Mantle Transition Zone Structure Research Articles

Mantle transition zone topography and structure beneath the central Tien Shan orogenic belt

In this study we calculated receiver functions (RFs) from teleseismic P waveforms recorded by stations of four seismic networks to determine the topography of the mantle transition zone (MTZ) beneath the central Tien Shan. We converted the RFs from the time domain to the depth domain and selected the depths of 410 and 660 km discontinuities after stacking the RFs in narrow raypath bins. To better determine the MTZ topography, we applied an updated RF method to invert the depth differences between the 410 and 660 km discontinuities in each RF for lateral depth variations of the two discontinuities. Using this approach, we detected several anomalies in the 410 and 660 km discontinuity depths beneath the central Tien Shan region. Extensive synthetic tests were carried out to confirm the main features of the result. Beneath the south and east of Lake Issyk‐Kul, the 410 km discontinuity becomes shallower while the 660 km discontinuity becomes deeper, leading to a thicker MTZ with a lower temperature, possibly caused by pieces of thickened lithosphere dropping down to at least the bottom of the MTZ. Beneath the northwest of Lake Issyk‐Kul, the 410 km discontinuity becomes deeper while the 660 km discontinuity becomes shallower, resulting in a thinner MTZ with a higher temperature, which may reflect a small‐scale hot upwelling from the lower mantle.

Mantle transition zone beneath the Caribbean-South American plate boundary and its tectonic implications

We analyzed receiver-function data recorded by a temporary broadband array deployed as part of the BOLIVAR project and the permanent seismic network of Venezuela to study the mantle transition zone structure beneath the Caribbean-South American plate boundary and Venezuela. Significant topography on both the 410-km and the 660-km discontinuities was clearly imaged in the CCP (common-conversion-point) stacked images. Beneath the southeastern Caribbean, the 410-km is featured by a narrow (∼ 200 km EW) ∼ 25-km uplift extending in the NS direction around 63° west, while the 660-km is depressed by ∼ 20 km in a narrow region slightly west to the uplift, a scenario that is more consistent with westward descent of the oceanic South American plate rather than a break-off of NNW dipping proto-Caribbean oceanic lithosphere along the El Pilar Fault. We also found a thick transition zone beneath the Falcon region in northwestern Venezuela, possibly associated with the subducted Nazca plate. A flat 410-km was observed beneath the Guayana shield, suggesting that the shield has a stable and moderately deep keel, which has little effect on the underlying transition zone structure.

Seismic signatures of detached lithospheric fragments in the mantle beneath eastern Himalaya and southern Tibet

In this study, we investigate the mantle transition zone (MTZ) structure beneath eastern Himalaya and southern Tibet using ~ 9000 high quality receiver functions from 96 broadband stations spanning the entire region. The pervasive early arrival times(~ 1.7 s compared to IASP91) of the P-to-s conversions from the 410 km discontinuity are attributed to the high shear wave velocities associated with the subducted Indian (and Asian) lithosphere(s). Global and regional shear wave velocity models obtained from seismic tomography evince such high velocity anomalies both in the uppermost mantle and within the MTZ, in this complex collision environment. In contrast, the conversions from the 660 km discontinuity are either normal or delayed (up to 1 s) providing evidences for a thickened MTZ beneath most of the study region. Although presence of water in the MTZ can result in such a thickening, issues like 1) low amplitudes of the P410s and P660s conversions, 2) small dependence of their amplitudes with frequency, 3) absence of a detectable low velocity layer atop 410 and 4) lack of evidence for a 520 km discontinuity, preclude such an interpretation. Instead, we attribute this thickening to lowered temperatures affected by the possible presence of detached cold and dense lithospheric slabs within the MTZ. Such a detachment might have been facilitated either by convection or gravity removal of the lithosphere thickened due to continued subduction of the Indian plate since the Mesozoic.

Mantle transition zone structure beneath Kenya and Tanzania: more evidence for a deep-seated thermal upwelling in the mantle

SUMMARY Here we investigate the thermal structure of the mantle beneath the eastern Branch of the East African Rift system in Kenya and Tanzania. We focus on the structure of the mantle transition zone,asdelineatedbystackingofreceiverfunctions.Thetopofthetransitionzone(the410km discontinuity) displays distinctive topography, and is systematically depressed beneath the rift in Kenya and northern Tanzania and adjacent volcanic fields. This depression is indicative of a localized ! 350 C thermal anomaly. In contrast, the bottom of the transition zone (the 660km discontinuity) is everywhere depressed. This region-wide depression is best explained as a Ps conversion from the majorite‐perovskite transition of anomalously warm mantle. We interpret this structure of the transition zone as resulting from the ponding of a mantle plume (possibly the deep-mantle African Superplume) at the base of the transition zone, which then drives localized thermal upwellings that disrupt the top of the transition zone and extend to shallow mantle depths beneath the rift in Kenya and northern Tanzania.

Mantle transition zone structure along a profile in the SW Pacific: thermal and compositional variations

Events from the South Pacific, recorded in the Tien Shan region are studied with a migration method, to measure discontinuity depth along a profile between the Mariana and Molucca Sea subduction zones. Deflections of the upper-mantle discontinuities within and between the subduction zones are investigated using PP precursors filtered to periods of 3 to 10 s. We find P-wave reflections near the 410, 520 and 660 km discontinuities. The 410 km discontinuity is elevated in the subduction region and near the predicted depth (410 km) in the average velocity region. In some cases, we find negative polarities of the reflection from the 410 km discontinuity, which may indicate water-induced melting above the olivine to wadsleyite transition. The 520 km discontinuity is shallow between the two subduction zones, and deepens towards the Mariana backarc region: this discontinuity topography may be due to detection of both the wadsleyite to ringwoodite transition in a depleted downwelling (elevated region) and the formation of calcium-perovskite in a more fertile upwelling (depressed region). The 660 km discontinuity is split in the Halmahera slab with one reflection each near 600 and 700 km depth, consistent with a cold sinking slab and detection of phase transitions for both garnet and olivine to perovskite, respectively. In the region between the subduction areas, we find a downward deflection of the 660 km discontinuity with short length-scale (100–300 km) undulations that may reflect compositional variation.

Variable seismic structure near the 660 km discontinuity associated with stagnant slabs and geochemical implications

This study reports a compilation of reflectivity synthetic modeling for the structure of the upper mantle transition zone with high velocity anomalies (HVA's), associated with the northwestern Pacific subduction zones. Here, the employed method of reflectivity synthetics effectively determines the structure of flattened HVA, i.e. stagnant slab, as triplicated regional body waves are very sensitive to the velocity discontinuities in the transition zone. Results show a distribution of HVA's with or without a depression of the “660 km” discontinuity depth, which indicates a possible variation of geochemical properties at the bottom of the upper mantle. A hypothesis is proposed for this implication, i.e. that the structural variation may represent the contrast between a hydrous garnet-rich layer (subducted crust) and bulk peridotite associated with a stagnant slab, which is supported by the results of recent laboratory experiments. The garnet-rich layer can flow and descend faster than bulk peridotite as hydrous garnet is weaker and denser than peridotite in the transition zone. Given that the Clapeyron slope for hydrous garnet–perovskite is positive at ∼660 km, two zones of HVA with, and without a depression in the discontinuity depth may exist next to each other at the bottom of the transition zone. This variation of the discontinuity depths coincides with the segmentation identified at deeper depths (>629 km) in a P-wave travel-time tomography model although the resolution of the tomography inversion is limited to elaborate the discontinuity structure.

Mantle transition zone structure and upper mantle S velocity variations beneath Ethiopia: Evidence for a broad, deep‐seated thermal anomaly

Ethiopia has been subjected to widespread Cenozoic volcanism, rifting, and uplift associated with the Afar hot spot. The hot spot tectonism has been attributed to one or more thermal upwellings in the mantle, for example, starting thermal plumes and superplumes. We investigate the origin of the hot spot by imaging the S wave velocity structure of the upper mantle beneath Ethiopia using travel time tomography and by examining relief on transition zone discontinuities using receiver function stacks. The tomographic images reveal an elongated low‐velocity region that is wide (>500 km) and extends deep into the upper mantle (>400 km). The anomaly is aligned with the Afar Depression and Main Ethiopian Rift in the uppermost mantle, but its center shifts westward with depth. The 410 km discontinuity is not well imaged, but the 660 km discontinuity is shallower than normal by ∼20–30 km beneath most of Ethiopia, but it is at a normal depth beneath Djibouti and the northwestern edge of the Ethiopian Plateau. The tomographic results combined with a shallow 660 km discontinuity indicate that upper mantle temperatures are elevated by ∼300 K and that the thermal anomaly is broad (>500 km wide) and extends to depths ≥660 km. The dimensions of the thermal anomaly are not consistent with a starting thermal plume but are consistent with a flux of excess heat coming from the lower mantle. Such a broad thermal upwelling could be part of the African Superplume found in the lower mantle beneath southern Africa.

Constraining seismic velocity and density for the mantle transition zone with reflected and transmitted waveforms

We examine stacks of several seismic phases having different sensitivities to mantle transition zone structure. When analyzed separately, underside P and S reflections (PdP and SdS) are suggestive of very different structures despite similar raypaths and data coverage. By stacking the radial component of PdP rather than the vertical PdP, we show that this difference does not result from interference from other more steeply inclined phases such as PKP and Ppdpdiff. In general, stacks of P‐to‐S converted phases (Pds) appear to lack evidence of a 520‐km discontinuity when examined without other phases. When these phases and stacked topside P reflections (Ppdp) are analyzed jointly using a nonlinear inversion method, consistent but nonunique, seismological models emerge. These models show that a discontinuity at ∼653 km depth has smaller contrasts in density and velocity than found in most previous studies. A sub‐660 gradient can account for the majority of this difference. A 1.6 ± 0.5% P‐velocity contrast and a 2.2 ± 0.3% density contrast at ∼518 km depth without a S‐velocity contrast can explain the lack of a P520s, together with robust Pp520p and S520S phases. For models parameterized with a finite thickness for each discontinuity, the 410‐km discontinuity is consistently ∼3 times thicker than the 660‐km discontinuity.

Mantle transition zone topography and structure beneath the Yellowstone hotspot

The depths of the 410 and 660 km discontinuities beneath the Yellowstone hotspot are constrained using common conversion point imaging of P‐wave receiver functions. The mean depth of the 410 and 660 is 411 ± 1.5 km and 656 ± 1.6 km, with 36–40 km of peak to peak topography. This topography is spatially uncorrelated, providing no evidence for a lower mantle plume currently beneath the hotspot. The topography suggests that ±200°C thermal anomalies exist with respect to an average mantle adiabat. Two warmer than normal regions are found: at the 410 to the NNW of the hotspot and at the 660 to the NE. Colder temperatures exist at the 410 under central Wyoming. Upper mantle convection and/or intermittent heat and mass transfer across the 660 may be responsible for the uncorrelated topography. Three negative arrivals about the 410 and 660 are observed that display correct Pds moveout.

Upper mantle transition zone structure beneath the Philippine Sea Region

Seismic structure of the upper mantle transition zone beneath the Philippine Sea region is studied using waveform modeling. Models AZ41 and AZ40 were derived to represent the structure beneath the north central part and northwestern regions, respectively. AZ41 has high‐velocity anomalies from 510 km to 660 km 3% faster than a global model ak135, a depression of the ‘660’ discontinuity to 690 km, and a small (+2%) discontinuity at 510 km. Model AZ40 is the same as AZ41 except there is no depression of the ‘660’. On the other hand, the waveforms which sampled the southern region can be explained with the global model ak135. These results support the interpretation that subducted Pacific lithosphere is stagnant above the ‘660’ in the northern part, whereas in the southern part it penetrates into the lower mantle. In accord with earlier suggestions, the different configuration of the subducted slab suggested above could be due to the migration of the trench.

Seismic evidence for a thinner mantle transition zone beneath the South Pacific Superswell

Broadband seismograms recorded by the seismic stations deployed on oceanic islands in the South Pacific for two deep earthquakes in 1998 are used to investigate the mantle transition zone structure beneath the South Pacific, where a large‐scale hot plume might ascend from the core‐mantle boundary (CMB). By stacking S waves reflected at the mantle transition zone discontinuities, we find clear signals associated with the 410‐km and 660‐km discontinuities. The transition zone thickness, which is constrained from the travel time difference between the reflected waves from the 410‐km and 660‐km discontinuities, is observed to be approximately 15 km thinner beneath the South Pacific Superswell than that beneath other regions. The result suggests that the transition zone temperature beneath French Polynesia is approximately 100∼200 K higher than the surrounding mantle.

Mantle transition zone structure beneath Tanzania, east Africa

We apply a three‐dimensional stacking method to receiver functions from the Tanzania Broadband Seismic Experiment to determine relative variations in the thickness of the mantle transition zone beneath Tanzania. The transition zone under the Eastern rift is 30–40 km thinner than under areas of the Tanzania Craton in the interior of the East African Plateau unaffected by rift faulting. The region of transition zone thinning under the Eastern rift is several hundred kilometers wide and coincides with a 2–3% reduction in S wave velocities. The thinning of the transition zone, as well as the reduction in S wave velocities, can be attributed to a 200–300°K increase in temperature. This thermal anomaly at >400 km depth beneath the Eastern rift cannot be easily explained by passive rifting and but is consistent with a plume origin for the Cenozoic rifting, volcanism and plateau uplift in East Africa.

Evidence of high velocity anomalies in the transition zone associated with Southern Kurile Subduction Zone

We address the use of regional broad‐band waveform data to better constrain the upper mantle transition zone structure. We then show evidence of high velocity anomalies in the transition zone associated with the southern Kurile subduction zone. Modeling of the triplicated regional waveforms supports that high velocity anomalies exist in the deeper part of the transition zone (over 500 km in depth), and the “660” discontinuity may be depressed to about 690 km depth. Our results suggest there is some degree of flattening of the slab in the southern Kuriles.