Abstract
Vertical velocities obtained from uplifted river terrace dating near mountain fronts are commonly converted into overthrusting slip rates assuming simple geometry of the fault at depth. However, the lack of information on the dip angle of these shallow structures can lead to misinterpretation in the accommodation of convergence, and thus to erroneous conclusions on the transfer of shortening to the emergent thrust faults. Here, to assess the impact of fault geometry, we focus on the eastern Himalayan region in the south Central Bhutan, where the topographic frontal thrust (TFT) has been already documented by GPS, paleoseismic, geomorphic and geological studies. This study is based on high-resolution near-surface geophysical investigations, including electrical resistivity, seismic and gravity measurements. Using a similar stochastic inversion approach for all data sets, new quantitative constraints on both fault geometry and petrophysical parameters are obtained to image shallow depths, in the upper ca. 80 m. The combined results from both surface observations and geophysical measurement provide a TFT geometry that is dipping northwards with a shallow angle at the top (0–5 m), steeply dipping in the middle (5–40 m) and flattening at deeper depths (>40 m). Together, our new constraints on the fault geometry allow us to estimate a minimum overthrusting slip rate of 10 ± 2 mm yr−1, which is only a part of the ca. 17 mm yr−1 GPS convergence. This suggests that, in the study area, significant deformation partitioning on several faults including TFT and the Main Boundary Thrust cannot be ruled out. More importantly, assuming constant slip rate, the obtained dip angle variations lead to uplift rate changes with distance to the TFT. This underlines that taking into account uplift rate from terrace dating only at the front location and assuming a constant dip angle fault geometry based on surface observations may significantly bias the slip rate estimates.
Highlights
The Himalaya that stretches ca. 2500 km from the Hazara-Kashmir syntaxis in the west to the Namcha Barwa syntaxis in the east constitutes one of the most seismically active regions of the world
Compared to commonly used approaches based on the search for the simplest model, the main advantages of our method include its ability (1) to assess the fault geometry because no smoothing is applied, (2) to provide a measurement of the uncertainties on the obtained dip angle and (3) to allow trade-off analysis between geometric and either electrical resistivity, velocity or density properties
We take advantage of the various scales of investigation coming from Electrical resistivity tomography (ERT), seismic and gravity methods to obtain an accurate description of shallow structures and fault geometry at depth, which can be subdivided in three main zones: (1) a very shallow part up to 5 m depth well-constrained by both field observations and seismic data considering the ray coverage; (2) an intermediate depth part well-imaged by ERT sections between 5 and 40 m depth due to high-resistivity contrasts; (3) a deeper part documented by gravity measurements below 40 m depth
Summary
The Himalaya that stretches ca. 2500 km from the Hazara-Kashmir syntaxis in the west to the Namcha Barwa syntaxis in the east constitutes one of the most seismically active regions of the world. In central Nepal and Arunachal Pradesh, estimates of Holocene horizontal shortening rates have been already obtained from studies of uplifted river terraces (Lave & Avouac 2000; Burgess et al 2012) nearby the Main Frontal Thrust (MFT), which is the most recent surface expression of the MHT (Schelling & Arita 1991; Pandey et al 1995) In these areas, it has been reported that the MFT absorbs most of the shortening rate across the Himalaya, whereas the MFT is locked over interseismic periods (Ader et al 2012). This leads to the current understanding of seismic cycle in central Himalaya where most of the interseismic deformation deficit is released during M > 8 earthquakes that rupture the MFT up to the surface (Bilham et al 1997; Cattin & Avouac 2000).
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