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Grandin R.,CNRS ENS Geology Laboratory | Doin M.-P.,CNRS ENS Geology Laboratory | Bollinger L.,CEA DAM Ile-de-France | Pinel-Puyssegur B.,CEA DAM Ile-de-France | And 3 more authors.
Geology | Year: 2012

The rise and support of the ~5000 m topographic scarp at the front of Indian-Eurasian collision in the Himalaya involves long-term uplift above a mid-crustal ramp within the Main Himalayan Thrust (MHT) system. Locking of the shallower portion of the fl at-ramp-fl at during the interseismic period also produces transient uplift above the transition zone. However, spatial and temporal relationships between permanent and transient vertical deformation in the Himalaya are poorly constrained, leading to an unresolved causal relationship between the two. Here, we use interferometric synthetic aperture radar (InSAR) to measure interseismic uplift on a transect crossing the whole Himalaya in central Nepal. The uplift velocity of 7 mm/yr at the front of the Annapurna mountain range is explained by an 18-21 mm/yr slip rate on the deep shallow-dipping portion of the MHT, with full locking of the mid-crustal ramp underlying the High Himalaya. The transient uplift peak observed by InSAR matches spatially with the long-term uplift peak deduced from the study of trans-Himalayan river incision, although models of the seismic cycle involving thrusting over a ramp of fi xed geometry predict an ~20 km separation between the two peaks. We argue that this coincidence indicates that today's mid-crustal ramp in central Nepal is located southward with respect to its average long-term location, suggesting that mountain growth proceeds by frontward migration of the ramp driven by underplating of material from the Indian plate under the Himalaya. © 2012 Geological Society of America.

Girault F.,University Paris Diderot | Perrier F.,University Paris Diderot | Gajurel A.P.,Tribhuvan University | Bhattarai M.,National Seismological Center | And 4 more authors.
Geochimica et Cosmochimica Acta | Year: 2012

Effective radium concentration (ECRa) of 622 rock samples from 6 different sites in the Nepal Himalayas was measured in the laboratory using radon accumulation experiments. These sites, located from Lower Dolpo in Western Nepal to Eastern Nepal, are divided into 9 transects which cut across the Main Central Thrust zone (MCT zone) separating low-grade metamorphic Lesser Himalayan Sequence (LHS) units to the south and higher-grade metamorphic Greater Himalayan Sequence (GHS) units to the north. This boundary remains difficult to define and is the subject of numerous debates. ECRa values range from 0.03±0.03 to 251.6±4.0Bqkg-1, and appear to be representative of the formation and clearly related to the local lithology. For example, for the Upper Trisuli and Langtang Valleys site in Central Nepal, the most studied place with 350 available ECRa values, LHS rocks are characterized by a mean value of 5.3±1.3Bqkg-1 while GHS rocks of Formations I and II show significantly lower values with a mean value of 0.69±0.11Bqkg-1, thus leading to a LHS/GHS ECRa ratio of 7.8±2.2. This behavior was systematically confirmed by other transects (ratio of 7.9±2.2 in all other sites), with a threshold ECRa value, separating LHS from GHS, of 0.8Bqkg-1, thus bringing forward a novel method to characterize, within the MCT shear zone, which rocks belong to the GHS and LHS units. In addition, Ulleri augen gneiss, belonging to LHS rocks, occurred in several transects and were characterized by high ECRa values (17.9±4.3Bqkg-1), easy to distinguish from the GHS gneisses, characterized by low ECRa values at the bottom of the GHS, thus providing a further argument to locate the MCT. The measurement of ECRa data, thus, provides a cost-effective method which can be compared with neodymium isotopic anomalies or estimates of the peak metamorphic temperature. This study, therefore, shows that the measurement of ECRa provides additional information to discriminate different geological formations, and can be particularly useful in areas where geology mapping is not straightforward or still remains controversial. © 2012 Elsevier Ltd.

Richon P.,CEA DAM Ile-de-France | Richon P.,CNRS Paris Institute of Global Physics | Richon P.,University Paris Diderot | Perrier F.,CNRS Paris Institute of Global Physics | And 6 more authors.
Journal of Environmental Radioactivity | Year: 2011

Temporal variation of radon-222 concentration was studied at the Syabru-Bensi hot springs, located on the Main Central Thrust zone in Central Nepal. This site is characterized by several carbon dioxide discharges having maximum fluxes larger than 10 kg m-2 d-1. Radon concentration was monitored with autonomous Barasol™ probes between January 2008 and November 2009 in two small natural cavities with high CO2 concentration and at six locations in the soil: four points having a high flux, and two background reference points. At the reference points, dominated by radon diffusion, radon concentration was stable from January to May, with mean values of 22 ± 6.9 and 37 ± 5.5 kBq m-3, but was affected by a large increase, of about a factor of 2 and 1.6, respectively, during the monsoon season from June to September. At the points dominated by CO2 advection, by contrast, radon concentration showed higher mean values 39.0 ± 2.6 to 78 ± 1.4 kBq m-3, remarkably stable throughout the year with small long-term variation, including a possible modulation of period around 6 months. A significant difference between the diffusion dominated reference points and the advection-dominated points also emerged when studying the diurnal S1 and semi-diurnal S2 periodic components. At the advection-dominated points, radon concentration did not exhibit S1 or S2 components. At the reference points, however, the S2 component, associated with barometric tide, could be identified during the dry season, but only when the probe was installed at shallow depth. The S1 component, associated with thermal and possibly barometric diurnal forcing, was systematically observed, especially during monsoon season. The remarkable short-term and long-term temporal stability of the radon concentration at the advection-dominated points, which suggests a strong pressure source at depth, may be an important asset to detect possible temporal variations associated with the seismic cycle. © 2010 Elsevier Ltd.

Bollinger L.,CEA DAM Ile-de-France | Sapkota S.N.,National Seismological Center | Tapponnier P.,Nanyang Technological University | Klinger Y.,CNRS Paris Institute of Global Physics | And 6 more authors.
Journal of Geophysical Research B: Solid Earth | Year: 2014

The return times of large Himalayan earthquakes are poorly constrained. Despite historical devastation of cities along the mountain range, definitive links between events and specific segments of the Main Frontal Thrust (MFT) are not established, and paleoseismological records have not documented the occurrence of several similar events at the same location. In east central Nepal, however, recently discovered primary surface ruptures of that megathrust in the A.D. 1255 and 1934 earthquakes are associated with flights of tectonically uplifted terraces. We present here a refined, longer slip history of the MFT's two overlapping strands (Patu and Bardibas Thrusts) in that region, based on updated geomorphic/neotectonic mapping of active faulting, two 1.3 km long shallow seismic profiles, and logging of two river-cut cliffs, three paleoseismological trenches, and several pits, with constraints from 74 detrital charcoals and 14 cosmogenic nuclide ages. The amount of hanging wall uplift on the Patu thrust since 3650 ± 450 years requires three more events than the two aforementioned. The uplift rate (8.5 ± 1.5 mm/yr), thrust dip (25° ± 5°N), and apparent characteristic behavior imply 12-17.5 m of slip per event. On the Bardibas thrust, discrete pulses of colluvial deposition resulting from the coseismic growth of a flexural fold scarp suggest the occurrence of six or seven paleo-earthquakes in the last 4500 ± 50 years. The coeval rupture of both strands during great Himalayan earthquakes implies that in eastern Nepal, the late Holocene return times of such earthquakes probably ranged between 750 ± 140 and 870 ± 350 years. © 2014. American Geophysical Union. All Rights Reserved.

Girault F.,CNRS Paris Institute of Global Physics | Poitou C.,CNRS Paris Institute of Global Physics | Perrier F.,CNRS Paris Institute of Global Physics | Koirala B.P.,National Seismological Center | Bhattarai M.,National Seismological Center
Natural Hazards and Earth System Sciences | Year: 2011

Low-field magnetic susceptibility χm and effective radium concentration ECRa, obtained from radon emanation, have been measured in the laboratory with 129 soil samples from Nepal. Samples along horizontal profiles in slope debris or terrace scarps showed rather homogeneous values of both χm and ECRa. One sample set, collected vertically on a lateritic terrace scarp, had homogeneous values of ECRa while χm increased by a factor of 1 to 10 for residual soils and topsoils. However, for a set of samples collected on three imbricated river terraces, values of ECRa, homogeneous over a given terrace, displayed a gradual increase from younger to older terraces. By contrast, χm showed more homogeneous mean values over the three terraces, with a larger dispersion, however, for the younger one. Similarly, Kathmandu sediments exhibited a large increase in ECRa from sand to clay layers, while χm increased moderately. The combination of χm and ECRa, thus, provides a novel tool to characterize quantitatively various soil groups and may be of interest to distinguish modes of alteration or deposition histories. © 2012 Author(s).

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