Pn Velocity Research Articles

Uppermost mantle structure and dynamics of the Tanlu fault zone: New insights from Pn anisotropic tomography

Whether the segmentation of the Tanlu fault zone and mechanism of strong crustal earthquakes are related to the uppermost-mantle structure is still unclear. Here a new high-resolution uppermost-mantle velocity and anisotropy model of the Tanlu fault zone and surrounding areas is determined by using 13,045 Pn arrival-time data. These data are handpicked from seismograms recorded at 120 newly deployed TanluArray portable seismic stations and 337 Chinese provincial seismic stations. The pattern of Pn velocity and anisotropy agrees well with the surface geological and tectonic features. Obvious low-velocity (low-V) anomalies are observed in tectonically active areas, such as the Taihang mountain orogenic belt, the eastern Dabie mountain orogenic belt, and the Sulu fold belt, whereas areas with stable tectonics, such as the North China Basin and the South Yellow Sea Basin, exhibit high-velocity (high-V) anomalies. The fast propagation directions (FPDs) of Pn waves in the study area exhibit a complex distribution, but the FPDs show a rotating feature around the margin of the South Yellow Sea Basin. The block feature of the high-V anomalies beneath the North China Basin and segmentation of the low-V anomalies along the Tanlu fault zone may be related to lithospheric delamination and thermal erosion caused by hot and wet upwelling flows in the big mantle wedge. The 1668 Tancheng earthquake (M8.5) occurred near a low-V anomaly, and recent moderate and strong earthquakes (M > 4.0) took place along the boundaries of the velocity anomalies. These results offer new perspectives on the mantle dynamics and mechanism of strong earthquakes in and around the Tanlu fault zone.

Uppermost Mantle Pn Velocity and Anisotropy Structures Beneath the Sakhalin–Kuril–Kamchatka Region

AbstractIn this study, we used the Pn tomography method to obtain detailed velocity and anisotropy structures of the uppermost mantle beneath Sakhalin–Kuril–Kamchatka region for improving the understanding of plate subduction, arc–arc collision, and volcanism. We found low Pn velocities beneath volcanoes and areas characterized by pronounced tectonic activity and high Pn velocities with strong anisotropy in the subducting plate. Low Pn velocity anomalies beneath southern Sakhalin connected the low‐velocity anomalies in the mantle and crust, indicating the ascent of fluid or melt, and may provide a magmatic source for the rear‐arc Rishiri volcano. In the absence of plate subduction, low‐velocity anomalies and north–south Pn anisotropy manifested beneath northern Kamchatka, revealing the lateral propagation of mantle flow beneath this northern region. We suggest that the eastern boundary of the slab window at the Kamchatka–Aleutian junction is likely to be located near the Komandorsky Islands.

Pn Velocity and Anisotropy Tomography With Nonuniform Grid Beneath the Ryukyu–Taiwan–Philippines Region

AbstractTo enhance our comprehension of the dynamic processes associated with complex plate subduction, volcanic magmatism, and lithosphere deformation beneath the Ryukyu–Taiwan–Philippines region, we utilized nonuniform inversion grids for Pn velocity and anisotropy tomography, and obtained the uppermost mantle structure of this area. The results demonstrate remarkable characteristics: cold oceanic subducting plates display high Pn velocities, whereas volcanic arcs, the extinct mid‐ocean ridge in the South China Sea, and the Palawan region exhibit low Pn velocities. The extinct mid‐ocean ridge also displays a low‐velocity anomaly, indicating that residual heat persists even after approximately 15 million years since seafloor spreading. The occurrence of a discontinuous low‐velocity beneath the Ryukyu arc supports the presence of a slab window at approximately 123°E. The volcanic arcs of all subduction zones within the study area displayed trench‐parallel Pn anisotropy. The observed Pn fast directions beneath the Taiwan orogenic belt are consistent with crustal anisotropy, providing evidence for the crust–mantle coupled deformation. Moreover, our results shed light on the deep structural characteristics of the complex subduction zones beneath the Philippines region. Plate subduction causes partial melting due to dehydration; then, the melt ascends and accumulates at the uppermost mantle, showing low Pn velocities. However, the low‐velocity anomaly is not widely distributed but corresponds to a narrow band. In particular, the east–west bidirectional subduction of the Philippines and Negros formed separate low‐velocity anomalies. Low Pn velocities and trench‐parallel anisotropy indicate the location of different subduction zones and finely characterize their impact on the uppermost mantle structure.

New Constraints on the Complex Subduction and Tearing Model of the Cocos Plate Using Anisotropic Pn Tomography

AbstractTo better understand the structure and process of the Cocos plate subduction, we obtained the first Pn velocity and anisotropy model of the uppermost mantle in southern Mexico using the Pn tomography method and new ISC‐EHB data. This study confirms that the subducted Cocos oceanic plate exhibits mid‐oceanic ridge‐perpendicular anisotropy, and there is an arc‐parallel anisotropic structure beneath the subduction arc. The variation of the Pn velocity and anisotropy structure at the east and west ends of the flat‐slab subduction zone in the Guerrero area provide new and clear seismological images for a plate‐tearing model which might be caused by the Orozco and O'Gorman fracture zones. The Pn low‐velocity and trench‐perpendicular anisotropic structure found north of the flat‐slab subduction zone support the slab retreat model of the Central Cocos plate. The Pn structure supports the existence of tearing corresponding to Teuhantepec Ridge in the Cocos plate beneath the Isthmus of Tuxtlas. Our study provides a reliable tomography basis and new constraints for a complicated subduction dynamics process in the southern Mexico subduction zone.

Seismic Architecture of the Lithosphere-Asthenosphere System in the Western United States from a Joint Inversion of Body- and Surface-wave Observations: Distribution of Partial Melt in the Upper Mantle

Quantitative evaluation of the physical state of the upper mantle, including mapping temperature variations and the possible distribution of partial melt, requires accurately characterizing absolute seismic velocities near seismic discontinuities. We present a joint inversion for absolute but discontinuous models of shear-wave velocity (Vs) using 4 types of data: Rayleigh wave phases velocities, P-to-s receiver functions, S-to-p receiver functions, and Pn velocities. Application to the western United States clarifies where upper mantle discontinuities are lithosphere-asthenosphere boundaries (LAB) or mid-lithospheric discontinuities (MLD). Values of Vs below 4 km/s are observed below the LAB over much of the Basin and Range and below the edges of the Colorado Plateau; the current generation of experimentally based models for shear-wave velocity in the mantle cannot explain such low Vs without invoking the presence of melt. Large gradients of Vs below the LAB also require a gradient in melt-fraction. Nearly all volcanism of Pleistocene or younger age occurred where we infer the presence of melt below the LAB. Only the ultrapotassic Leucite Hills in the Wyoming Craton lie above an MLD. Here, the seismic constraints allow for the melting of phlogopite below the MLD.

Olivine Fabrics in the Oceanic Lithosphere Constrained by Pn Azimuthal Anisotropy

AbstractWe constrain olivine fabrics in the oceanic lithosphere using active and passive seismic observations of Pn azimuthal anisotropy. We first analyze active‐source data derived from a broadband ocean bottom seismometer array that was deployed in the Northwestern Pacific. We infer the azimuthal dependence of the Pn velocity, including the 2ϕ and 4ϕ terms of the sinusoidal functions, where ϕ is the back azimuth. We observe a skewed azimuthal dependence of the Pn velocity, with a large peak‐to‐peak amplitude of about 12%. Similar features are observed using an independent data set of teleseismic Pn waves. We constrain the direction of the crystallographic axes of olivine to explain the observed azimuthal dependence and identify A‐type olivine with a slightly dipping a‐axis and slightly tilting b‐axis being compatible with our observations. In contrast, we find that D‐type olivine with any direction of crystallographic axes cannot explain our observations. Secondary deformation and recrystallization in the older Pacific may be responsible for this strong and complex seismic anisotropy in the study region.

Pn velocity and anisotropy at the lithospheric mantle wedge beneath the middle-eastern Gangdese Metallogenic Belt, southern Tibet, China

The lithospheric mantle wedge beneath the middle-eastern Gangdese Metallogenic Belt (GMB), located at the forefront of the India-Eurasia collision-subduction zone retains many traces of structure evolution, which can be revealed by the velocity and anisotropy structure of the uppermost mantle of this region. In this study, high-resolution structures of the Pn velocity and anisotropy beneath the middle-eastern GMB are presented using interstation Pn differential travel-time data recorded by a dense broadband seismic array. The average Pn-wave velocity of 8.13 km/s beneath the GMB is close to that of the peridotite. The Pn velocity and anisotropy show prominent differences across the Jiali dextral strike-slip fault (JLF, ∼31° N) on the east of Shenza-Xietongmen rift, as indicates that the JLF (31°N) is an uppermost mantle trace of the Shiquan River-Nam Tso Mélange Zone. Low Pn velocity (<8.0 km/s) and a nearly EW-oriented Pn fast-wave direction to the north of the JLF, suggesting the EW extension of the Eurasian plate. NE-oriented Pn fast-wave direction with strange intensity (>0.03) to the south of the JLF indicates the direct influence of the Indian plate northward subduction. The rifts located at the boundary of high and low-velocity anomalies imply that the rift system formed under the influence of lithospheric mantle activity. Based on the NE compression caused by the Indian plate’s northeastward subduction, some of the forced upwellings of thermal material generated within the mantle wedge tip caused an EW extension of the crust. Subsequently, the rifts system was locally formed.

Pn Anisotropic Tomography of Hainan Island and Surrounding Areas: New Insights Into the Hainan Mantle Plume

AbstractA high‐resolution velocity and anisotropy model of the uppermost mantle under the Hainan Island and surrounding areas is obtained by a tomographic inversion of 8192 Pn arrival‐time data hand‐picked from high‐quality seismograms recorded at a total of 137 seismic stations of our recently deployed HAVESArray portable seismic network and Chinese provincial stations. Our new model shows obvious velocity and anisotropic structural features related to the distribution of Quaternary volcanic rocks on the surface. Extensive low Pn velocity anomalies are detected under the Weizhou Island, northern Hainan Island, southern Leizhou Peninsula, southern South China, and around the Pearl River Mouth Basin (PRMB), whereas high Pn velocity anomalies are imaged under the PRMB and areas of 108°–109°E and between the Baise‐Hepu fault and the Wuchuan‐Sihui fault. The Pn fast propagation directions exhibit a radiated pattern around the PRMB. Integrated with previous global and regional tomographic models, our present results indicate that the Hainan plume is upwelling around the PRMB in the upper mantle, and then plume‐related flow is split into several branches under the volcanoes in the northern Hainan Island, southern Leizhou Peninsula and Weizhou Island. This flow pattern could be associated with the westward deep subduction of the Philippine Sea and Pacific plates and eastward deep subduction of the Indian plate. Our results provide new insights into the formation mechanism of the Hainan‐Leizhou and Weizhou volcanoes.

Velocity and Anisotropic Structure of the Uppermost Mantle Beneath Central America and Northwestern South America From Pn Tomography

AbstractWe constructed the first Pn velocity and anisotropy model of the uppermost mantle in Central America and northwestern South America using the Pn tomography method and new ISC‐EHB data. Significantly low Pn velocities and arc‐parallel anisotropic structures are observed beneath the volcanic arc in Central America. The separate, low Pn velocity beneath the back‐arc volcano in Nicaragua shows that the hot material upwelling in the uppermost mantle beneath this volcano is likely separated from the main volcanic arc. The stable Caribbean Plate is characterized by high Pn velocities. The southward subduction and dehydration of the Caribbean Plate seem to be potential causes of the low Pn velocities in eastern Panama. Based on our Pn velocity, anisotropy and previous geological observations, we suggest a dynamic model of the Colombia‐Ecuador region including mantle flow and magmatic gap: there is arc‐parallel anisotropy in the uppermost mantle beneath the Colombia‐Ecuador volcanic arc, arc‐perpendicular mantle flow in the deeper mantle beneath the subducted plate, and a magmatic gap near the Caldas Tear.

Pn Anisotropic Tomography of Northeast Asia: New Insight Into Subduction Dynamics and Volcanism

AbstractWe present a new high‐resolution model of P‐wave anisotropic tomography of the uppermost mantle beneath NE Asia obtained by inverting 88,303 high‐quality manually picked Pn arrival times. Our model reveals strong lateral heterogeneities in both isotropic velocity and azimuthal anisotropy. Distinct high Pn velocities are visible under the basins in NE China, and eastern and western Japan basins. Very high Pn velocities with trench‐normal fast Pn anisotropy are revealed near the Kurile‐Japan trench, representing the incoming Pacific plate. Prominent low Pn velocity anomalies are revealed under active intraplate volcanoes in NE China and arc volcanoes on the Japan Islands. Very low Pn velocities with trench‐parallel fast Pn anisotropy are observed along the volcanic front in Japan, reflecting compressed mantle flows. An NW‐SE oriented low‐velocity (low‐V) zone from the Wudalianchi volcano to the Changbaishan volcano meets the E‐W oriented low‐V zone around the Changbaishan volcano and the N‐S low‐V zone east of the Sanjiang basin, which converge to a broad N‐S low‐V zone. This zone passes through the central Japan basin to the junction of the Japan and Izu‐Bonin trenches, where it meets the nearly E‐W low‐V zone from the Changle volcano. These low‐V anomalies exhibit zone‐parallel fast Pn anisotropy and may indicate channels for material and energy exchanges among the volcanoes, driven by mantle flows in the big mantle wedge. These results provide new insights into interplate and intraplate volcanism and subduction dynamics in the study region.

Anisotropic Pn Tomography of Alaska and Adjacent Regions

AbstractWe use Pn tomography with coordinate rotation to image in Alaska and adjacent areas and obtain a new Pn velocity and anisotropy model. The observed Pn velocities correlate well with the local geological structures. Low Pn velocities are observed beneath the Aleutian volcanic arc, which may reveal partial melting related to plate dehydration. However, it is separated by a distinct high‐velocity anomaly, suggesting a slab tear. The slightly low Pn velocities beneath the Denali volcanic gap suggest that a small amount of magma is produced but is insufficient to form surface volcanoes. We observe sharp bending of the fast directions on the two sides of the northeast edge of the Yakutat Plate, which reveals the plate’s bending. Large‐scale low velocities are observed beneath the northwestern Cordillera, which may be due to the upwelling of hot material from the deep mantle. The Pn fast directions are parallel to the Queen Charlotte‐Fairweather fault system, consistent with the shear strain direction of the faults, reflecting that the influence of the plate boundary is far‐reaching and that the deformation of the crust and the upper mantle is coupled. Our study provides new constraints on the complex deformation beneath Alaska and adjacent areas.

Sn attenuation tomography of southeastern Tibet: new constraints on lithospheric mantle deformation

SUMMARY We have formulated a 2-D Sn attenuation tomographic model to investigate the uppermost mantle shear wave Q and its tectonic implications beneath southeastern Tibet near Namche Barwa. To achieve our objective, we first compute interstation Q values using the two station method (TSM) analysis on 618 station pairs obtained from 26 regional earthquakes (Mw ≥5.5) with epicentral distances ranging from 5° to 15° recorded at 47 seismic stations belonging to the Namche Barwa network (XE network, 2003−2004). Furthermore, the QSn tomographic model is generated by utilizing these interstation Q values. QSn values are varying from 101 to 490 in the region. The tomography image reveals high attenuation (≤200 Q values) in the central region. Regions of low attenuation (&amp;gt;200 Q values) are observed in the southern part and in some small regions beneath the northern side of the study area. Consecutive high-low-high QSn values have been observed in the south part of the Lhasa block. The obtained QSn values, along with the prior isotropic Pn velocity model of the study area, indicate that the scattering effect is causing significant Sn wave energy dissipation due to structural heterogeneity present in the uppermost mantle beneath the region. This may be the result of the break-up of the subducting Indian Plate beneath the area.

Pn anisotropic tomography and mantle dynamics underneath the South China Sea and surrounding areas

Abstract In the present study, we present a new Pn anisotropic tomographic model of the uppermost mantle underneath the South China Sea and surrounding areas through the inversion of 23,042 high-quality hand-picked Pn arrival-time data. Our model shows obvious lateral heterogeneities in velocity and anisotropy that are closely correlated to surface geological structure in the study region. High Pn velocity anomalies are generally observed in the structurally stable areas, such as the South China block and Pearl-River Mouth basin. Obvious low Pn velocity anomalies are imaged underneath the Tengchong volcano, the Hainan volcano, and other volcanoes along the Ryukyu, Luzon, Philippine, and Java arcs, whereas the low-to-high-velocity transition zone is visible underneath the volcanoes in the Sumatra arc. Most large crustal earthquakes occurred in the low Pn velocity areas in the Ryukyu, Luzon, and Philippine arcs and the low-to-high Pn velocity transition zones in the Sumatra and Java arcs. These results suggest that these volcanism and earthquake occurrences may be closely related to the deep dynamics of the subduction of the Philippine Sea and Australian plates. High Pn velocity areas with trench-perpendicular fast directions of Pn propagation are revealed in the Ryukyu, Philippine, Sumatra, and Java trenches, presenting the subducting Philippine Sea and Australian plates, whereas low Pn velocity areas with trench-parallel fast directions are visible in the arc areas, suggesting the compressed mantle flows there. Our results provide new seismological evidence for understanding the mantle dynamics beneath the South China Sea and surrounding areas.

Pn tomography and anisotropic study of the Indian shield and the adjacent regions

High-resolution P-wave velocity and anisotropy structure of the hitherto elusive uppermost mantle beneath the Indian shield and its surrounding regions are presented to unravel the tectonic imprints in the lithosphere. We inverted high quality 19,500 regional Pn phases from 172 seismological stations for 4780 earthquakes at a distance range of 2° to 15° with a mean apparent Pn velocity of 8.22 km/s. The results suggest that the Pn velocity anomalies with fast anisotropic directions are consistent with the collision environments in the Himalaya, Tibetan Plateau, Tarim Basin, and Burmese arc regions. The higher Pn anomalies along the Himalayan arc explicate the subducting cold Indian lithosphere. The cratonic upper mantle of the Indian shield is characterized by Pn velocity of 8.12–8.42 km/s, while the large part of the central Indian shield has higher mantle-lid velocity of ~8.42 km/s with dominant anisotropic value of 0.2–0.3 km/s (~7.5%) suggesting the presence of mafic ‘lava pillow’ related to the Deccan volcanism. The impressions of the rifts and the mobile belts are conspicuous in the velocity anomaly image indicating their deep seated origin. The Pn anisotropy in the Indian shield exhibits a complex pattern and deviates from the absolute plate motion directions derived from the SKS study, demonstrating the presence of frozen anisotropy in the Indian lithospheric uppermost mantle, due to the large scale tectonic deformation after its breakup from the Gondwanaland. Whereas, Pn and SKS anisotropic observations are well consistent in Tarim basin, Tibetan regions, eastern Himalayan syntaxis and the Burmese arc. The modeled anisotropic Pn clearly manifests a lower velocity anomaly bounded by 85°E and 90°E ridges in the southern Bay of Bengal. Further, 85°E ridge spatially separates the BoB lithosphere into faster and slower regions consistent with the body wave tomography and free-air gravity observation.

New Uppermost Mantle Pn Velocity and Anisotropy Model of the Eastern Caribbean

AbstractUsing the Pn tomography method and new ISC‐EHB data, we obtained a new Pn velocity and anisotropy model of the uppermost mantle in eastern Caribbean. The consistency between the Pn fast directions and SKS splitting fast wave directions indicates that the anisotropic structure of the uppermost mantle in the area of the collisional plate boundary is the main cause of shear‐wave splitting. The results show that the seismic velocity of uppermost mantle beneath the Aves Ridge is not anomalous, implying that this area is no longer affected by the upwelling of mantle material. We observe high Pn velocities in the uppermost mantle beneath eastern Caribbean Sea, which are higher than in the deeper parts (100–200 km in depth) of the upper mantle; comparing these results with similar findings in the Japan Sea at the northwestern Pacific subduction zone, we believe the uppermost mantle beneath the back‐arc region is not or little affected by hot material from the deeper mantle.

Seismicity and Pn Velocity Structure of Central West Antarctica

AbstractWe have located 117 previously undetected seismic events mainly occurring between 2015 and 2017 that originated from glacial, tectonic, and volcanic processes in central West Antarctica using data recorded on Polar Earth Observing Network (POLENET/ANET) and UK Antarctic Network (UKANET) seismic stations. The seismic events, with local magnitudes (ML) ranging from 1.1 to 3.5, are predominantly clustered in four geographic regions; the Ellsworth Mountains, Thwaites Glacier, Pine Island Glacier, and Mount Takahe. Eighteen of the events are in the Ellsworth Mountains and can be attributed to a mixture of glacial and tectonic processes. The largest event noted in this study was a mid‐crustal (∼19 km focal depth; ML 3.5) normal mechanism earthquake beneath Thwaites Glacier. We also located 91 glacial events near the grounding zones of Thwaites Glacier and Pine Island Glacier that are predominantly associated with time periods of significant calving activity. Eight events, likely arising from volcano‐tectonic processes, occurred beneath Mount Takahe. Using Pn travel times from the seismic events, we find laterally variable uppermost mantle structure in central West Antarctica. On average, the Ellsworth Mountains are underlain by a faster mantle lid (VPn = ∼8.4 km/s) compared to the Amundsen Sea Embayment region (VPn = ∼8.1 km/s). Within the Amundsen Sea Embayment itself, we find mantle lid velocities ranging from ∼8.05 to 8.18 km/s. Laterally heterogeneous uppermost mantle structure, indicative of variable thermal and rheological structure, likely influences both geothermal heat flux and glacial isostatic adjustment spatial patterns and rates within central West Antarctica.

Uppermost Mantle Velocity beneath the Mid-Atlantic Ridge and Transform Faults in the Equatorial Atlantic Ocean

ABSTRACT Seismic rays traveling just below the Moho provide insights into the thermal and compositional properties of the upper mantle and can be detected as Pn phases from regional earthquakes. Such phases are routinely identified in the continents, but in the oceans, detection of Pn phases is limited by a lack of long-term instrument deployments. We present estimates of upper-mantle velocity in the equatorial Atlantic Ocean from Pn arrivals beneath, and flanking, the Mid-Atlantic Ridge and across several transform faults. We analyzed waveforms from 50 earthquakes with magnitude Mw&amp;gt;3.5, recorded over 12 months in 2012–2013 by five autonomous hydrophones and a broadband seismograph located on the St. Peter and St. Paul archipelago. The resulting catalog of 152 ray paths allows us to resolve spatial variations in upper-mantle velocities, which are consistent with estimates from nearby wide-angle seismic experiments. We find relatively high velocities near the St. Paul transform system (∼8.4 km s−1), compared with lower ridge-parallel velocities (∼7.7 km s−1). Hence, this method is able to resolve ridge-transform scale velocity variations. Ray paths in the lithosphere younger than 10 Ma have mean velocities of 7.9±0.5 km s−1, which is slightly lower than those sampled in the lithosphere older than 20 Ma (8.1 km±0.3 s−1). There is no apparent systematic relationship between velocity and ray azimuth, which could be due to a thickened lithosphere or complex mantle upwelling, although uncertainties in our velocity estimates may obscure such patterns. We also do not find any correlation between Pn velocity and shear-wave speeds from the global SL2013sv model at depths &amp;lt;150 km. Our results demonstrate that data from long-term deployments of autonomous hydrophones can be used to obtain rare and insightful estimates of uppermost mantle velocities over hundreds of kilometers in otherwise inaccessible parts of the deep oceans.

Lithospheric mantle underneath the Tibetan Plateau does not escape southeastward

A mass of lithospheric shortening occurred beneath the Tibetan Plateau due to the collision between the Indian and Eurasian plates. The extrusion of the upper crust beneath the Tibetan Plateau has been widely accepted, but whether the lithospheric mantle escapes laterally by creep deformation or rigid-block extrusion remains unclear. Based on the Pn travel time data, the velocity structure of the uppermost mantle in eastern and southeastern Tibetan Plateau was obtained by using the interstation travel-time difference method. The results show an obviously high Pn velocity beneath the Sichuan Basin and Qaidam Basin, indicating that the areas are stable blocks. The high Pn velocity beneath the eastern Lhasa terrane may be related to the underplating of the Indian lithospheric plate to the north. The Qilian block located in the north of the Kunlun fault appears as a normal Pn velocity, which indicates a mean lithospheric mantle thickness. A significantly low Pn velocity with an average of ~7.8 km/s in eastern and southeastern Tibetan Plateau, including the eastern Songpan–Ganzi terrane, the Chuan–Dian Diamond Block, and the northern part of Yunnan Province, indicates that the lithospheric mantle is relatively thin in these regions. Combined with other geologic and geophysical results, we propose that the lithospheric mantle of the plateau did not escape to eastern and southeastern Tibetan Plateau where the crust has been thickened by adding crustal material from the interior of the plateau.

Seismic velocity and anisotropy of the uppermost mantle beneath Madagascar from Pn tomography

SUMMARY The lithosphere of Madagascar records a long series of tectonic processes. Structures initially inherited from the Pan-African Orogeny are overprinted by a series of extensional tectonic and magmatic events that began with the breakup of Gondwana and continued through to the present. Here, we present a Pn-tomography study in which Pn traveltimes are inverted to investigate the lateral variation of the seismic velocity and anisotropy within the uppermost mantle beneath Madagascar. Results show that the Pn velocities within the uppermost mantle vary by ±0.30 km s–1 about a mean of 8.10 km s–1. Low-Pn-velocity zones (&amp;lt;8.00 km s–1) are observed beneath the Cenozoic alkaline volcanic provinces in the northern and central regions. They correspond to thermally perturbed zones, where temperatures are estimated to be elevated by ∼100–300 K. Moderately low Pn velocities are found near the southern volcanic province and along an E–W belt in central Madagascar. This belt is located at the edge of a broader low S-velocity anomaly in the mantle imaged in a recent surface wave tomographic study. High-Pn-velocity zones (&amp;gt;8.20 km s–1) coincide with stable and less seismically active regions. The pattern of Pn anisotropy is very complex, with small-scale variations in both the amplitude and the fast-axis direction, and generally reflects the complicated tectonic history of Madagascar. Pn anisotropy and shear wave (SKS) splitting measurements show good correlations in the southern parts of Madagascar, indicating coherency in the vertical distribution of lithospheric deformation along Pan-African shear zone as well as coupling between the crust and mantle when the shear zones were active. In most other regions, discrepancies between Pn anisotropy and SKS measurements suggest that the seismic anisotropy in the uppermost mantle beneath Madagascar differs from the vertically integrated upper mantle anisotropy, implying a present-day vertical partitioning of the deformation. Pn anisotropy directions lack the coherent pattern expected for an incipient plate boundary within Madagascar proposed in some kinematic models of the region.

Pn uppermost mantle tomography of Central Tibet: Implication for mechanisms of N-S rifts and conjugate faults

The north-south convergence between the Indian and Eurasian continents since Cenozoic has produced the Tibetan plateau, and east-west extension structures have also developed since late Cenozoic. Whether these extensional structures are limited to the crust or extend to the deep lithosphere remains controversial. The uppermost mantle serves as a link between the crust and deep lithosphere, the velocity structure of which provides an important constraint on exploring the mechanism of extensional structures. Based on Pn traveltime data recorded by the SANDWICH network in central Tibet, we use the interstation traveltime difference method to obtain the uppermost mantle velocity structure. Our results show significant velocity differences for the uppermost mantle beneath the conjugated strike-slip faults and rifts, which indicates their different formation mechanisms. The high Pn velocity beneath the conjugated strike-slip faults support an eastward horizontal shear at the base of the upper crust rather than the lower crust has contributed to the development of the conjugated strike-slip faults. The low Pn velocity beneath the rifts in Lhasa may provide evidence for the rifts cutting through the entire crust and reflect the connection between the formation of the rifts and deep tectonic activities. The most prominent features in our results are the low Pn velocity zones beneath the Tangra Yum Co Rift (TYR), the Pumqu-Xianza Rift (PXR) and the zone between the two rifts, and we explain the low Pn velocity as the top of the mantle wedge upon the subducting Indian lithospheric slab.