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Tectonics & Geophysics

Tectonics

    Bangladesh and the Ganges Brahmaputra Delta lies at the junction of three plates:  the Indian Plate, the Eurasian Plate and the Burma Platelet. The long thin sliver of the Burma platelet (Fig. 1) is both advancing over the Indian Plate and sliding sideways.  Its exact motion in this region is still uncertain.  Where the Platelet encounters the Ganges-Brahmaputra Delta north of 18°N latitude, it widens into a broad folded prism of offscraped sediments (Figs. 2, 3).  To the north, at the corner of Himalaya and Burma Arcs, the Shillong Plateau is advancing to the south (Fig. 4) and may represent the beginning of a forward jump of the Himalayas.

    We use mapping of the sediment layers that record the tectonic history, geodesy, seismology and field geologic mapping to determine the nature of the faulting and active tectonics across these boundaries.

           


Figure 2. Schematic profile from the Indian Shield to the Burma Arc at Bangladesh.  Pink is continental basement rock,  purple is subducting oceanic basement, and yellow is sediment. The lines in the sediments show the folds and thrusts that are deforming the delta sediments.

Figure 3.  Oblique view of topography across the Burma Arc foldbelt. Elevation profile is shown in blue.  Higher, more rigid part of foldbelt is in Myanmar.  Low elevation folds are made of recent delta sediments.  Front of folbelt is buried benath the delta and extends to the yellow line. D = location of Dhaka.


Geophysical Methods
 
    In order to determine the structure of the region, we will employ a variety of geophysical methods.

1. Resistivity Imaging.
To aid correlation of sedimentary features between wells, we will use a resistivity system. Using an array of electrodes, the tool induces electrical ground currents and measures voltage between pairs of points on the surface. Making thousands of measurements with different electrode distances and spacings, the data are inverted to image the spatial pattern of subsurface resistivity, primarily controlled by sand:mud ratios of the underlying strata (Fig. 5).


2. Marine’ Multichannel Seismics. We will image the stratigraphy to a few hundred meters depth by conducting a marine multichannel seismic survey (Fig. 6). Our partners from Bremen University will bring their portable micro-GI gun and a short streamer for a high-resolution survey from local boats on the rivers. 


3. GPS geodesy.  We have established a network of 18 sites where we continuously measure positions precise enough to see tectonic motions and ground subsidence (Fig. 7).  We will also use precision GPS to measure locations of all well sites and topographic profiles of geologic features.


4. Seismology. We are operating both a 6-station permanent network and a 6-station portable network that is relocated every 1-2 years.  The local seismicity (Fig. 8)  comes from the basal detachment below the accretionary wedge; imbricate thrust-folds and dextral faults rooted in this detachment; the Dauki reverse fault; and intraplate faults in the Indian basement, primarily near the hinge zone.  We also using teleseismic earthquakes to obtain additional information on the sediment and crustal thickness beneath Bangladesh. 


5. Compaction measurements. To better understand the subsidence of the delta, we will measure sediment compaction versus depth at two sites (Fig. 9). To do this we have drilled suites of 4-6 boreholes to depths from 20 to 300 m at each site. Each well has stretched optical fibers installed whose length are measured weekly. With each fiber measuring compaction to a different depth, together they provide a depth profile of compaction for the duration of the experiment. 
 
      
 

Figure 1.  Plate tectonics of the area of Bangladesh.  The thick black lines show the major plate boundaries.  Thrust faults have triangles on the upper plate.  The inset shows the outline of the Burma Platelet, a long this sliver between the larger Indian and Asian Plates.  The rupture zones of major earthquakes along the subduction boundary is also shown.


Figure 4. Westward view of the southern front of the Shillong Plateau.  Indian Shield basement rock outcrops in the higher central part of the Plateau.  It is covered by sedimentary strata including syntectonic sediments.  The younger sediments in the Sylhet Basin steepen to the south suggesting that the Dauki Fault is probably blind and lies farther south than the Plateau within the Basin.



Figure 5.  Resistivity image from Bangladesh.  High reistivity (red colors) layers correspond to sands and low resistivity (blue colors) layers correspond to clays.

Figure 6. Example seismic image from Lake Balaton, Hungary collected using the Bremen portable seismic system showing sedimentary layers and river channels.


Figure 7.  Plot of daily GPS positions vs time from site DHAK at Dhaka University.  The measurements show the site to be moving to the north and east with the Indian plate.  The vertical shows seasonal loading from the monsoon and subsidence, mainly due to groundwater pumping beneath Dhaka.










Figure 8.  Example of seismic records from local earthquake in Bangladesh.












Figure 9.  Cartoon illustrating the plan for each compaction system.  A local collaborator makes weekly measurements at each well.  Wells will be periodically surveyed to GPS monument.