An official website of the United States government
We utilized shear wave splitting analysis of teleseismic SKS, SKKS, and PKS phases to infer upper mantle deformational fabrics across a substantial area of Southeast Asia, where splitting measurements were previously limited. We used newly available permanent and temporary broadband seismic networks deployed across the Indo-Burma subduction zone and the eastern Indochina peninsula. The resulting 492 well-constrained splitting and 654 null measurements from 185 stations reveal clear large-scale patterns in the mantle deformational fabrics in response to the highly oblique active subduction and a large transform plate boundary. We identified two distinct domains of mantle deformation fabrics in the western Burma microplate and the eastern Indochina peninsula. In the former, trench parallel N-S fast polarization directions with an average lag time (δt) of 1.9 s are observed beneath the Indo-Burman Ranges. We suggest the observed splitting is partly due to anisotropy in the sub-slab region and relates to shear induced by the north moving Indian plate. The lithospheric fabric within the Indo-Burman Ranges and underlying subducting slab fabric contribute to produce the observed average δt of 1.9 s. The δt value decreases to an average of 1.0 s towards the back-arc until we reach the dextral Sagaing fault. In the second domain, starting approximately 100 km east of the Sagaing fault, we observe a consistent E-W fast direction with an average δt of 1.10 s in the eastern Shan-Thai and Indochina blocks. We interpret the E-W fabric as due to the deformation associated with the westward spreading of the Hainan mantle plume, possibly driven by overriding plate motion. Low velocities in the shallow mantle and late Cenozoic intraplate volcanism in this region support the plume-driven asthenospheric flow model in the Indochina peninsula. The sudden transition of the fast polarization direction from N-S to E-W along the eastern edge of the Burma microplate indicates the Sagaing fault acts as a mantle flow boundary between the subduction dominated trench parallel flow to the west and plume induced asthenospheric flow to the east. We also observed no net splitting beneath the Bengal basin which is most likely due to the presence of frozen vertical fabric resulting from the Kerguelen plume activity during Early Cretaceous.
more »
« less
Crust and Mantle Flow From Central Tibetan Plateau to the Indo‐Burma Subduction Zone
Abstract The extremely oblique Indo‐Burma subduction zone exhibits dextral strike‐slip faulting along the Sagaing, Kabaw, and Churachandpur‐Mao Faults as well as east‐west shortening between the Sagaing Fault and Bengal Basin. Through regional stress analysis, considering areas from central Tibet, around the eastern Himalaya Syntaxis, to Burma, it has been determined that the principal compressive stress directions align with the principal strain rates. The northeast‐southwest oriented compressive stress direction from the western Shan Plateau continues into Burma. Notably, P axes align with the topographic gradients, and T axes are sub‐parallel to the topographic contours in the Shan Plateau region south of 27°N. These stress patterns are consistent with a gravitational potential energy induced crustal and mantle flow. The alignment of the fast shear wave with the maximum strain rate and the colinear NW‐SE to E‐W fast direction of the SKS wave and T axis determined from focal mechanisms in the Shan Plateau suggest that the mantle lithosphere deforms in concert with the crust. We suggest crust and mantle flow south of the Red River Fault has resulted in widening of the lithosphere in the Shan Plateau in an east‐west direction. Therefore, the Sagaing Fault has bowed approximately 50–100 km westward if we assume that the Sagaing Fault was originally straight. Our results of regional stress inversion are consistent with late Miocene to present E‐W shortening in the Indo‐Burma subduction zone resulting from the release of gravitational potential energy from the central Tibetan Plateau.
more »
« less
- Award ID(s):
- 1928786
- PAR ID:
- 10546120
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 129
- Issue:
- 10
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Lithospheric shortening can be described by one of two end-member modes: indentation of the lithosphere and subduction of the lithospheric mantle. Deciphering the difference between these modes is crucial in the interpretation of past and present orogens and in predicting their structural architecture at depth. It is therefore important to establish how observable upper crustal proxies reflect deep lithospheric kinematics and dynamics. Over the last few decades, geological and geophysical data have provided valuable constraints on the northern margin of the Tibetan Plateau. This margin is defined by the Qilian Shan thrust belt, which developed in response to the far-field convergence between the Indian and Eurasian plates. The primary mechanism for this development is the southward subduction of the Asian lithospheric mantle beneath the Tibetan Plateau. We conducted numerical modelling to simulate the kinematics and response of the upper crust to the southward subduction of the lithospheric mantle. Our results show that subduction of the lithospheric mantle can result in upper crustal deformation that matches the records in the Qilian Shan, where pure shear shortening alone does not generate similar upper crust proxies, including the broad width and architecture of the bivergent orogenic wedge, the timing of fault initiation and evolution, seismicity and fault activity, the topography and geomorphology. The geometry of the subducting lithosphere impacts the width and asymmetry of the bivergent orogenic wedge. Our results demonstrate how records of crustal strain can be used to better interpret the deep structural architecture of past and present orogenic wedges.more » « less
-
Tectonic plate convergence is accommodated across the continental lithosphere via discrete lithospheric subduction or distributed shortening and thickening. These end-member deformation modes control intra-plate mountain building, but their selection mechanism remains unclear. The variable composition of the continental crust and lithospheric mantle, which impacts its density and rheology, can be inferred by the distribution of magnetic-indicated crustal iron. Here we demonstrate that vertically coherent pure-shear shortening dominated the active Tian Shan orogen, central Asia, based on high-resolution aeromagnetic imaging and geophysical-geodetic observations. Integrating these findings with thermomechanical collisional models reveals that the mode of intracontinental deformation depends on contrasts in lower crust composition and mantle lithosphere depletion between the converging continents and central orogenic region. Distributed shortening prevails when the converging continents have a more iron-enriched mafic crust and iron-depleted mantle lithosphere when compared to the intervening orogenic region. Conversely, continental subduction occurs without such lithospheric contrasts. This result explains how the Tian Shan orogen formed via distributed lithospheric thickening without continental subduction or underthrusting. Our interpretations imply that iron distribution in the crust correlates with lithospheric compositional, density, and rheological structure, which impacts the preservation and destruction of Earth’s continents, including long-lived cratons, during intracontinental orogeny.more » « less
-
null (Ed.)Recent GPS studies show that the Indo-Burma subduction system is locked with the implication of a potential large-magnitude earthquake. To inform better seismic hazard models in the region, we need an improved understanding of the crustal structure and the dynamics of the Indo-Burma subduction system. The Bangladesh-India-Myanmar (BIMA) tripartite project deployed 60 broadband seismometers across the subduction system and have been continuously recording data for ~2 years. In this study, we computed receiver functions from 30 high-quality earthquakes (M≥5.9) with epicentral distances between 30º and 90º recorded by the array. The algorithm utilized ensures the uniqueness of the seismic model and provides an uncertainty estimate of every converted wave amplitude. We stacked all the receiver functions produced at each station along the entire transect to generate a cross-sectional model of the average crustal structure. The level of detail in the image is improved by computing higher frequency receiver functions up to 4 Hz. The results represent some of the strongest constraints on crustal structure across the subduction system. Beneath the Neogene accretionary prism's outer belt, we observe a primary conversion associated with the Ganges Brahmaputra Delta that ranges in depth from ~10 km near the deformation front up to ~12 km at the eastern boundary. From the eastern end of the Neogene accretionary prism to the Sagaing Fault, we image the Indian subducting slab and the Central Myanmar basin. The depth-extent of seismicity associated with the Wadati-Benioff zone is consistent with the locations of primary conversions from the subducting plate. We further verify the converted phases of the slab by analyzing azimuthal moveout variations. The Central Myanmar basin is roughly bowl-shaped in cross-section with a maximum thickness of ~15 km about halfway between the Kabaw and Sagaing faults. The average crustal thickness beneath the Ganges-Brahmaputra delta is ~20 km, most likely representing a transitional crust formed from thinning of the continental crust intruded and underplated by igneous rocks. In contrast, the average thickness of the continental crust beneath the Central Myanmar basin is ~40 km. Our results provide a baseline model for future geophysical investigations of the Indo-Burma subduction zone.more » « less
-
Abstract The Indo‐Burma subduction zone is a highly oblique subduction system where the Indian plate is converging with the Eurasian plate. How strain is partitioned between the Indo‐Burma interface and upper plate Kabaw Fault, and whether the megathrust is a locked and active zone of convergence that can generate great earthquakes are ongoing debates. Here, we use data from a total of 68 Global Navigation Satellite System (GNSS) stations, including newly installed stations across the Kabaw Fault and compute an updated horizontal and vertical GNSS velocity field. We correct vertical rates for fluctuating seasonal signals by accounting for the elastic response of monsoon water on the crust. We model the geodetic data by inverting for 11,000 planar and non‐planar megathrust fault geometries and two geologically viable structural interpretations of the Kabaw Fault that we construct from field geological data, considering a basin‐scale wedge‐fault and a crustal‐scale reverse fault. We demonstrate that the Indo‐Burma megathrust is locked, converging at a rate ofmm/yr, and capable of hosting >8.2Mwmegathrust events. We also show that the Kabaw Fault is locked and accommodating strike‐slip motion at a rate ofmm/yr and converging at a rate ofmm/yr. Our interpretation of the geological, geophysical, and geodetic datasets indicates the Kabaw Fault is a crustal‐scale structure that actively absorbs a portion of the convergence previously ascribed to the Indo‐Burma megathrust. This reveals a previously unrecognized seismic hazard associated with the Kabaw Fault and slightly reduces the estimated hazard posed by megathrust earthquakes in the region.more » « less
