Segmentation of the Himalayas as revealed by arc-parallel gravity anomalies

International audienceLateral variations along the Himalayan arc are suggested by an increasing number of studies and carry important information about the orogen’s segmentation. Here we compile the hitherto most complete land gravity dataset in the region which enables the currently highest resolution plausible analysis. To study lateral variations in collisional structure we compute arc-parallel gravity anomalies (APaGA) by subtracting the average arc-perpendicular profile from our dataset; we compute likewise for topography (APaTA). We find no direct correlation between APaGA, APaTA and background seismicity, as suggested in oceanic subduction context. In the Himalayas APaTA mainly reflect relief and erosional effects, whereas APaGA reflect the deep structure of the orogen with clear lateral boundaries. Four segments are outlined and have disparate flexural geometry: NE India, Bhutan, Nepal & India until Dehradun, and NW India. The segment boundaries in the India plate are related to inherited structures, and the boundaries of the Shillong block are highlighted by seismic activity. We find that large earthquakes of the past millennium do not propagate across the segment boundaries defined by APaGA, therefore these seem to set limits for potential rupture of megathrust earthquakes

Slip deficit in central Nepal: omen for a repeat of the 1344 AD earthquake?

International audienceIn 1255, 1344, and 1408 AD, then again in 1833, 1934, and 2015, large earthquakes, devastated Kathmandu. The 1255 and 1934 surface ruptures have been identified east of the city, along comparable segments of the Main Frontal Thrust (MFT). Whether the other two pairs of events were similar is unclear. Taking into account charcoal's age inheritance, we revisit the timing of terrace offsets at key sites to compare them with the seismic record since 1200 AD. The location, extent, and moment of the 1833 and 2015 events imply that they released only a small part of the regional slip deficit on a deep thrust segment that stopped north of the Siwaliks. By contrast, the 1344 or 1408 AD earthquake may have ruptured the MFT up to the surface in central Nepal between Kathmandu and Pokhara, east of the surface trace of the great 1505 AD earthquake which affected western Nepal. If so, the whole megathrust system in Nepal broke in a sequence of earthquakes that lasted less than three centuries, with ruptures that propagated up to the surface from east to west. Today's situation in the Himalayan seismic sequence might be close to that of the fourteenth century

Surface ruptures of large Himalayan earthquakes in Western Nepal: Evidence along a reactivated strand of the Main Boundary Thrust

International audienceThe chronology of the seismic ruptures along the active faults of Western Nepal remains almost unconstrained despite their high seismogenic potential. We present here a slip history of one of these structures, a 120 km-long reactivated segment of the Main Boundary Thrust named the Surkhet–Gorahi fault. This slip history is based on geomorphologic and neotectonic mapping of active faults deduced from the analysis of a high resolution total station digital elevation model and 15 detrital charcoals radiocarbon ages constraining the age of deposition or abandonment of 4 alluvial terraces of the Bheri river in Botechaur. Our results show that the last two earthquakes occurred on this fault after 1860 and 640 BP, respectively, and accommodated slip greater than 8 m each, a value corresponding to the incremental vertical offset of the terraces. Such events released a significant part of the slip deficit accumulated on the Main Himalayan thrust fault. However, given the geometry of this fault system as well as the date of occurrence of the last events, the ruptures could be associated with major earthquakes also rupturing the Main Frontal Thrust, such as the great 1505 earthquake

Dipolar self-potential anomaly associated with carbon dioxide and radon flux at Syabru-Bensi hot springs in central Nepal

International audienceThe Syabru-Bensi hot springs are located at the Main Central Thrust (MCT) zone in central Nepal. High carbon dioxide and radon exhalation fluxes (reaching 19 kg m À2 d À1 and 5 Bq m À2 s À1 , respectively) are associated with these hot springs, making this site a promising case to study the relationship between self-potential and fluids (gas and water) exhalation along a fault zone. A high-resolution self-potential map, covering an area of 100 m by 150 m that surrounds the main gas and water discharge spots, exhibits a dipolar self-potential anomaly with a negative peak reaching À180 mV at the main gas discharge spot. The positive lobe of the anomaly reaching 120 mV is located along the terraces above the main gas and water discharge spots. Several electrical resistivity tomograms were performed in this area. The resistivity tomogram crossing the degassing area shows a dipping resistive channel interpreted as a fracture zone channeling the gas and the hot water. We propose a numerical finite difference model to simulate the flow pattern in this area with the constraints imposed by the electrical resistivity tomograms, the self-potential data, the position of the gas vents, and hot water discharge area. This study provides insights on the generation of electrical currents associated with geothermal circulation in a geodynamically active area, a necessary prerequisite to study, using self-potentials, a possible modulation of the geothermal circulation by tectonic activity. Citation: Byrdina, S., et al. (2009), Dipolar self-potential anomaly associated with carbon dioxide and radon flux at Syabru-Bensi hot springs in central Nepal

High carbon dioxide flux associated with radon-222 gas exhalation and dipolar self-potential anomaly at the Syabru-Bensi hot springs in central Nepal

Gas discharges have been identified at the Syabru-Bensi hot springs, located at the Main Central Thrust zone in Central Nepal and characterized by a water temperature reaching 61°C, high salinity and high alkalinity. The gas is mainly dry carbon dioxide, marked by a δ13C isotopic anomaly of -0.8‰. The diffuse carbon dioxide exhalation flux, mapped by the accumulation chamber method, reaches 19 000 g×m-2×day-1, comparable with values measured on active volcanoes. Radon exhalation flux at the soil surface has been measured at more than sixty points in the vicinity of the main gas discharge. Extreme values, larger than 2 Bq×m-2×s-1, similar to peak values measured in volcanic areas or above uranium waste piles, are observed in association with the larger values of the carbon dioxide exhalation flux. This high radon exhalation thus results from emanation at depth, producing a radon concentration in the pore space varying from 25 000 to more than 50 000 Bq×m-3, transported to the surface by the flow of carbon dioxide. The high radon-222 content of the carbon dioxide offers an interesting tracing method and an additional practical tool for long term monitoring, for example to study transient changes preceding large earthquakes. An extended dipolar self-potential anomaly has also been found, with a negative pole reaching -180 mV at the main gas discharge, and a wide positive lobe on the terrace above. This dipolar anomaly, the largest reported so far, is interpreted in a hydroelectrical numerical model assuming a primary upward fluid flow associated with the gas, coupled with a secondary flow towards the springs, taking into account the resistivity structure obtained from profiles of electrical resistivity tomography. Thus, the Syabru-Bensi hot springs provide a unique opportunity to study the generation of electrical currents associated with biphasic fluid flow in a geodynamically active area. A pilot multidisciplinary team has now undertaken a multidisciplinary study of the geological, geophysical and geochemical properties of the Syabru-Bensi geothermal system. Studying the spatial and temporal variations of the gas discharges and the associated properties of the hot springs may lead to important clues on the presence and displacements of crustal fluids in relation with the nucleation of large earthquakes in the Nepal Himalayas