Forearc Deformation Within the Indo-Burman Fold-Thrust Belt: Constraining the Transition from Plane Strain Shortening to Noncoaxial Dextral Transpression Near the Churachandpur-Mao Fault
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Mathews, Austin
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Abstract
Subduction margins account for more than 90% of the total seismic energy released by shallow earthquakes worldwide (Pacheco & Sykes, 1992). Megathrust earthquakes release stored elastic strain energy that has accumulated on the interface between the converging plates. The magnitude and distribution of seismicity depends on the convergence vector and partitioning of strain among forearc fault systems. This study capitalizes on the subaerial exposure of the Indo-Burman Ranges, an accretionary wedge resulting from the oblique (70°) collision of India and Eurasia, to characterize forearc structures that form during highly oblique subduction. This study utilizes field data to carry out structural analyses and kinematic modeling of faults and folds to constrain the transition from west-directed shortening in the Outer Belt of the Indo-Burman Ranges to dextral transpression within the Inner Belt. Results indicate that structures in the Outer Belt record WNW-ESE directed plane strain and accommodate margin-normal shortening as fault-propagation folds. These Outer Belt structures have accommodated
26.6 km of shortening and form above a folded décollement which deepens westward from 3.0 to 5.6 km depth due to flexural subsidence in the Outer Belt. Within the Inner Belt, transpressional fault splays strike northwest from the Churachandpur-Mao Fault and modify structures from the fold-thrust belt in a 20 km wide zone of transpressional shear. East of the Churachandpur-Mao Fault, transpressional deformation intensifies, manifested by a pervasive sub-vertical cleavage fabric with prominent tight to isoclinal folding. Field evidence for transpressional deformation in the core of the Indo-Burman Ranges accretionary wedge spans eastward to the Kabaw Fault, the backstop of the accretionary prism. Structural and geodetic modeling indicates that the Kabaw Fault is a crustal-scale dextral-oblique thrust fault. Together, the Churachandpur-Mao Fault and Kabaw Fault form a contractional left stepover between the two right-lateral faults. Fault systems in the Indo-Burman Ranges forearc are partitioned such that the Outer Belt absorbs margin-normal convergence, and the Inner Belt primarily accommodates marginparallel convergence in a zone of distributed, dextral and transpressional shear. Results from this thesis quantify the fault kinematics and map distribution of active structures in the Indo-Burman Ranges to better inform regional seismic hazard assessments.
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Seismic Hazard, Structural Geology, Indo-Burman Ranges, Forearc Deformation, Himalayan Tectonics, Subduction Zones