UC Berkeley

The Indo-Burma Range (IBR) is a highly oblique subduction zone that accommodates both strike-slip and convergent motions. Previous short-term tectonic investigations of geodetic and microseismic have been unable to fully explain the active tectonics in this region, as they are most sensitive to the elastic component of earthquake cycle deformation. The current distribution of interseismic deformation and background seismicity does not capture the contributions of large earthquake ruptures, postseismic deformation and inelastic strain to the development of geologic structures and the topography of the range. This is evident in the ongoing controversy among different research groups on whether the subduction thrust is active, and whether the non-uniqueness of existing elastic block models is/can be constrained only by interseismic deformation measurements. Since the IBR is the only subaerial segment along this convergent boundary, it is an interesting region to investigate how landscapes evolve in this tectonic setting. Therefore, this dissertation aims to investigate both short-term (stress orientations and plate driving forces) and long-term tectonics (geomorphology and structure), which is the integrated result of active deformation over many earthquake cycles.

For my short-term tectonics study, the objective is to improve the understanding of the plate driving forces and subduction dynamics across the IBR boundary zone. I combined 189 focal mechanisms from available catalogs and publications from 1950 until 2019 and divided them to subdomains before performing stress inversions. As a result, I find that the maximum principal stress (σ1) always is orientated approximately NS, subparallel to the subducting slab, even at deeper depths, while the intermediate principal stress (σ2) is plunging toward the west at different angles for distinct subdomains. The minimum principal stress (σ3) is plunging eastward nearly following the Indian slab dip angles, suggesting downdip tension. The σ1 orientation is always consistent with depth, indicating that the NS compression is due to the slab pushing northward through the mantle. Also, the slab-dip parallels σ3 at all depths below 30 km, demonstrating that the net slab pull is a primary driving force of this subduction zone. According to these observations, I propose that we cannot rule out that the megathrust in this region is tectonically active and able to produce major events (> M8) in the future.

To study the long-term tectonics by investigating the morphometric expression, I first assessed the stability of the major drainage divides and investigated tectonic uplift employing 30 × 30 SRTM digital elevation model (DEM) to generate watersheds, streams and drainage divides. To examine the divide stability, I used Gilbert's metrics (including channel elevation, relief and gradient) and χ of the channel heads located on both sides of the divide segments. Then I calculated the basin-scale (hypsometry and relief) and stream-scale (normalized steepness index, ksn) geomorphic indices to exhibit the pattern of the relative tectonic uplift rates. I find that most of the drainage divides are static, allowing us to explore uplift rates using the morphometric indices while also considering the variation in lithology and precipitation rates. Both basin- and stream-scale indicators are not well correlated with first-order variations of the lithology and precipitation rates, but they suggest greater relative uplift rates in the eastern, inner-belt region. I infer that out-of-sequence thrusts in the inner belt might play an important active role in generating greater relative uplift rates in this region even though the deformation front has long migrated westward.

According to the previous findings, it is possible that there is an out-of-sequence reactivation of older antiforms in the western outer belt of the IBR. Therefore, I employed the Earth surface dynamics models (Landlab) to investigate four possible, first-order thrust propagation scenarios. I first thoroughly investigated the characteristics of the young western antiforms and a total of 20 tributaries of four rivers (Feni, Karnaphuli, Sangu and Matamuhuri Rivers) in the outer belt region, utilizing the DEM employed previously. I created the fluvial profiles of 20 channels, which mostly are antecedent streams, and determined knickpoint locations along the streams. Next, I used the geomorphic observations of drainages and antiforms in the outer belt to generate four first-order surface dynamics models consisting of a set of three antiforms. The modeling scenarios include sequential thrust propagation, partial reactivation, continuous reactivation of older structures, and synchronous thrusting style. Then, I compared the antecedent stream profiles and knickpoint locations of the modeled cases to the observed channels in the outer belt of the IBR. The sequential propagation with continuous reactivation model, invoking continued growth of the eastern anticlines, correlates best with the outer-belt IBR, and synchronous thrust activity and growth also likely occur in the westernmost region. This means as the deformation front has migrated westward, the older eastern antiforms remain tectonically active. The IBR bivergent antiform structures and numerical modeling results of weak décollement also support my findings of out-of-sequence reactivated splay faults in the outer belt. This suggests an active megathrust in this region. Although it is still inconclusive if the megathrust will fully rupture causing > M8 events, it is more likely that frequent, intermediate earthquakes due to independent failures of reactivated subsidiary faults will occur in the future.

In summary, for my short-term study of the three-dimensional distribution of stress in the IBR, I cannot clearly support or reject ongoing subduction and megathrust slip across the IBR, while the long-term tectonics investigations help further elucidate the active tectonics across this region. The observation of greater relative uplift in the eastern inner belt and reactivated out-of-sequence splay faults in the western outer belt suggest active distributed uplift across the region. Even though the stress orientations in this area do not suggest EW shortening, slab pull might have contributed to the EW convergent motion and active splay faults that are possibly rooted from the megathrust generating out-of-sequence thrust activity. This leads to continuing active tectonics in the inner and outer belts of the IBR. Although currently I cannot conclude whether the megathrust is fully locked and capable of producing the major events in the future, it is more likely that small and intermediate earthquakes will occur frequently in the interior of the system due to the failures of the active splay faults across the IBR. Since the structures and tectonics in the IBR are especially complex, more field surveys and other detailed studies, such as chronological dating and high-resolution tomography, are needed to further improve our understanding of the tectonics and earthquake hazards in this region.