Understanding the Deformation Structures and Tectonics of the Active Orogenic Fold-Thrust Belt: Insights from the Outer Indo-Burman Ranges

Abstract

The tectonic deformation of the outer Indo-Burman Ranges (i.e., Chittagong Tripura Fold Belt, CTFB) is associated with the oblique convergence of Indo-Burmese plates since the latest Miocene. This article presents detailed field evidence of deformation structures and their kinematics in the exposed Tertiary successions in the CTFB. We combine observations made in this study with the published structural, geodetic, and seismic data sets to present an overview of the active tectonic framework of the region and its strain partitioning. To determine the kinematic evolution, décollement depth, and amount of strain, we combined geologic field mapping, structural analysis of fifteen anticlines, fracture/lineament analysis, and paleostress analysis of faults that define the ∼100 km wide CTFB. Structural data and kinematic analyses suggest subhorizontal plane strain with approximately 10% east-west shortening (oriented ~65°) that is perpendicular to the axial plane (oriented ~155°) of the CTFB anticlines. No evidence of significant transpression or strike-slip faulting has been observed in the CTFB and, therefore, suggests that full slip-partitioning is normal to the outer belt and parallel to the inner belt of the IBR. Paleostress analysis results are in good agreement with the present-day stress regime, and this implies that past and present deformation is dynamically related with the normal component of India-Burma oblique vector velocity motion.

Quantitative tectonic geomorphic study from bedrock rivers of the Fold and Thrust Belt in the Bengal Basin

“…Therefore, the NNE–SSW trending and fold‐related lineaments are lesser in number in the lineament map and rose diagrams (Figure 3). Fortunately, some of our remote sensing‐based mapped structures are in good agreement with some of the structures mapped by integrating remote sensing and field studies of the recent study by Hossain et al (2022). They showed both axial fracture sets (NNW–SSE trends related to fold structures) and oblique/cross‐axial fracture sets (NE–SW and E‐W trends, which are fold‐related conjugate fractures) in only selected anticlines, such as Basin 2 of this study.…”

“…The well‐known earthquake events include the Chittagong earthquake of 1762 (R 8.5+), the Sirajganj earthquake of 1787 (MM X), the Cachar earthquake of 1869 (R 7.5), the Bengal earthquake of 1885 (M w 6.8), Great Indian/Assam earthquake of 1897 (M w 8.1), Srimangal earthquake of 1918 (M w 7.6), Meghalaya earthquake of 1923 (M w 7.1), Dubri earthquake of 1930 (M w 7.1), Bihar‐Nepal earthquake of 1934 (M w 8.1), Assam earthquake of 1950 (R 8.5), and Nepal earthquake of 2015 (M w 7.8) (Hossain et al, 2019). The recent tectonic activity of the study area was also demonstrated in the recent detailed investigation from field mapping, fold structural analysis, fracture/lineament analysis, and palaeostress and kinematic analysis of faults by Hossain et al (2022). Also, several new lineaments of dominant ENE–WSW to E–W trends with secondary NNE–SSW and WNW–ESE trends were mapped in our study (Figure 3).…”

“…The ongoing tectonics in the Bengal Basin is well established based on historical and recent observations of increasing trends of seismic activity and the possible development of new faults/fractures from seismically quiet zones or the resurgence of some faults/fractures (Hossain et al, 2019, 2022; Khan & Chouhan, 1996). Records of past large‐magnitude and high‐intensity earthquakes in the area are shown in Figure 10a, which caused catastrophic damages, shifted the course of large rivers, and changed the dynamics of the landscape (Baro & Kumar, 2015; Hossain et al, 2019; Wang et al, 2014).…”

“…Geological data, particularly lineament distributions, including their orientation, types, and patterns, are required to understand the tectonics of a region (Masoud & Koike, 2006). Remote sensing data, including DEM, have been proven to be useful for the identification of lineaments and interpretation of tectonic deformation in a particular region (Bhattarai et al, 2021; Caran et al, 1981; Gani & Abdelsalam, 2006; Gani et al, 2009; Hossain et al, 2022). The remote sensing‐based extraction of lineaments is built on various distinct geomorphologic features such as linear segments of a river channel, drainage deflection, radar shadow‐illumination, ridges, and valleys, and scarp faces, among others (e.g., Gani & Abdelsalam, 2006; Prabu & Rajagopalan, 2013; Rajasekhar et al, 2018).…”

“…The Bengal Basin, the largest peripheral collisional foreland basin of the Himalayas, is crucial due to its proximity to several distinct geological features such as the Main Boundary Thrust Fault (MBT) of the Himalayas and the Shillong Plateau, the Bengal Delta—the world's largest delta drained by the Ganges and Brahmaputra river network, the Bay of Bengal and the Bengal Deep‐Sea Fan‐world's largest submarine fan, the Chittagong‐Tripura Fold Belt (CTFB) and the Indo‐Burman Range (IBR) (e.g., Alam et al, 2003; Gosh, 2022; Figure 1a–c). Moreover, the ongoing (since Late Miocene) oblique subduction/convergence of the Indian Plate under the Burmese microplate in the north and north‐east makes this region critical in terms of seismicity and intraplate crustal deformation (Gani & Alam, 1999; Hossain et al, 2022; Steckler et al, 2016). This active continental plate‐to‐plate interaction of this complex subduction affected the eastern part of the Basin and adjacent regions’ topography by forming primarily contractional and strike–slip faults such as macro‐scale fold and thrust belts at its eastern and northern parts.…”