Burma Arc Research Articles

Recent felt earthquakes (Mw 5.0–5.9) in Mizoram of north‐east India region: Seismotectonics and precursor appraisal

Eight medium magnitude (Mw 5.0–5.9) severely felt earthquakes, mostly at a shallower depth (<35 km), occurred in a short period of 7 months only, April–October, 2020, in Mizoram State, north‐east region (NER) of India, and it created much panic in the state. The increasing population growth as well as rapid urbanization in Aizawl, the capital city of Mizoram, may be under severe threat for an impending large earthquake in the region. The NER, India, is jawed between two arcs, the Himalayan Arc collision zone to the north and the Indo–Burma (Myanmar) Arc subduction zone to the east, and witnessed two great earthquakes (Mw ≥ 8.0, 1897 and 1950) and a total of 25 large earthquakes (Mw ≥ 7.0) in the past since 1869. The 2020 sudden burst of seismic activity in Mizoram, along with a strong (Mw 6.7) earthquake in Manipur in 2016 and a medium magnitude (Mw 5.7) earthquake in 2017 in Tripura, adjacent to the Mizoram State, are critically examined to understand the ongoing seismotectonics and the precursor implications, if any. The 2016 Manipur earthquake is interpreted to be an intra‐plate earthquake triggered by the transverse Kopili Fault by strike‐slip motion. The 2017 Tripura earthquake is also interpreted to be typically an intra‐plate strike‐slip event in the eastern margin of the Bengal Basin. The seismic activity in Mizoram, on the other hand, is found to have occurred in the accretionary Indo–Burma Wedge, between the Churachandpur Mao Fault and Eastern Boundary Thrust. The centroid moment tensor solutions of the events suggest the dextral‐slip motion of the wedge along the Indo–Burma Arc. A preliminary analysis of the Mizoram seismicity recognizes a precursor anomaly, a temporal variation of much higher seismic activity, for an impending large earthquake in the study area.

Interseismic fault slip deficit and coupling distributions on the Anninghe-Zemuhe-Daliangshan-Xiaojiang fault zone, southeastern Tibetan Plateau, based on GPS measurements

The Anninghe-Zemuhe-Daliangshan-Xiaojiang (AZDX) fault zone is an important boundary between the South China block and southeastern (SE) Tibetan Plateau. Its strong earthquake potential has been highly concerned, because remarkable left lateral strike-slip motion has been observed in this fault zone. Using the linear spherical block model constrained by a refined GPS horizontal velocity field, we have derived the detailed partial slip deficits and the coupling fraction distributions along the AZDX fault zone, as well as self-consistent long-term slip rates on major active faults in SE Tibetan Plateau. Our results identified three remarkable slip deficit regions with high fault coupling fractions (0.6–1.0), including the southern segment of the Anninghe Fault (north of Xichang), the southern segment of the Daliangshan Fault (Butuo-Qiaojia), and southern terminus of the Xiaojiang Fault. These regions spatially coincide with the seismic gaps inferred from the spatiotemporal distributions of historical earthquake ruptures and paleoearthquake surveys, jointly implying their higher strong earthquake potential. Furthermore, combining with analyses of GPS data, previous geological and numerical simulation studies, we tentatively discussed the possible effect range of the Burma arc tectonic domain.

Mantle Transition Zone Structure Beneath Myanmar and Its Geodynamic Implications

AbstractLinking the India‐Tibet collision to the north and the Andaman oceanic subduction to the south, Myanmar occupies a crucial position in the India‐Eurasia convergence system. Various seismological studies have indicated that the Indian plate is obliquely subducted along the Burma arc. However, the depth extent and continuity of the subducted slab remain enigmatic. With seismic recordings collected from 114 recently deployed seismic stations, we map the topographies of the mantle transition zone (MTZ) boundaries, that is, the 410‐ and 660‐km discontinuities, beneath Myanmar using receiver functions. Regional 3‐D velocity models were adopted to account for the lateral velocity heterogeneity. The 410‐km discontinuity is uplifted by over 15 km within 95°E‐97°E and 21°N‐24°N beneath Myanmar. This feature correlates well with the east‐dipping high‐velocity anomaly in the tomographic models, with a velocity increase of 0.9%–1.2% at the 410‐km discontinuity depth, suggesting that the subducted slab has reached the MTZ. The uplift of the 410‐km discontinuity terminates to the south at ∼21°N, indicating a distinct change in slab geometry. Our results also reveal a depressed 660‐km discontinuity, which is spatially offset to the southwest of the uplifted 410‐km discontinuity. We propose that the offset between the 410‐km discontinuity uplift and the 660‐km discontinuity depression could indicate a slab break‐off and tearing beneath Myanmar, which was triggered by the northward motion of the Indian plate during the eastward subduction. We further speculate that the slab tear could mark the transition from oceanic to continental plate subduction.

Lithospheric dynamics in the vicinity of the Tengchong volcanic field (southeastern margin of Tibet): an investigation usingPreceiver functions

SUMMARYTengchong volcanic field (TVF) in the northern Indochina block lies in a critical area for understanding complex regional dynamics associated with continent–continent convergence between the Indian and Eurasian plates, including northeastward compression generated by subduction of the Indian Plate beneath the Burma Arc, and southeastward lateral extrusion of the crust from below central Tibet. We gathered 3408 pairs of P receiver functions with different frequencies and calculated the splitting parameters of the Moho-converted Pms phase. An anisotropic H-κ stacking algorithm was used to determine crustal thickness and Vp/Vs ratios. We also inverted for the detailed S-velocity structure of the crust and upper mantle using a two-step inversion technique. Finally, we mapped the topography of the lithosphere–asthenosphere boundary. Results show fast-wave polarization directions with a dominant NE–SW orientation and delay times varying between 0.19 and 1.22 s, with a mean of 0.48 ± 0.07 s. The crustal Vp/Vs ratio varies from 1.68 to 1.90 and shows a maximum value below the central part of the TVF, where there is relatively thin crust (∼35–39 km) and a pronounced low-velocity anomaly in the middle–lower crust. The depth of the lithosphere–asthenosphere boundary ranges from 53 to 85 km: it is relatively deep (∼70–85 km) in the vicinity of the TVF and relatively shallow in the south of the study area. In the absence of low shear wave velocity in the upper mantle below the TVF, we propose that the low-velocity anomaly in the lower crust beneath the TVF derives from the upper mantle below the neighbouring Baoshan block.

Is there a big mantle wedge under eastern Tibet?

The big mantle wedge (BMW) model was originally proposed to explain the origin of the intraplate Changbaishan volcano in NE China where the subducting Pacific slab becomes stagnant in the lower part of the mantle transition zone (MTZ). The BMW is defined as a broad region in the upper mantle and upper part of the MTZ overlying the long stagnant Pacific slab. Under eastern Tibet, prominent low-velocity (low-V) anomalies are revealed in the upper mantle, whereas an obvious and broad high-velocity (high-V) anomaly is imaged in the MTZ from the Burma arc northward to the Kunlunshan fault zone and eastward to the Xiaojiang fault zone. In addition, receiver-function analyses clearly illustrate a thickened MTZ in a similar area. Hence we conclude that there is a BMW structure under eastern Tibet, which can explain the deep origin of the Tengchong volcano and generation of large crustal earthquakes in the region. The Tengchong volcano is caused by hot and wet mantle upwelling in the BMW and fluids from dehydration reactions of the stagnant Indian slab in the MTZ. The 2008 Wenchuan earthquake (Ms 8.0) took place in a transitional belt from the low-V Songpan-Ganzi block to the high-V Sichuan basin, suggesting that the occurrence of the Wenchuan earthquake was affected by the BMW structure. These results shed new light on the mantle structure and dynamics under eastern Tibet.

Scarps Generated by Erosion and Sedimentation

The Ganges–Brahmaputra Delta (GBD), the world’s largest delta, is composed of sediments eroded from the collision zone of Himalayas. These sediments moved the margin of the Indian Subcontinent for ~400 km and formed the thickest sedimentary sequence, which is subducted within the subduction zone of the Burma arc. A critical facility is located in the GBD central part. Although it is located for more than 300 km from the Indo-Burma subduction zone in the east and the continental collision zone in the north, these planetary structures significantly affects the seismic activity and seismic hazard of the GBD central part. Scarps exposed by trenches in the vicinities of this object were found in the surface and studied. The trench walls exhibit inclined layers with small vertical displacement, which could indicate a former paleoearthquake, however, they are underlain by horizontal layers without displacement. This pattern is interpreted as a result of erosion (neotectonic) activity related to repeated fluctuations of the Ganges River bed, which are traced in all trenches.

Holocene eruptions of Mt. Popa, Myanmar: Volcanological evidence of the ongoing subduction of Indian Plate along Arakan Trench

In the western part of Myanmar the geodetic and seismic data indicate the ongoing highly oblique subduction of Indian Plate under Eurasian Plate. Volcanoes of Burma arc, however, did not produce eruptions in the recorded history and are considered extinct. Such perception questions the ongoing subduction, as well as keeping local officials unaware of possibility of future volcanic eruptions in the country. We have investigated the youngest lava flows and pyroclasts of Mt. Popa, which is the best-preserved polygenetic volcanic edifice in Myanmar. The Ar/Ar dating of the youngest products of the volcano provided very young radiometric ages which were unable to be measured accurately. The radiocarbon dating of paleosols intercalated with the most recent ash layers of the volcano has shown that Mt. Popa produced several eruptions in the beginning of Holocene. The youngest eruption occurred ~8000 BP and included the 1.3 km3 gravitational collapse of the volcanic cone with deposition of the 11-km-long debris avalanche, immediately followed by emplacement of 0.1 km3 of pyroclastic flow of calc-alkaline basaltic andesite composition. The collapse direction as well as complex morphology of the resulted crater were prearranged by geometry of the listric fault, which was formed during the pre-collapse asymmetric gravitational spreading of the volcanic cone. The recently erupted products are geochemically similar to products of young monogenetic volcanoes of Monywa area located 150 km north Mt. Popa, and both display patterns consistent with magma generation at an active subduction system. Low magma production rate of Mt. Popa (3 × 10−5 km3/year averaged for the 1 Ma of the volcano history) is in agreement with the highly oblique angle and slow rate of the subduction. The fact of the Holocene eruptions of calc-alkaline composition in Burma arc represents volcanological evidence of the ongoing subduction in this part of the collision zone between India and Asia.

Deep structure of the Longmenshan fault zone and mechanism of the 2008 Wenchuan earthquake

Since the occurrence of the 2008 Wenchuan earthquake ( M s8.0), many researchers have conducted extensive seismological and geophysical observations and investigations and obtained important results about the Longmenshan fault zone. Crustal structure inferred from local tomography shows that seismic velocity exhibits significant changes across the Wenchuan earthquake hypocenter from the south to the north. To the south,obvious low-velocity (low-V) anomalies exist, whereas strong lateral heterogeneities are revealed to the north, which may explain why the aftershocks extend northeastward. The Wenchuan earthquake occurred at the boundary between high-velocity (high-V) and low-V anomalies and a significant low-V zone is revealed below the mainshock hypocenter, suggesting that the nucleation of the Wenchuan earthquake was related to partial melts and/or fluid effects and associated with the reduction of effective normal stress on the fault plane, due to hightemperature and highpressure in the Longmenshan fault zone caused by the India-Asia collision. The upper-mantle structure inferred from teleseismic tomography shows that the Longmenshan fault zone is located in the transition zone from low-V anomalies beneath the Songpan-Ganzi block to high-V anomalies beneath the Sichuan basin. This structural feature extends down to 200−300 km depths. High-V anomalies in the mantle transition zone are connected with those in the upper mantle beneath the Burma arc, indicating that the Wenchuan earthquake could be associated with upwelling of hot and wet materials in the big mantle wedge formed by the deep subduction of the Indian plate. These results suggest that the generation of the Wenchuan earthquake was related to structural heterogeneities in not only the crust but also the upper mantle. In addition,high-poisson’s ratio and high-conductivity anomalies are revealed beneath the Wenchuan earthquake source area, which may also reflect lower crustal flow that is compressed and moving eastward. When the eastward flow encountered the strong Sichuan basin block around the Longmenshan fault zone, the flow is further compressed upward along the margin of the Sichuan basin, which could have caused the fault ruptureand generated the Wenchuan earthquake. Some geologists found a vertical co-seismic displacement of 6.5 m in the Longmenshan fault zone, suggesting that there exists an imbricate structure due to the crustal shortening. Such a shortening mechanism could rupture the seismogenic fault leading to the large earthquake. The Zipingpu reservoir is located very close to the Wenchuan earthquake epicenter. It is still debated whether the reservoir triggered the 2008 Wenchuan earthquake or not. Although some simulation results suggest that the Wenchuan earthquake could be triggered by the reservoir, there are large differences in the estimated stress in the hypocentral area due to large uncertainties of the physical parameters adopted in the numerical simulations. Hence, the physical parameters should be determined precisely in future investigations so as to clarify the effect of the reservoir on the seismogenesis. We also discuss the causal mechanism of the 2013 Lushan earthquake ( M s7.0) and future seismic risk in the gap between the aftershock zones of the Wenchuan and Lushan earthquakes.

Structure of the mantle transition zone under the Yunnan region and its geodynamic implications

The mantle transition zone (MTZ) bounded by physical discontinuities at 410 and 660 km depth is believed to play a key role in controlling mantle flow. Mineral physics experiments reveal that phase changes from olivine to β phase and γ -spinel to perovskite+magnesiowustite, at pressures equivalent to the existing at those mantle discontinuities, largely explain the variations in seismic velocity observed by the seismologists. The globally observed seismic velocity discontinuity at 660 km depth marks the bottom of the MTZ, which is the natural boundary between the upper and lower mantle. This discontinuity is considered an important mantle boundary, since it is especially associated with the mantle dynamics, the source of mantle plumes and the sinking of the subducted plates. On the other hand, temperature variations are the cause of thickening and thinning of the MTZ in correspondence with the positive and negative slopes of the Clapeyron curve at the 410 and 660 km mantle discontinuities, respectively. Receiver function analysis is a straightforward and commonly accepted method of studying the crust and upper-mantle structure through the use of teleseismic waveforms recorded at three-component seismic stations. This method requires the rotation of the ZNE displacement components to the radial and transversal components in the horizontal plane; and later the transformation of the radial and vertical components, which lie in the same vertical plane, into the L and Q components of the ground motion. By deconvolving the L component from the Q -component, both the source, far field path and instrument effects are removed. The resulting receiver function waveform is the ground’s impulse response. The analysis of each receiver function recorded at an array station provides, after moveout correction, a detailed estimation of the depth of the interface where the P-to-S seismic phase conversion is produced. However, the converted Ps phases at the discontinuities of 410 and 660 km depth are so weak that stacking of a large amount of waveforms is needed to enhance the signal-to-noise ratio in the receiver functions. In this study, we obtained 8600 teleseismic P-wave receiver functions recorded at 48 permanent broadband stations deployed in Yunnan and surroundings, which later were properly stacked in a single trace in 1°×1°-sized grid cells. Before stacking, the receiver functions were migrated from time domain to depth domain with the help of a reference earth model. Finally, we obtained 6 stacking images of receiver functions at latitude of 28°, 27°, 26°, 25°, 24° and 23°N, being the stacking depth in the 0–800 km range. The results indicate: (1) further north of 26°N, the average depths of the 410 and 660 km mantle discontinuities are in a range of 407–408 and 663–670 km, respectively, while the average thickness of the MTZ is within the range of 255–269 km, which are values very close to the global average thickness of 250 km. (2) South of 26°N, the average depths of the 410 and 660 km discontinuities are in a range of 412–426 and 675–703 km, respectively, and the average thickness of the MTZ is within the range of 262–279 km, which is obviously larger than the global average value of 250 km. The deepening of the 410 and 660 km discontinuities in the Yunnan region is obviously associated with the eastward subduction of the Indian plate below the Burma Arc; then, based on the greater thickness of the MTZ beneath the Yunnan region, we suggest that the eastward subduction of the Indian plate occurs mainly south of 26°N in the Yunnan region.

Pn anisotropic tomography and mantle dynamics beneath China

We present a new high-resolution Pn anisotropic tomographic model of the uppermost mantle beneath China inferred from 52,061 Pn arrival-time data manually picked from seismograms recorded at provincial seismic stations in China and temporary stations in Tibet and the Tienshan orogenic belt. Significant features well correlated with surface geology are revealed and provide new insights into the deep dynamics beneath China. Prominent high Pn velocities are visible under the stable cratonic blocks (e.g., the Tarim, Junngar, and Sichuan basins, and the Ordos block), whereas remarkable low Pn velocities are observed in the tectonically active areas (e.g., Pamir, the Tienshan orogenic belt, central Tibet and the Qilian fold belt). A distinct N-S trending low Pn velocity zone around 86°E is revealed under the rift running from the Himalayan block through the Lhasa block to the Qiangtang block, which indicates the hot material upwelling due to the breaking-off of the subducting Indian slab. Two N-S trending low Pn velocity belts with an approximate N-S Pn fast direction along the faults around the Chuan-Dian diamond block suggest that these faults may serve as channels of mantle flow from Tibet. The fast Pn direction changes from N-S in the north across 27°N to E-W in the south, which may reflect different types of mantle deformation. The anisotropy in the south could be caused by the asthenospheric flow resulted from the eastward subduction of the Indian plate down to the mantle transition zone beneath the Burma arc. Across the Talas-Fergana fault in the Tienshan orogenic belt, an obvious difference in velocity and anisotropy is revealed. To the west, high Pn velocities and an arc-shaped fast Pn direction are observed, implying the Indo-Asian collision, whereas to the east low Pn velocities and a range-parallel Pn fast direction are imaged, reflecting the northward underthrusting of the Tarim lithosphere and the southward underthrusting of the Kazakh lithosphere. In most parts of eastern China, pronounced low Pn velocities and a complex anisotropy pattern are observed, implying the re-orientation of the olivine arrangement in the thin lithosphere due to the westward subduction of the Pacific plate.

Teleseismic P‐wave tomography and mantle dynamics beneath Eastern Tibet

We determined a new 3-D P-wave velocity model of the upper mantle beneath eastern Tibet using 112,613 high-quality arrival-time data collected from teleseismic seismograms recorded by a new portable seismic array in Yunnan and permanent networks in southwestern China. Our results provide new insights into the mantle structure and dynamics of eastern Tibet. High-velocity (high-V) anomalies are revealed down to 200 km depth under the Sichuan basin and the Ordos and Alashan blocks. Low-velocity (low-V) anomalies are imaged in the upper mantle under the Kunlun-Qilian and Qinling fold zones, and the Songpan-Ganzi, Qiangtang, Lhasa and Chuan-Dian diamond blocks, suggesting that eastward moving low-V materials are extruded to eastern China after the obstruction by the Sichuan basin, and the Ordos and Alashan blocks. Furthermore, the extent and thickness of these low-V anomalies are correlated with the surface topography, suggesting that the uplift of eastern Tibet could be partially related to these low-V materials having a higher temperature and strong positive buoyancy. In the mantle transition zone (MTZ), broad high-V anomalies are visible from the Burma arc northward to the Kunlun fault and eastward to the Xiaojiang fault, and they are connected upward with the Wadati-Benioff seismic zone. These results suggest that the subducted Indian slab has traveled horizontally for a long distance after it descended into the MTZ, and return corner flow and deep slab dehydration have contributed to forming the low-V anomalies in the big mantle wedge. Our results shed new light on the dynamics of the eastern Tibetan plateau.

GPS-derived heterogeneous inter- and intra-Indian plate deformation and its consequences

We have analyzed 18 permanent GPS stations in and around the Indian plate to understand inter- and intraplate deformation patterns of infinitesimal strain variations. Differential deformation pattern along and across strike of the Himalaya shows westward decrease in crustal shortening with strong influence of the local geology. Subduction rate toward the eastern Indian plate boundary decreases drastically from south to north, indicating the development of locking condition. Excluding the western boundary of the Indian plate, which is under strong deformation due to flexuring of crust, the rest part of the Indian plate is rigid. Indo-Gangetic plain plays a key role in mountain building processes. The proposed kinematic model suggests the dominance of tensional forces and compressional forces toward craton margin and mountain front, respectively, with conditions for the development of proto-thrust in the form of step-out structures, south of MFT. Strain analysis shows western and eastern ends of the Himalayan front are under strong extensional and compressional strain, indicating the probability of future earthquakes. Similar high strain conditions also prevail on the west coast of the Indian craton as well Ganga–Brahmaputra Delta and Burma Arc region toward the northeastern corner of the Indian craton.

Magma mixing recorded by Sr isotopes of plagioclase from dacites of the Quaternary Tengchong volcanic field, SE Tibetan Plateau

The Tengchong volcanic field east of the Burma arc in SW China comprises numerous Quaternary volcanoes. The volcanic rocks can be grouped into four units, numbered 1–4 from oldest to youngest. Units 1, 3 and 4 are composed of trachybasalt, basaltic trachyandesite and trachyandesite, respectively, whereas Unit 2 consists of hornblende dacite. Primary minerals in the dacite include plagioclase, clinopyroxene, amphibole and magnetite, all set in a glassy groundmass. Some of the dacites are moderately to highly weathered and show effects of low temperature alteration, with loss on ignition (LOI) up to 9.6wt% and positive correlations of LOI with Al2O3, but negative correlations with SiO2, CaO and Na2O. Fresh dacites (LOI <3wt%) belong to the calk-alkaline series and have pronounced negative Ti, Nb and Ta anomalies. They have whole-rock 87Sr/86Sr ratios ranging from 0.7090 to 0.7101, εNd values from −10.8 to −11.8, and εHf values from −5.1 to −6.8. Many of the large plagioclase crystals in the fresh dacites have reverse zoning, ranging from An55 in the cores to An72 in the rims. The smaller phenocrysts have relatively uniform An values between 56 and 50. One plagioclase crystal has an An value of 33.5.Plagioclase in the fresh dacite has relatively high Sr contents and 87Sr/86Sr ratios ranging from 0.7050 to 07125 except for one value of 0.7516, whereas plagioclase in the altered dacite has low Sr contents and highly variable Sr isotopic compositions (0.7039–0.7138), indistinguishable from those of the fresh rocks.Both An values and Sr isotopic compositions of the plagioclase indicate mixing of mantle- and crustally-derived magmas. We therefore propose that mantle-derived basaltic magma caused partial melting of the lower–middle crust and that mixing of the mafic and felsic magmas produced the dacite in staging magma chambers. The dacites contain minerals crystallized from both the mafic and felsic magmas, as well as a few xenocrysts plucked from the country rocks.

The plate contact geometry investigation based on earthquake source parameters at the Burma arc subduction zone

Accurately characterizing the three-dimensional geometric contacts between the crust of the Chinese mainland and adjacent regions is important for understanding the dynamics of this part of Asia from the viewpoint of global plate systems. In this paper, a method is introduced to investigate the geometric contacts between the Eurasian and Indian plates at the Burma arc subduction zone using earthquake source parameters based on the Slab1.0 model of Hayes et al. (2009, 2010). The distribution of earthquake focus depths positioned in 166 sections along the Burma Arc subduction zone boundary has been investigated. Linear plane fitting and curved surface fitting has been performed on each section. Three-dimensional geometric contacts and the extent of subduction are defined quantitatively. Finally, the focal depth distribution is outlined for six typical sections along the Burma arc subduction zone, combining focal mechanisms with background knowledge of geologic structure. Possible dynamic interaction patterns are presented and discussed. This paper provides an elementary method for studying the geometric contact of the Chinese mainland crust with adjacent plates and serves as a global reference for dynamic interactions between plates and related geodynamic investigations.

Heterogeneous mantle source and magma differentiation of quaternary arc-like volcanic rocks from Tengchong, SE margin of the Tibetan Plateau

The Tengchong volcanic field north of the Burma arc comprises numerous Quaternary volcanoes in the southeastern margin of the Tibetan Plateau. The volcanic rocks are grouped into four units (1–4) from the oldest to youngest. Units 1, 3 and 4 are composed of olivine trachybasalt, basaltic trachyandesite and trachyandesite, and Unit 2 consists of hornblende dacite. The rocks of Units 1, 3, and 4 form a generally alkaline suite in which the rocks plot along generally linear trends on Harker diagrams with only slight offset from unit to unit. They contain olivine phenocrysts with Fo values ranging from 65 to 85 mol% and have Cr-spinel with Cr# ranging from 23 to 35. All the rocks have chondrite-normalized REE patterns enriched in LREE and primitive mantle-normalized trace element patterns depleted in Ti, Nb and Ta, but they are rich in Th, Ti and P relative to typical arc volcanics. Despite minor crustal contamination, 87Sr/86Sr ratios (0.706–0.709), eNd values (−3.2 to −8.7), and eHf values (+4.8 to −6.4) indicate a highly heterogeneous mantle source. The Pb isotopic ratios of the lavas (206Pb/204Pb = 18.02–18.30) clearly show an EMI-type mantle source. The underlying mantle source was previously modified by subduction of the Neo-Tethyan oceanic and Indian continental lithosphere. The present heterogeneous mantle source is interpreted to have formed by variable additions of fluids and sediments derived from the subducted Indian Oceanic lithosphere, probably the Ninety East Ridge. Magma generation and emplacement was facilitated by transtensional NS-trending strike-slip faulting.

Accurate Relative Earthquake Hypocenters Reveal Structure of the Burma Subduction Zone

The Burma arc links the Himalaya to the Andaman–Sumatra trench, and comprehension of the geological features in the region is vital to understand the active tectonic processes in the area. Subduction on the Sumatra trench produced the devastating 2004 Sumatra earthquake ( M w∼9.0), but the mechanisms accommodating relative India–Sundaland motion in Burma are still unknown. Previous seismological studies of the area use only earthquake catalog hypocenters, so structural details of subducted material remain poorly understood. Here, accurate relative hypocenters for 81 earthquakes are estimated using first arrivals picked from regional and teleseismic recordings. Depth determination is improved using measured travel-time differences between P onsets and depth phases ( pP and sP ). The results clearly illustrate a slab 21°–25° N dipping ∼25° at 40–80 km deep, ∼40° at 80–120 km deep, and ∼60° at 120–160 km deep, and its strike follows the Indo–Burman ranges. At latitudes >25° N earthquakes become more shallow, and a diffuse picture of hypocenters with an east–west lateral discontinuity at depth is observed, indicating lateral deformation of the slab. Previous studies suggest there is a transition at ∼90 km deep from shallower strike-slip earthquakes to deeper thrust-type events. Here, accurate depth estimates, combined with confirmation of two Global Centroid Moment Tensor solutions using body-wave modeling, shows that thrust and strike-slip earthquakes both occur deeper than 100 km in the Burma arc.

Collision of the Ganges–Brahmaputra Delta with the Burma Arc: Implications for earthquake hazard

We take a fresh look at the topography, structure and seismicity of the Ganges–Brahmaputra Delta (GBD)–Burma Arc collision zone in order to reevaluate the nature of the accretionary prism and its seismic potential. The GBD, the world's largest delta, has been built from sediments eroded from the Himalayan collision. These sediments prograded the continental margin of the Indian subcontinent by ∼ 400 km, forming a huge sediment pile that is now entering the Burma Arc subduction zone. Subduction of oceanic lithosphere with > 20 km sediment thickness is fueling the growth of an active accretionary prism exposed on land. The prism starts at an apex south of the GBD shelf edge at ∼ 18°N and widens northwards to form a broad triangle that may be up to 300 km wide at its northern limit. The front of the prism is blind, buried by the GBD sediments. Thus, the deformation front extends 100 km west of the surface fold belt beneath the Comilla Tract, which is uplifted by 3–4 m relative to the delta. This accretionary prism has the lowest surface slope of any active subduction zone. The gradient of the prism is only ∼ 0.1°, rising to ∼ 0.5° in the forearc region to the east. This low slope is consistent with the high level of overpressure found in the subsurface, and indicates a very weak detachment. Since its onset, the collision of the GBD and Burma Arc has expanded westward at ∼ 2 cm/yr, and propagated southwards at ∼ 5 cm/yr. Seismic hazard in the GBD is largely unknown. Intermediate-size earthquakes are associated with surface ruptures and fold growth in the external part of the prism. However, the possibility of large subduction ruptures has not been accounted for, and may be higher than generally believed. Although sediment-clogged systems are thought to not be able to sustain the stresses and strain-weakening behavior required for great earthquakes, some of the largest known earthquakes have occurred in heavily-sedimented subduction zones. A large earthquake in 1762 ruptured ∼ 250 km of the southern part of the GBD, suggesting large earthquakes are possible there. A large, but poorly documented earthquake in 1548 damaged population centers at the northern and southern ends of the onshore prism, and is the only known candidate for a rupture of the plate boundary along the subaerial part of the GBD–Burma Arc collision zone.

Crust‐Mantle Velocity Structure of S Wave and Dynamic Process Beneath Burma Arc and Its Adjacent Regions

AbstractThe Rayleigh dispersion data of 530 selected paths crossing Burma Arc and its adjacent regions are determined by applying the MF‐FTAN (Matched‐Filter Frequency‐Time Analysis) technique to long period data recorded at SHIO, CHTO, KMI and LSA stations. The periods of the Rayleigh dispersion are in the range of 10.45~105.03 s. On the basis of this, a method of grid dispersion inversion is proposed to extract pure‐path dispersion in each 1° × 1° grid from mixed‐path dispersion. Then the S wave velocity structure to a depth of 200 km is inverted from pure‐path dispersion in each grid, and finally the 3‐D velocity structure of S wave is reconstructed beneath Burma Arc and its adjacent regions. The result shows that taking Sagain fault as the boundary, the velocity of crust on the west side of the fault is higher than that on the east side of the fault. The lithosphere thickness in the Indo‐Burma areas is in the range of 110~130 km and the S wave velocity at the top of upper mantle is 4.3~4.4 km/s beneath the Indo‐Burma regions. However there is a lower‐velocity column of uprising mantle about 150~200 km wide beneath Yunnan‐Burma‐Thailand Block on the east side of Burma Arc. The lithosphere thickness is 70~80 km and the velocity of S wave at the top of the upper mantle is 4.1~4.2 km/s in the area. On the other hand, the S wave velocity structure shows an NS (north‐south) variation, this feature corresponds well to the distribution of earthquake sources, strikes of the faults and volcanoes.

A present-day tectonic stress map for eastern Asia region

Based on the data of earthquake centroid moment tensor (CMT) solution, P-wave first motion focal mechanism solution and deep hole breakouts, a present-day tectonic stress map for eastern Asia region is compiled. The original stress data are smoothed for every 200 km × 200 km area by taking the average of all stress indicators within each sub-region. The stress map shows the spatial distribution of the orientation of principal stress axes and the stress regimes. An earthquake focal mechanism map for the eastern Asia is also given. The maps of orientation of principal stress axes show that, apart from the strong influence of the collision between the Indian Ocean plate and Eurasian plate, the present-day tectonic stress in eastern Asia is significantly affected by the back-arc extension of the subduction zones. The joint effect of the continental collision at the Himalaya arc and back-arc extension in the Burma arc region may be responsible for the remarkable rotation of the principal stress orientations in southeastern part of the Tibet plateau. The joint action of the collision between the Philippine Sea plate and Eurasian plate at Taiwan Island and the back-arc extension of the Ryukyu arc affect the stress field in eastern part of China. There are no strong earthquakes in the present day in the vast back-arc region of the Java trench subduction zone. The back-arc extension there may create a condition favorable to the southward flow of the lithosphere material in southeastern Asia. In the inner part of the Tibet plateau region, roughly demarcated by the Kunlun mountain, the northern and northeastern part is a broad intracontinental compressive zone, while the southern and southwestern part is generally in a normal-faulting stress state.

Slip-line field theory and large-scale continental tectonics

A simple analogy is made between the tectonics of Asia and deformation in a rigidly indented rigid–plastic solid. India is analogous to the indenter and the great strike-slip faults correspond to slip lines. For various indentation geometries, the sense and linearity (or curvature) of strike-slip faults, convergence at the Burma arc and the existence of the Himalayan Burman Syntax, the conjugate strike-slip faults in Mongolia and the extension at the Baikal and Shansi graben can be predicted. Given the horizontal force necessary to support Tibet, an average shear stress of a few to several hundred bars along faults in Asia is predicted, corresponding to the yield stress of rigid–plastic material.