Éditeur(s) : HAL CCSD Seismological Society of America
Résumé : With the assistance of the ASTER team (M. Arnold, G. Aumaître, D. Bourlès, and K. Keddadouche)
International audience
The Bulnay earthquake of 23 July 1905 (Mw 8.3–8.5), in north‐central Mongolia, is one of the world’s largest recorded intracontinental earthquakes and one of four great earthquakes that occurred in the region during the twentieth century. The 375 km long surface rupture of the left‐lateral, strike‐slip, N095°E‐trending Bulnay fault associated with this earthquake is remarkable for its pronounced expression across the landscape and for the size of features produced by previous earthquakes. Our field observations suggest that in many areas the width and geometry of the rupture zone is the result of repeated earthquakes; however, in those areas where it is possible to determine that the geomorphic features are the result of the 1905 surface rupture alone, the size of the features produced by this single earthquake are singular in comparison to most other historical strike‐slip surface ruptures worldwide. Along the 80 km stretch, between 97.18° E and 98.33° E, the fault zone is characterized by several meters width and the mean left‐lateral 1905 offset is 8.9±0.6  m with two measured cumulative offsets that are twice the 1905 slip. These observations suggest that the displacement produced during the penultimate event was similar to the 1905 slip. Morphotectonic analyses carried out at three sites along the eastern part of the Bulnay fault allow us to estimate a mean horizontal slip rate of 3.1±1.7  mm/yr over the Late Pleistocene–Holocene period. In parallel, paleoseismological investigations show evidence for two earthquakes prior to the 1905 event, with recurrence intervals of ∼2700–4000  yrs.
ISSN: 0037-1106

hal-01179837
https://hal.archives-ouvertes.fr/hal-01179837
https://hal.archives-ouvertes.fr/hal-01179837v2/document
https://hal.archives-ouvertes.fr/hal-01179837/file/rizza2015.pdf
DOI : 10.1785/0120140119

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
We studied the patterns, rates and evolution of fluvial terraces and fault system during the building process of an intracontinental transpressional mountain in the Gobi-Altay (Mongolia). By analyzing incisions and offsets of fluvial terraces and alluvial fans, we show that the massif has grown by outward migration of thrust faults through time. On the northern flank, the present bounding thrust fault began its activity ~600 ka ago, while a more internal sub-parallel fault was still active until ~200-100 ka. Vertical offset of an alluvial fan abandoned ~100 ka ago allows an estimate of 0.1 mm/yr Upper Pleistocene - Holocene uplift rate. The morphology of the catchment-piedmont system strongly suggests a periodical formation of the alluvial surfaces, controlled by the climatic pulses, at the beginning of the wet interglacial periods. The abandonment of the alluvial terraces lags by several thousand years the abandonment of the alluvial fans, showing a diachronous incision propagating upstream. The incision rate deduced from the different elevations of straths exceeds of one order of magnitude the rock uplift rate. This excess is mostly due to ongoing drainage network growth at the core of the massif, and incision due to alluvial apron entrenchment near the outlet. This implies that fluvial response is mainly controlled by drainage growth, interaction with piedmont and cyclic climatic variations, rather than by rock uplift.
ISSN: 0278-7407

hal-00323558
https://hal.archives-ouvertes.fr/hal-00323558
https://hal.archives-ouvertes.fr/hal-00323558v2/document
https://hal.archives-ouvertes.fr/hal-00323558/file/Vassallo2007.pdf
DOI : 10.1029/2006TC002081

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
We carried out morphotectonic studies along the left-lateral strike-slip Bogd Fault, the principal structure involved in the Gobi-Altay earthquake of 1957 December 4 (published magnitudes range from 7.8 to 8.3). The Bogd Fault is 260 km long and can be subdivided into five main geometric segments, based on variation in strike direction. West to East these segments are, respectively: the West Ih Bogd (WIB), The North Ih Bogd (NIB), the West Ih Bogd (WIB), the West Baga Bogd (WBB) and the East Baga Bogd (EBB) segments. Morphological analysis of offset streams, ridges and alluvial fans-particularly well preserved in the arid environment of the Gobi region-allows evaluation of late Quaternary slip rates along the different faults segments. In this paper, we measure slip rates over the past 200 ka at four sites distributed across the three western segments of the Bogd Fault. Our results show that the left-lateral slip rate is similar to 1 mm yr(-1) along the WIB and EIB segments and similar to 0.5 mm yr(-1) along the NIB segment. These variations are consistent with the restraining bend geometry of the Bogd Fault.;Our study also provides additional estimates of the horizontal offset associated with the 1957 earthquake along the western part of the Bogd rupture, complementing previously published studies. We show that the mean horizontal offset associated with the 1957 earthquake decreases progressively from 5.2 m in the west to 2.0 m in the east, reflecting the progressive change of kinematic style from pure left-lateral strike-slip faulting to left-lateral-reverse faulting. Along the three western segments, we measure cumulative displacements that are multiples of the 1957 coseismic offset, which may be consistent with a characteristic slip. Moreover, using these data, we re-estimate the moment magnitude of the Gobi-Altay earthquake at M-w 7.78-7.95.;Combining our slip rate estimates and the slip distribution per event we also determined a mean recurrence interval of similar to 2500-5200 yr for past earthquakes along the different segments of the western Bogd Fault. This suggests that the three western segments of the Bogd Fault and the Gurvan Bulag thrust fault (a reverse fault bounding the southern side of the Ih Bogd range that ruptured during the 1957 earthquake) have similar average recurrence times, and therefore may have ruptured together in previous earthquakes as they did in 1957. These results suggest that the western part of the Bogd Fault system, including the Gurvan Bulag thrust fault, usually behaves in a 'characteristic earthquake' mode.
ISSN: 0956-540X

hal-00639467
https://hal.archives-ouvertes.fr/hal-00639467
https://hal.archives-ouvertes.fr/hal-00639467v2/document
https://hal.archives-ouvertes.fr/hal-00639467/file/rizzaGJI2011.pdf
DOI : 10.1111/j.1365-246X.2011.05075.x

Éditeur(s) : HAL CCSD
Résumé : National audience
Geological Society of America Special Paper

insu-00402774
https://hal-insu.archives-ouvertes.fr/insu-00402774

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The dating of the uplift onset of the Mongolian mountain ranges, the northernmost relief associated with the India-Eurasia convergence, is a fundamental issue to better understand the mechanisms of propagation of the Cenozoic transpressive deformation in Central Asia. Using apatite fission tracks we determined the timing and strain rates of the tectonics affecting the Gurvan Bogd system, in the Gobi-Altay, since the Middle Mesozoic to the Late Cenozoic. The region was firstly affected by a Lower-Middle Jurassic tectonic phase, characterized by a vertical crustal movement larger than 2 km. Then followed a protracted period without major crustal vertical movements until the last uplift phase. The peneplanation of the Jurassic relief produced an erosional surface that has undergone negligible denudation or sedimentation for more than 100 Ma. This same surface corresponds to the present summit plateaux of the massifs, standing about 2000 m above the surrounding region, which corresponds to the vertical movement produced by the ongoing uplift. Modelling of fission track data from the massifs of Ih Bogd and Baga Bogd shows that this uplift phase probably started at 5 ± 3 Ma. The Gobi-Altay mountain range appears therefore as one of the youngest mountain ranges in Central Asia, which is consistent with the idea of a northward propagation of the transpressional deformation from the Himalayan front to the Siberian craton. The Cenozoic uplift rate of the massifs is estimated to be between 0.25 and 1 mm/yr, which is slightly higher than the upper Pleistocene vertical slip rates of the bordering faults. This suggests that thrust faults observed within the massifs would increase the uplift rate inside the massifs compared to the uplift rate determined at their boundaries.
ISSN: 0012-821X

insu-00191116
https://hal-insu.archives-ouvertes.fr/insu-00191116
DOI : 10.1016/j.epsl.2007.04.047

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The East Sayan ranges are a key area to understand the interactions between the transpressive deformation linked to the far-field effects of the India-Asia collision and the extension linked to the opening of the Baikal Rift System. The active deformation that affects this range is very recent (around 5 Ma) but occurs in a very complex morphotectonic setting and the understanding of the Tertiary deformation relies entirely on a detailed knowledge of the pre-deformation situation. Using apatite fission track thermochronology, cosmogenic 10Be and morphological study on Tertiary lava flows we demonstrate that prior to the Oligocene the morphology of the East Sayan area was characterized by a wide, constantly rejuvenated erosion surface. Apatite fission track thermal modelling indicates that this surface started to form at least in Late Jurassic-Early Cretaceous (140-120 Ma). The long-term exhumation rates (several tens of million years) derived from apatite fission track data (17.5 m/Ma) and the short-term erosion rates (over a few hundred thousand years) derived from cosmogenic 10Be data (12-20 m/Ma) are coherent implying a near constant mean erosion rate since Late Jurassic. This constant, slow erosion prevented the formation of a lateritic-kaolinic weathering crust on the planation surface. By Oligocene-early Miocene times a long wavelength uplift that remains to be explained, induced incision that created shallow valleys later filled by basaltic lava flows. Finally, the present short-wavelength topography initiated during the Pliocene.
ISSN: 0743-9547

insu-00805603
https://hal-insu.archives-ouvertes.fr/insu-00805603
DOI : 10.1016/j.jseaes.2011.09.017

Éditeur(s) : HAL CCSD Geological Society of America
Résumé : In Gobi Altay and Altay, Mongolia, several flat surfaces, worn through basement rocks and uplifted during the ongoing tectonic episode to a similar altitude of 4000 m, suggests disruption of a single large-scale surface. New thermochronology and field data show that the plateau surfaces represent uplifted parts of an ancient peneplain that formed during Jurassic time. The Gobi Altay and Altay flattopped massifs are tectonically and geomorphologically unique. Their preservation for ~150 m.y. implies that no further tectonic movements occurred before the onset of the last deformation episode, 5 ± 3 m.y. ago. It also suggests that very low erosion rates were maintained by a dry climate over millions of years.
ISSN: 0091-7613

hal-00406436
https://hal.archives-ouvertes.fr/hal-00406436
DOI : 10.1130/G23758A.1

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
ISSN: 0012-821X

hal-01671315
https://hal.inria.fr/hal-01671315
DOI : 10.1016/j.epsl.2017.10.051

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
This work describes the history of late Pleistocene glaciations in the central Sayan-Tuva Upland (southern Siberia). Geological and geomorphological analysis as well as 10Be surface-exposure dating revealed the glacier fluctuations in this continental area. The available published data show that the glaciers were formed in the MIS 6 and probably survived in the MIS 5. Data are also available concerning glacial advances in different periods of MIS 4, MIS 3 and MIS 2. ELAs were 2030-2230 m. Two distinct 10Be exposure ages groups are highlighted reflecting the time of formation of glacial deposits in the MIS 2 associated to the Big Sayan Ridge outlet glaciers. The Sentsa - Sailag group (terminal moraine) has a mean exposure age of 16.44 ± 0.38 ka. The Jombolok (terminal moraine) - Jombolok (outwash plain) group has a mean exposure age of 22.80 ± 0.56 ka. The last glaciation that occurred at MIS 2 is characterized by the absence of ice cap on the Azas volcanic Plateau and of ice field in the Todza Basin. The thickness of the valley glacier was 300-400 m. At MIS 2, the terminal moraines were ∼1300-1400 m a.s.l. in the Tissa, Sentsa, Jombolok and Sailag river valleys.
ISSN: 0277-3791

hal-00758082
https://hal.archives-ouvertes.fr/hal-00758082
DOI : 10.1016/j.quascirev.2012.06.005

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
An original karstic system in a coastal alluvial plain located in a carbonate environment (Cassis, Provence -- SE France) was studied using a multi-method approach (electrical resistivity tomography, gravimetric, passive seismic survey), combined with geomorphologic methods, surface observations and drilling. Limestone dissolution and/or cavity collapse led to the development of a polje, which was filled rapidly by erosion of Aptian marls. The combination of several dating and paleoenvironmental proxies indicates that the polje was filled during a glacial period (MIS 6, 8 or 10, i.e. in the 130-360 ka age range). This discovery has implications for the understanding of karst processes. The connection between the polje and the three km-long Bestouan underwater karstic conduit with submarine outlet is strongly suggested by sedimentological studies and geophysical prospections.
ISSN: 0169-555X

hal-01054334
https://hal.archives-ouvertes.fr/hal-01054334
DOI : 10.1016/j.geomorph.2014.02.021

Éditeur(s) : HAL CCSD National Academy of Sciences
Résumé : International audience
ISSN: 0027-8424

halsde-00335837
https://hal.archives-ouvertes.fr/halsde-00335837
DOI : 10.1073/pnas.0708015105

Éditeur(s) : HAL CCSD Geological Society of America
Résumé : International audience
How convergent systems distribute strain among frontal thrusts is a major concern regarding seismic hazard assessment. Along the 2500 km Himalayan arc, the seismic behavior of the Bhutan region is unknown, because it corresponds to the only portion of the arc where no evidence of major earthquakes has been reported. This can be due either to the fact that no active tectonic studies have been conducted or to continental shortening being absorbed by the Shillong plateau 150 km farther south. Analyzing offset fluvial terraces in south-central Bhutan shows that two major earthquakes ruptured the Himalayan frontal thrust during the last millennium, and that a comparable rate of Holocene deformation (∼20 mm/yr) is accommodated across the Himalaya in Bhutan as in central Nepal. Thus, the propensity for great earthquakes in Bhutan is similar to what is observed in neighboring portions of the Himalaya arc. This in turn suggests that the shortening process beneath the Shillong plateau has little effect on how strain accumulates within the Bhutanese Himalaya.
ISSN: 0091-7613

hal-01122799
https://hal.archives-ouvertes.fr/hal-01122799
DOI : 10.1130/G35162.1