Myanmar Geosciences Society

In the Indo-Burma subduction zone the subducted and hanging-wall lithospheres interact strongly with the asthenosphere due to India-Asia plate convergence. Imaging the lithosphere-asthenosphere boundary (LAB) is therefore crucial for an in-depth understanding of the convergence process. Our Sp receiver function (RF) images built with dense array data reveal a regionally shallow LAB at an average depth of ~70 km, except beneath the Indo-Burma Ranges, where the Indian LAB dips eastward at an average of ~20° down to ~140 km; this dipping geometry is resolved primarily through SKS-derived RFs. Farther east, a positive velocity gradient emerges ~50-80 km beneath the Burma LAB, together they define an upper-asthenospheric low-velocity layer. Waveform modeling further indicates a sharp LAB, characterized by a Vs drop up to ~6-10% over <~20 km. Our RF results support a melt-rich LAB, potentially linked to the asthenospheric flow induced by the Indian plate subduction and subsequent rollback and/or tearing.

Continental‐scale shear zones in Indochina record Cenozoic tectonic processes in SE Asia. Previous extrusion models link these shear zones to northward indentation of the Indian continent and conflict with distributed Oligocene conjugate strike‐slip pairs, formed by E‐W‐directed compression (present coordinates). This paper presents evidence of Oligocene shearing along the Mogok Metamorphic Belt in Myanmar, the western boundary of the Indochina block. The Kyanigan quarry at northern Mogok Metamorphic Belt exposes paragneiss, marble and quartzite schist with E‐W striking, which would be dragged from N‐S striking foliations of the main Mogok Metamorphic Belt by right‐lateral shearing of the brittle Sagaing Fault. After restoration, right‐lateral shearing ‘σ’ structures, cored with garnet, in paragneiss in the Kyanigan quarry are consistent with ‘σ’ and ‘δ’ structures in the Moulmein granitic mylonite to the south. U–Pb ages of metamorphic zircon of a sheared paragneiss and a leucogranite, cross‐cutting shear foliation in the Kyanigan quarry, and a biotite 39Ar‐40Ar age of mylonite at Moulmein constrain dextral shearing to 33–25.4 Ma, coeval with other 31–24.5 Ma shear zones in Indochina. The eleven NW‐striking left‐lateral shear zones and N‐S to NE‐striking right‐lateral shear zones define Oligocene conjugate fault patterns in Indochina. After restoration of ~80–45° clockwise rotation, these Oligocene conjugate shear zones, coupled with the anticlinal configuration of the Xuelongshan and Doi Inthanon‐Doi Suthep domes, reflect approximate N–S‐directed shortening. These structures to the north with coeval Myanmar‐Sumatra magmatic arc to the south demonstrate that they correspond to back‐arc contraction during northward subduction of the Indian Ocean. Oligocene continental‐scale intra‐continental shearing may be triggered by syn‐subduction compression in SE Asia.

Abstract Geological evidence has demonstrated the presence of an intra–Neo-Tethyan subduction system during the Cretaceous. However, when and how this intra-oceanic subduction was initiated, especially for the eastern Neo-Tethys, are still not well constrained. Here we present geochemical and geochronological analyses of the Indawgyi mafic rocks from the Central Ophiolite Belt in the West Burma Block (Myanmar), which record early forearc spreading during the intra–Neo-Tethyan subduction initiation. Zircon U-Pb ages of gabbros indicate the ophiolitic crust formation at ca. 120 Ma. Gabbros show mid-oceanic-ridge basalt–like rare earth element patterns and depleted Sr-Nd-Pb isotopic compositions with negative anomalies of high field strength elements (e.g., Nb, Ta, Zr, and Hf), similar to forearc basalt characteristics. Basalts show more slab-derived component signatures than the gabbros and represent mantle wedge magmas most likely formed between forearc spreading and arc maturation. These data, together with regional geological records and geophysical observations, suggest that the Indawgyi gabbros were derived from an intra–Neo-Tethyan forearc setting during the early stage of subduction initiation. Considering the timing of supra-subduction zone ophiolites and metamorphic sole in the Indo-Burma Range, we propose that spontaneous subduction initiation and sinking of the eastern Neo-Tethyan lithosphere during the Early Cretaceous (ca. 120 Ma) led to formation of the Indawgyi forearc crust, whereas subsequent mature subduction resulted in the Middle Cretaceous (ca. 108–90 Ma) arc magmatism in the West Burma Block. These findings confirm the double-subduction model of the Neo-Tethys Ocean and shed new light on the intra–Neo-Tethyan subduction initiation.

Although Myanmar is an earthquake-prone country, there has not been proposed an official national seismic hazard map. Thus, this study conducted a probabilistic seismic hazard assessment for Myanmar and some of its metropolitan areas. Performing this assessment required a set of databases that incorporates both earthquake catalogs and fault parameters. We obtained seismic parameters from the International Seismological Centre, and the fault database includes fault parameters from paper reviews and the database. Based on seismic activities, we considered three categories of seismogenic sources—active fault source, shallow area source, and subduction zone source. We evaluated seismic activity of each source based on the earthquake catalogs and fault parameters. Evaluating the ground-shaking behaviors for Myanmar requires evaluation of ground-shaking attenuation; therefore, we validated existing ground motion prediction equations (GMPEs) by comparing instrumental observations and felt intensities for recent earthquakes. We then incorporated the best fitting GMPEs into our seismic hazard assessments. By incorporating the Vs30 (the average shear velocity down to 30 m depth) map from an analysis of topographic slope, we utilized site effect and assessed national probabilistic seismic hazards for Myanmar. The assessment shows highest seismic hazard levels near those faults with high slip rates, including the Sagaing Fault and along the Western Coast of Myanmar. We also assessed seismic hazard for some metropolitan cities, including Bagan, Bago, Mandalay, Sagaing, Taungoo and Yangon, in the forms of hazard curves and disaggregation by implementing detailed Vs30 maps from micro-tremor surveys. The city-scale assessments show higher hazards for sites close to an active fault or/and with a low Vs30, demonstrating the importance of investigating site conditions. The outcomes of this study will be beneficial to urban planning on a city scale and building code legislation on a national scale.

Aims: Coronavirus disease 2019 (COVID-19) has very high mortality in severe forms of disease, where immunopathology plays an important role. Use of immunomodulating therapies including 6 to 12 mg of dexamethasone is well established. Higher doses of corticosteroids were used with reported success in some settings. This study aims to explore the role of pulse dose methylprednisolone therapy in very severe COVID-19 patients in preventing the need for ICU care and death in resource-limited setting. Study Design: Retrospective case series study. Place and Duration of Study: Oak-ta-chat-thal-ta-pwint COVID-19 treatment center in Yangon, Myanmar between September 2021 to December 2021. Methodology: This study included 13 confirmed COVID-19 patients with severe to critical illness, who were treated with pulse dose methylprednisolone therapy. We reviewed the patients’ demographics, comorbidities, and disease severity before starting pulse dose methylprednisolone therapy and changes in oxygen requirement, chest X-ray scores, inflammatory markers, development of significant clinical events, and 28 days mortality after therapy. Results: Before pulse dose methylprednisolone therapy, all 13 patients had very severe disease (mean SPO2/FiO2 = 173 mmHg, mean SPO2 = 88.54%, mean CRP = 115 mg/L, mean ferritin = 1,295.5 ng/mL and mean Brixia Score = 6.54). They received 3-7 days (mean = 5.5 days) of pulse dose methylprednisolone. Ten patients (76%) survived in a setting with limited ICU care. High ferritin was a significant predictor of mortality. Improvement in oxygen requirement was noticeable after 1-11 days (mean = 5.6 days). Hyperglycemia was common and confirmed bacterial infection was found in 3 patients, but all patients received empirical antibiotics therapy. Conclusion: Pulse-dose methylprednisolone therapy may be an effective salvage therapy in a carefully selected subset of very severe COVID-19 patients. It might be a feasible alternative to other more expensive immunomodulating agents and organ support treatments in a resource-limited setting.

Abstract Myanmar occupies a complex region in the active Indo-Burma subduction system. To illuminate the upper 10 km crustal structure of central Myanmar and obtain new insight into the subduction system, we jointly use P-wave polarizations and receiver functions (RFs) to construct a high-resolution VS profile based on a Bayesian Markov chain Monte Carlo approach. This obtained profile clearly delineates six tectonic units and their boundaries, including the Indo-Burman ranges (IBR), the IBR-fore-arc basin boundary, the fore-arc basin, the volcanic arc, the back-arc basin, and the Sunda plate. The Sunda plate has relatively higher upper crustal VS (&amp;gt;3.0 km/s) and thinner sedimentary cover (∼1 km) compared with the Central Myanmar basin in the Burma plate. The fore-arc basin, containing thick sediments (&amp;gt;10 km), and the back-arc basin, with thinner sediments (∼1–6 km), are separated by a region with higher VS (∼3.0 km/s), which represents crystallized magma beneath the volcanic arc. A narrow zone of relatively high-VS (∼2.6–2.7 km/s) ophiolites is situated between the fore-arc basin and the IBR. We also find a narrow zone of high-VS (∼2.9 km/s) metamorphic rocks contained within the low-VS (≲2.3 km/s) IBR. This study suggests that the proposing joint inversion of two types of single-station measurements, that is, P-wave polarizations and RFs, can robustly and computationally efficiently image the shallow VS structure and provide a reliable uncertainty estimation.

The Mogok metamorphic belt (MMB) in central Myanmar is well known for its complex tectonics, magmatism, and metamorphism in the framework of Tethyan subduction and India–Asia collision. It is also world renowned due to the gemstone‐class rubies and sapphires. Identification and discrimination of those petrological units in this region therefore are important for understanding the gemstone mineralization. Ultramafic massifs also crop out together with the gemstone‐bearing metamorphic rocks in the MMB. It is still poorly constrained about their origin (i.e., mantle peridotites of Jurassic–Cretaceous ophiolites or cumulate rocks) and their relationship with the gemstone generation. Here, we report petrological and geochemical data for the Mogok ultramafic rocks. Petrographic observations show that they have typical cumulate textures, with a crystallization order of olivine/spinel‐orthopyroxene‐clinopyroxene. They have low whole‐rock Al2O3 and CaO, and extremely high spinel Cr# (=Cr/[Cr + Al]; up to 0.86). Clinopyroxenes of these rocks show light rare earth element enriched patterns when normalized to CI chondrite. The results suggest that the Mogok ultramafic rocks have different compositions, both whole‐rock and mineral, from mantle peridotites of Myanmar ophiolites, such as the Kalaymyo and Myitkyina ophiolites, but resemble typical Alaskan‐type ultramafic cumulates. Therefore, the Mogok ultramafic rocks are cumulates generated by high‐pressure crystallization of hydrous arc melts, probably during the subduction of the Tethyan oceanic slab. We argue that the emplacement of these arc melts may have provided additional heat for the earlier magmatic rocks and regional high‐temperature metamorphism, although there is a need to further constrain the ages of these rocks. This, along with magmatism and metamorphism during post‐collisional extension, probably collectively contributed to the generation of world‐class coloured gemstone mineralization.