Subsurface Structural Variability of the Himalayan Foreland Basin, Nepal

  • Author / Creator
    Duvall, Michael
  • The Himalayan orogen is segmented along its strike at the scale of hundreds of kilometres. This segmentation is apparent from along-strike changes in earthquake rupture patterns, thickness changes, deformation styles, topographic gradients, thermal evolution, and faults at oblique angles to the orogen. Understanding the driving mechanisms of this longitudinal variability could further our understanding of the orogen, and potentially improve earthquake hazard assessment. This thesis attempts to explain longitudinal variability of the Himalaya by observing how the Himalayan foreland basin varies along-strike, and determining what mechanisms drive the observed changes. The geometry of the Nepali part of the Ganga Basin is assessed through interpretation of two-dimensional seismic reflection data. The study area covers ~26 000 km2 of the foreland basin, and incorporates 5134 km of seismic reflection data imaged by 181 profiles. The data have been depth-converted using time-depth relationships derived from wells, so as to highlight structures. Regionally interpreted surfaces include: two horizons internal to the Cenozoic succession; an angular unconformity at the base of the Cenozoic succession where older stratified units, ranging in age from Proterozoic to possibly Mesozoic, are truncated; and the nonconformity that separates sedimentary strata from acoustic basement representing Archean granitoids and Proterozoic gneisses. In addition, faults and damage zones are identified in both the foreland basin strata and the underlying basement. Because much of the foreland basin fill was deposited in continental environments close to sea level, thickness can be used as a proxy for subsidence rate. The interpreted horizons have been used to produce isopach maps for each of the main stratigraphic intervals. The seismic interpretations show that basement depth fluctuates dramatically, ranging from > 12 km to < 3 km. These variations define two sets of depressions and ridges, and several large graben oriented at high angles to the orogen. The Cenozoic succession thins and thickens in step with the basement below. Thickness maps of units within the Cenozoic succession reveal maxima above basement depressions, and minima above basement ridges, indicating that the basement features were inherited by the foreland basin, and controlled accommodation in the foreland basin. A system of six tear faults oriented NNE-SSW and NNW-SSE is spatially localized in the sedimentary succession above a basement ridge in eastern Nepal. An E-W striking blind thrust fault is interpreted in the subsurface, ~2.8 km below sea level, bounded by two tear faults, soling on a detachment surface below the top of the Lower Siwalik subgroup. The geometry of the fault system is interpreted as a series of blocks soling on a basal décollement, with strike-slip movement on tear faults accommodating differential slip between the blocks. The faults represent the subsurface propagation of shortening associated with the Main Himalayan Thrust into the Ganga Basin that is recognized for the first time as a ‘New Frontal Thrust’. The faults are interpreted to be localized by basement ridges that influence lithofacies, basin thickness, overpressure and/or the topography of the basin floor. Cumulative slip between 42 and 124 m is calculated to have accumulated on these faults over the last ~0.5 Ma. Present-day fault development in the Himalayan foreland basin may have implications for seismicity patterns varying along the strike of the orogen. Basement-controlled differential subsidence has played (and likely continues to play) a significant role in the development of the Ganga foreland basin. Accommodation generation varied along strike during foreland basin fill, and was controlled by NE-SW oriented basement ridges and their bounding faults. Spatial localization of tear faults above basement ridges suggests that the ridges may influence lateral ramp and transfer zone distribution in the developing Himalayan foreland thrust belt.

  • Subjects / Keywords
  • Graduation date
    Fall 2019
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
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