multiple rifting episodes spanning ∼80 Myr, reveal local asymmetries along strike. Rift structures from the broad, late Jurassic (165–145 Ma) rift zone are partially overprinted by a narrower, mid‐to‐late Cretaceous rift zone (∼100 Ma), which evolved in highly extended crust. This late‐stage rift zone is located within a region of heterogeneous crust with faults that cut late syn‐rift strata, interpreted as a continent ocean transition zone. This late stage transitional rift is populated by seismically identified rift‐parallel basement highs and intracrustal bodies with corresponding positive Bouguer gravity and magnetic anomalies. These undrilled features can be interpreted as exposures of exhumed mantle rocks, lower crustal rocks and/or as discrete magmatic bodies. Our results suggest that strain across an initially broad Australo‐Antarctic rift system (165–145 Ma) migrated to a narrow rift zone with some magmatism at 100–83 Ma. Breakup did not occur until ∼53 Ma within the eastern Bight‐Wilkes and Otway‐Adélie margin sectors, suggesting a west to east propagation of seafloor spreading. The prolonged eastward propagation of seafloor spreading processes and the increased asymmetry of the Australian‐Antarctic margins coincides with a change from rift‐perpendicular to oblique rifting processes, which in turn coincide with along‐strike variations in cratonic to Palaeozoic lithosphere.
Key Points
margins reveal local asymmetries
Australo‐Antarctic rift system gave way to a late stage, narrow rift zone
Within the narrow younger rift we interpret a new segmentation pattern
P and S wave tomographic models have been developed for the northern Malawi rift and adjacent Rungwe Volcanic Province (RVP) using data from the Study of Extension and maGmatism in Malawi aNd ...Tanzania project and data from previous networks in the study area. The main features of the models are a low‐velocity zone (LVZ) with δVp = ~−1.5–2.0% and δVs = ~−2–3% centered beneath the RVP, a lower‐amplitude LVZ (δVp = ~−1.0–1.3% and δVs = ~−0.7–1%) to the southeast of the RVP beneath the center and northeastern side of the northern Malawi rift, a shift of the lower‐amplitude anomaly at ~−10° to −11° to the west beneath the central basin and to the western side of the rift, and a fast anomaly at all depths beneath the Bangweulu Craton. The LVZ widens further at depths >~150–200 km and extends to the north beneath northwestern Malawi, wrapping around the fast anomaly beneath the craton. We attribute the LVZ beneath the RVP and the northern Malawi rift to the flow of warm, superplume mantle from the southwest, upwelling beneath and around the Bangweulu Craton lithosphere, consistent with high 3He/4He values from the RVP. The LVZ under the RVP and northern Malawi rift strongly indicates that the rifted lithosphere has been thermally perturbed. Given that volcanism in the RVP began about 10 million years earlier than the rift faulting, thermal and/or magmatic weakening of the lithosphere may have begun prior to the onset of rifting.
Plain Language Summary
P and S wave tomographic models have been developed for the northern Malawi rift and adjacent Rungwe Volcanic Province (RVP) using data from the Study of Extension and maGmatism in Malawi aNd Tanzania project and data from previous networks in the study area. A low‐velocity anomaly is imaged under the RVP and northern Malawi rift. We attribute the low‐velocity anomaly to flow of warm mantle from the African superplume to the southwest of the study area, which has migrated around the side of thick Bangweulu Craton lithosphere and upwelled beneath the thinner mobile belt lithosphere to the east of the Bangweulu Craton. The observation that volcanism began in the RVP prior to the onset of rifting suggests that the lithosphere beneath the Malawi rift may have been thermally weakened prior to rifting.
Key Points
Low‐velocity anomaly is imaged under Rungwe Volcanic Province and northern Malawi rift
Low‐velocity anomaly is attributed to upwelling of warm mantle around side of Bangweulu Craton lithosphere
Lithosphere beneath the Malawi rift may have been weakened prior to rifting
Recent large basaltic eruptions began after only minor surface uplift and seismicity, and resulted in caldera subsidence. In contrast, some eruptions at Galápagos Island volcanoes are preceded by ...prolonged, large amplitude uplift and elevated seismicity. These systems also display long-term intra-caldera uplift, or resurgence. However, a scarcity of observations has obscured the mechanisms underpinning such behaviour. Here we combine a unique multiparametric dataset to show how the 2018 eruption of Sierra Negra contributed to caldera resurgence. Magma supply to a shallow reservoir drove 6.5 m of pre-eruptive uplift and seismicity over thirteen years, including an Mw5.4 earthquake that triggered the eruption. Although co-eruptive magma withdrawal resulted in 8.5 m of subsidence, net uplift of the inner-caldera on a trapdoor fault resulted in 1.5 m of permanent resurgence. These observations reveal the importance of intra-caldera faulting in affecting resurgence, and the mechanisms of eruption in the absence of well-developed rift systems.
The Galápagos Archipelago represents an opportunity to investigate the properties of young oceanic lithosphere, the effects of a hotspot anomaly on lithospheric thickness, and melting dynamics in a ...hotspot-ridge interaction. Here we use data recorded by the SIGNET array and permanent station PAYG on the Islands Santa Cruz and Isabela, respectively. We used P-to-S (Ps) and S-to-P (Sp) receiver functions to constrain crust and mantle structure. A simultaneous deconvolution method was used to constrain 1-D structure and also for the modeling of robust features. A migrated extended multitaper method was used to investigate 3-D structural variations. Ps images a velocity increase with depth at 11±7 km, probably the base of the pre-plume crust, or old Moho. Sp imaging and modeling images a second, deeper velocity increase at 37±7 km depth. A velocity decrease with depth is imaged on average at 75±12 km likely associated with the lithosphere–asthenosphere boundary. This discontinuity is imaged deeper, 82 km, in the southwest and shallower, 66 km, in the northeast near the spreading ridge. Although the trend is consistent with lithospheric thickening with age, the thickness is much larger than predicted by conductive cooling models of 0–10 My oceanic lithosphere. We infer a compositional contribution to velocity variations. Finally, a velocity increase with depth is imaged at ∼125 to 145±15 km depth that is likely associated with the onset of melting. The discontinuity is imaged deeper in 3 sectors of the Galápagos platform-ridge region, all coincident with the slowest surface wave shear velocity anomalies in the upper 100 km. One is located in the southwest in a hypothesized plume location. The other two are to the northwest and northeast, possibly illuminating multiple plume diversions related to complex plume–ridge interactions.
•We image the crust and mantle beneath Galápagos with receiver functions and the SIGNET array.•A Moho phase at 11 km and a deeper velocity increase near 37 km depth are imaged.•The lithosphere–asthenosphere boundary is imaged at 66–82 km, deeper than thermal models (∼50 km).•The onset of melting is imaged at 125–145 km, deepest by the hotspot and 2 regions near the ridge.•A compositional lithosphere and multiple plume diversions explain the observations.
Models for the formation of the archetypal rift-rift-rift triple junction in the Afar depression have assumed the synchronous development of the Red Sea- Aden-East African rift systems soon after ...flood basaltic magmatism at 31 Ma, but the timing of intial rifting in the northern sector of the East African rift system had been poorly constrained. The aims of our field, geochronology, and remote sensing studies were to determine the timing and kinematics of rifting in the 3rd arm, the Main Ethiopian rift (MER), near its intersection with the southern Red Sea rift. New structural data and 10 new SCLF super(40)Ar/ super(39)Ar dates show that extension in the northern Main Ethiopian rift commenced after 11 Ma, more than 17 My after initial rifting in the southern Red Sea and Gulf of Aden. The triple junction, therefore, could have developed only during the past 11 My, or 20 My after the flood basaltic magmatism. Thus, the flood basaltic magmatism and separation of Arabia from Africa are widely separated in time from the opening of the Main Ethiopian rift, which marks the incipient Nubia-Somalia plate boundary; triple junction formation is not a primary feature of breakup above the Afar mantle plume. The East African rift system appears to have propagated northward from the Mesozoic Anza rift system into the Afar depression to cut across Oligo-Miocene rift structures of the Red Sea and Gulf of Aden, in response to global plate reorganisations. Structural patterns reveal a change from 130 degree E-directed extension to 105 degree E-directed extension sometime in the interval 6.6 to 3 Ma, consistent with predictions from global plate kinematic studies. The along-axis propagation of rifting in each of the three arms of the triple junction has led to a NE-migration of the triple junction since 11 Ma.
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