Active Growth Pattern of Adjacent Thrust-sheets in the Indo-Myanmar Fold-Thrust Belt, Northeast India

This study presents a comprehensive geomorphic investigation to delineate the growth patterns of the Nungba and Barak-Makru thrust sheets within the Indo-Myanmar fold-thrust belt in Northeast India. Employing well-established geomorphic indices, like Normalized Steepness Index (Ksn), Hypsometric Integral (HI), and Transverse Topographic Symmetry Factor (T), the study provides a nuanced understanding of recent tectonic activities as well as intricate growth dynamics of these thrust sheets. The Ksn and HI reveal distinct patterns of growth in the northern segments of both the Nungba thrust sheet (NTS) and Barak-Makru thrust sheet (BMTS), exhibiting more intense deformation compared to their southern parts; nevertheless, the NTS is experiencing more intense activity. Field observations, including patterns of stream incision and geomorphic anomalies such as fault scarps, drainage deflections, and disposition of strath terraces, corroborate these findings and provide a tangible context to the geomorphic data. Variations in stream incision patterns from north to south across the thrust sheets offer a lens into the spatial variation in growth patterns, suggesting a north-to-south decrease in tectonic uplift of the terrane. Furthermore, the spatial distributions of T values elucidate differing mechanisms of down-tilting patterns in the thrust sheets, with NTS tilting northward and BMTS southward. This bidirectional tilting pattern is attributed to the variable uplift rates at the basal structural discontinuities of the thrust sheets, resulting from the differential strain accommodation, influenced by the ongoing collisional dynamics within the fold-thrust belt. This study underscores the importance of geomorphic investigations in understanding the complex and variable nature of tectonic processes within a growing orogen, highlighting that the adjacent thrust sheets exhibit distinct growth patterns even under similar principal stress regimes. The research not only provides vital insights into the active tectonic processes shaping the Indo-Myanmar fold-thrust belt but also illuminates the importance of integrating geomorphic and field investigations in understanding the ongoing orogenic dynamics.