Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
[1] This article presents (U-Th)/He thermochronological data from the Longmen Shan belt, eastern Tibet. Located between the Songpan-Garze terrane and the Yangtze craton, this mountain range is one of the steepest margins of the Tibetan Plateau and an important area for the comprehension of the mechanisms that control the dynamics of such plateau borders in terms of spatial distribution of deformation or timing of topographic building. We describe several age-elevation transects and perform forward modeling of our data to derive quantitative information on the exhumation of the range. A major phase of exhumation started at 8–11 Ma, with an average rate of $0.65 mm a À1. Comparison of zircon and apatite ages indicates that the eastern part of the range may have experienced a significant decrease in exhumation since 2–3 Ma. We use the distribution of finite exhumation across the major faults of the area to quantify their dip-slip throw rate over the last 10 Ma. The Beichuan Fault, which was activated during the 2008 Sichuan earthquake, is the major active structure of the Longmen Shan since the late Miocene, with an average thrusting slip rate between 0.4 and 1 mm a À1. Conversely, over the same time period, only minor dip-slip activity occurred on the Wenchuan Fault Zone. This distribution in space and time of exhumation and deformation is discussed and compared to the different proposed models for the geodynamical evolution of the eastern Tibetan margin. It also provides an important long-term perspective to put in context the destructive 2008 Sichuan earthquake that struck the central Longmen Shan.
ISSN: 0278-7407

hal-01577059
https://hal-amu.archives-ouvertes.fr/hal-01577059
https://hal-amu.archives-ouvertes.fr/hal-01577059/document
https://hal-amu.archives-ouvertes.fr/hal-01577059/file/godard2009late.pdf
DOI : 10.1029/2008TC002407

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
No major earthquake occurred in North Chile since the 1877 Mw 8.6 subduction earthquake that produced a huge tsunami. However, geodetic measurements conducted over the last decade in this area show that the upper plate is actually deforming, which reveals some degree of locking on the subduction interface. This accumulation of elastic deformation is likely to be released in a future earthquake. Because of the long elapsed time since 1877 and the rapid accumulation of deformation (thought to be 6-7 cm yr−1), many consider this area is a mature seismic gap where a major earthquake is due and seismic hazard is high. We present a new Global Positioning System (GPS) velocity field, acquired between 2008 and 2012, that describes in some detail the interseismic deformation between 18°S and 24°S. We invert for coupling distribution on the Nazca-South America subduction interface using elastic modelling. Our measurements require that, at these latitudes, 10 to 12 mm yr−1 (i.e. 15 per cent of the whole convergence rate) are accommodated by the clockwise rotation of an Andean block bounded to the East by the subandean fold-and-thrust belt. This reduces the accumulation rate on the subduction interface to 56 mm yr−1 in this area. Coupling variations on the subduction interface both along-strike and along-dip are described. We find that the North Chile seismic gap is segmented in at least two highly locked segments bounded by narrow areas of weak coupling. This coupling segmentation is consistent with our knowledge of the historical ruptures and of the instrumental seismicity of the region. Intersegment zones (Iquique, Mejillones) correlate with high background seismic rate and local tectonic complexities on the upper or downgoing plates. The rupture of either the Paranal or the Loa segment alone could easily produce a Mw 8.0-8.3 rupture, and we propose that the Loa segment (from 22.5°S to 20.8°S) may be the one that ruptured in 1877.
ISSN: 0956-540X

hal-00941279
https://hal.archives-ouvertes.fr/hal-00941279
DOI : 10.1093/gji/ggt183

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Zandjan province is situated in the NW of Iran, at the junction between the southern termination of the Tabriz right-lateral strike-slip fault and the western end of the Alborz active mountain range. Despite this key location, in terms of active tectonics, there is no destructive earthquake reported in available historical seismicity catalogues. Even if a few low magnitude instrumental earthquakes have been recorded indicating that this area is actively deforming, the Zandjan region can be considered as a seismic gap. Based on detailed field investigations, digital elevation models (DEMs), satellite image and aerial photo interpretations, we have identified a major active fault network affecting this regions. It is mainly constituted by several west and est verging thrust faults, trending NW-SE to N-S, that clearly cut Late Quaternary to Holocene deposits. Evidences in the morphology indicate also that they accommodate some right-lateral slip component. This fault network can be divided into four major fault zones affecting Zandjan city (present day population over 500,000) and part of the Zandjan-Mianeh basin more to the west. Based on fault geometrical characteristics (fault length, segmentation ... ) and kinematics, the Zandjan fault system is certainly capable to produce moderate to large earthquakes (Mw = 6.5-7.0) and serious damages in the Zandjan city and surrounding areas. The study of these faults is, then, of major interest for seismic hazard assessment in Zandjan province where total population exceed 1.5 million but also to better understand present day geodynamic of NW Iran.
ISSN: 0040-1951

hal-00617526
https://hal.archives-ouvertes.fr/hal-00617526
DOI : 10.1016/j.tecto.2011.04.012

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
The 2004 Aceh and 2005 Nias events are the two greatest earthquakes of the past 40 years with a total rupture of 1700 km long and a coseismic slip reaching up to 25 m. These two earthquakes have caused large stress perturbations which significantly altered seismic activity in the Sumatra-Andaman region. Using both detailed mapping of failure planes and various slip distributions, we calculate this stress change along the Sumatra-Andaman-Sagaing fault system from central Sumatra to southern Myanmar. The static Coulomb stress change Delta CFF and the observed seismic activity are in very good agreement with a Coulomb index similar to 20% greater than the one obtained for random events. Compared to previous studies, this high Coulomb Index confirms two important issues on the use of static stress change criterion: unsuited to study near-field aftershocks and only relevant for aftershocks analysis on large and mature faults at a time scale of several months. The calculated Delta CFF distribution suggests that the 2004 and 2005 earthquakes inhibit failure on the North Andaman rift and on the Sagaing fault, while failure is encouraged along the transform Andaman zone, the central Andaman rift, the West Andaman fault, the Sumatra fault system, and the offshore thrust faults west of Sumatra Island. The maximum value of similar to 15-20 bar (1.5-2 MPa) for Delta CFF is reached in the northern part of the Sumatra fault system. This high value together with the lack of major earthquake in the last 170 years result in a high seismic hazard for this region. Our results are also consistent with temporal evolution of both earthquakes' location and focal mechanism prior to and after the events. In particular, we explain the occurrence and the mechanism of seismic swarms observed in the central Andaman rift and along the west Andaman fault. Finally, our calculations reveal that the seismicity in the Andaman rift zone can only be explained if mu' > 0.5. This result leads to two end-member models: one with a constant and high fault friction and one with spatial variations, for which friction may depend on either the nature of the lithosphere (oceanic versus continental) or the fault type.
ISSN: 1525-2027

hal-00456027
https://hal.archives-ouvertes.fr/hal-00456027
DOI : 10.1029/2008GC002167

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
P>The Strait of Hormuz area is a transition zone between the continental collision of the Zagros (west) and the subduction of an oceanic part of the Arabian Plate beneath the Makran wedge (east). Geology and recent GPS measurements indicate that about 15 mm yr-1 of relative motion in N10 degrees E direction is accommodated by two major fault systems: (1) the NNW-trending Minab-Zendan-Palami (MZP) fault system that connects the Main Zagros Thrust (MZT) to the inner Makran thrust system and the Frontal subduction thrust and (2) the N-trending Sabzevaran-Kahnuj-Jiroft (SKJ) fault system that bounds the Jazmurian depression to the west. We use dense GPS measurements along four transects across these fault systems in order to determine the strains spatial distribution. The northern GPS transect confirms the total fault slip rates for both fault systems estimated by the tectonic analyses (about 10 and 7.3 mm yr-1 in N10 degrees direction across the MZP and SKJ fault systems, respectively). For both fault systems, the elastic deformation spreads over shear zones that are several tens of kilometres wide. However, transects located close to latitude 27 degrees N reveal a much narrower shear zone (similar to 10 km) for the MZP fault system. Moreover, we confirm that most of the present-day strain is transferred towards the frontal subduction thrust rather than towards the inner Makran thrusts. In order to complement this new GPS velocity field with spatially dense measurements, we processed a set of ERS radar images by the radar interferometry (InSAR) technique. We used both a 'stacking' and a 'persistant-scatterers' approach to differentiate the ground deformation signal which spatial gradient is expected to be very low, from the atmospheric signal. Results from these interferograms appear to be relatively in agreement with the GPS-determined strain distribution. Nevertheless, they confirm the absence of any superficial creep behaviour since no sharp discontinuity on interferometric phase can be noted on any interferogram. Finally, we use a purely kinematic 'block model' inversion process to calculate slip rates and locking depths for each fault system from our GPS measurements. These models suggest that the relative quiescence over the last 200 yr has certainly produced a slip deficit as high as 2 m. So, we may wonder if the MZP fault system is not late in the interseismic phase of its earthquake cycle.
ISSN: 0956-540X

hal-00429386
https://hal.archives-ouvertes.fr/hal-00429386
DOI : 10.1111/j.1365-246X.2009.04321.x

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
A network of 54 survey GPS sites, 28 continuous GPS stations and three absolute gravity (AG) observation sites have been set up in the Alborz mountain range to quantify the present-day kinematics of the range. Our results allow us to accurately estimate the motion of the South Caspian block (SCB) for the first time, and indicate rotation of the SCB relative to Eurasia, accounting for the left lateral motion in the Alborz range. In light of these new results, it clearly appears that deformation rates vary along the range, the eastern part accommodating mainly left lateral strike slip (2 mm yr(-1) south of the range and 5 mm yr(-1) north of the range) with a very low range normal shortening rate on the Khazar thrust fault (similar to 2 mm yr(-1)), and the western part accommodating range normal shortening (similar to 6 mm yr-1) on the Khazar thrust fault with a left lateral component of similar to 2 mm yr(-1) north of the range and 1 mm yr(-1) south of the range. These present-day kinematics agree with geomorphologic estimated slip rates, but not the long-term deformation, corroborating the idea that the kinematics of the range have changed recently due to the change of SCB motion.;Modelling of the interseismic deformation suggests a deep locking depth on the central-western segment of the Khazar fault (similar to 30 km) in agreement with the Baladeh earthquake rupture and aftershock ranging between 10 and 30 km. Given this unusual deep locking depth and the 34 degrees dip of the thrust, a large part of the Alborz range is located above the seismically coupled part of the fault. Based on our AG measurements this part of the range seems to uplift at a rate of 1-5 mm yr(-1), in agreement with terrace uplift.
ISSN: 0956-540X

hal-00617146
https://hal.archives-ouvertes.fr/hal-00617146
https://hal.archives-ouvertes.fr/hal-00617146v2/document
https://hal.archives-ouvertes.fr/hal-00617146/file/djamour2010.pdf
DOI : 10.1111/j.1365-246X.2010.04811.x

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
We analyze focal mechanisms of shallow-intermediate earthquakes in a NW-SE transect along the western Betic Cordillera and Alboran Sea, and deep earthquakes located in the central Betics to constrain the state of stress at the Gibraltar Arc slow convergence area. Shallow earthquakes (< 40 km) are preferably clustered at the mountain front. A general NW-SE horizontal compression is compatible with the convergence, and NW plunging compression axes are in agreement with frontal thrust activity. Toward the Alboran Sea the earthquakes are progressively deeper, reaching intermediate depths (40-120 km) near the coast and supporting the present-day activity of subduction only in this area. The Iberian continental crust concentrates most of the intermediate seismicity and is forced to partially sink into the mantle, probably through the pull of the oceanic slab. This downdip pull together with the buoyancy of the Iberian continental crust produces the slab curvature, downdip extension in the external arc of the continental slab, and downdip compression in the inner arc. T axes highly dipping to the southeast at 90-120 km depth occur at the oceanic/continental transition. Deep earthquakes (> 620 km) show very similar focal mechanisms, fitting the general slab behavior of resistance to further descent at the 660 km discontinuity. Seismicity features evidence the present-day stress distribution in a context of transition from subduction to continental collision.
ISSN: 1525-2027

hal-00669889
https://hal.archives-ouvertes.fr/hal-00669889
DOI : 10.1029/2011GC003824

Éditeur(s) : HAL CCSD American Geophysical Union (AGU) American Geophysical Union
Résumé : International audience
GPS data reveal that the Brahmaputra Valley has broken from the Indian Plate and rotates clockwise relative to India about a point a few hundred kilometers west of the Shillong Plateau. The GPS velocity vectors define two distinct blocks separated by the Kopili fault upon which 2–3 mm/yr of dextral slip is observed: the Shillong block between longitudes 89 and 93°E rotating clockwise at 1.15°/Myr and the Assam block from 93.5°E to 97°E rotating at ≈1.13°/Myr. These two blocks are more than 120 km wide in a north-south sense, but they extend locally a similar distance beneath the Himalaya and Tibet. A result of these rotations is that convergence across the Himalaya east of Sikkim decreases in velocity eastward from 18 to ≈12 mm/yr and convergence between the Shillong Plateau and Bangladesh across the Dauki fault increases from 3 mm/yr in the west to >8 mm/yr in the east. This fast convergence rate is inconsistent with inferred geological uplift rates on the plateau (if a 45°N dip is assumed for the Dauki fault) unless clockwise rotation of the Shillong block has increased substantially in the past 4–8 Myr. Such acceleration is consistent with the reported recent slowing in the convergence rate across the Bhutan Himalaya. The current slip potential near Bhutan, based on present-day convergence rates and assuming no great earthquake since 1713 A.D., is now ~5.4 m, similar to the slip reported from alluvial terraces that offsets across the Main Himalayan Thrust and sufficient to sustain a Mw ≥ 8.0 earthquake in this area.
ISSN: 0148-0227

hal-01115084
https://hal.archives-ouvertes.fr/hal-01115084
DOI : 10.1002/2014JB011196

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
The North Tehran Fault (NTF) is located at the southernmost piedmont of Central Alborz and crosses the northern suburbs of the Tehran metropolis and adjacent cities, where ∼15 million people live. Extending over a length of about 110 km, the NTF stands out as a major active fault and represents an important seismic hazard for the Iranian capital after historical seismicity. In order to characterize the activity of the NTF in terms of kinematics, magnitude and recurrence intervals of earthquakes, we carried out a first paleoseismological study of the fault within its central part between Tehran and Karaj cities. We opened a trench across a 3 m-high fault scarp affecting Quaternary deposits. Our study shows that the scarp is the result of repeated events along a main N115°E trending shallow dipping thrust fault, associated with secondary ruptures. From the trench analysis and Infrared Stimulated Luminescence (IRSL) dating of fault-related sediments, we interpreted between 6 and 7 surface-rupturing events that occurred during the past 30 kyrs. Their magnitudes (estimated from the displacements along the faults) are comprised between 6.1 and 7.2. The two last events - the largest - occurred during the past 7.9 ± 1.2 ka, which yields a Holocene slip rate of ∼0.3 mm/yr. The 7 earthquakes scenario suggests a regular periodicity with a mean recurrence interval of ∼3.8 kyrs. However, the two most recent events could correspond to the two largest historical earthquakes recorded in the area (in 312-280 B.C. and 1177 A.D.), and therefore suggest that the NTF activity is not regular.
ISSN: 0148-0227

hal-00757568
https://hal.archives-ouvertes.fr/hal-00757568
https://hal.archives-ouvertes.fr/hal-00757568v2/document
https://hal.archives-ouvertes.fr/hal-00757568/file/ritz2012.pdf
DOI : 10.1029/2012JB009147

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

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
After the January 12, 2010, Haiti earthquake, we deployed a mainly offshore temporary network of seismologic stations around the damaged area. The distribution of the recorded aftershocks, together with morphotectonic observations and mainshock analysis, allow us to constrain a complex fault pattern in the area. Almost all of the aftershocks have a N‐S compressive mechanism, and not the expected left‐lateral strike‐slip mechanism. A first‐order slip model of the mainshock shows a N264°E north‐dipping plane, with a major left‐lateral component and a strong reverse component. As the aftershock distribution is sub‐parallel and close to the Enriquillo fault, we assume that although the cause of the catastrophe was not a rupture along the Enriquillo fault, this fault had an important role as a mechanical boundary. The azimuth of the focal planes of the aftershocks are parallel to the north‐dipping faults of the Transhaitian Belt, which suggests a triggering of failure on these discontinuities. In the western part, the aftershock distribution reflects the triggering of slip on similar faults, and/or, alternatively, of the south‐dipping faults, such the Trois‐Baies submarine fault. These observations are in agreement with a model of an oblique collision of an indenter of the oceanic crust of the Southern Peninsula and the sedimentary wedge of the Transhaitian Belt: the rupture occurred on a wrench fault at the rheologic boundary on top of the under‐thrusting rigid oceanic block, whereas the aftershocks were the result of the relaxation on the hanging wall along pre‐existing discontinuities in the frontal part of the Transhaitian Belt.
ISSN: 0094-8276

insu-00643489
https://hal-insu.archives-ouvertes.fr/insu-00643489
https://hal-insu.archives-ouvertes.fr/insu-00643489/document
https://hal-insu.archives-ouvertes.fr/insu-00643489/file/GRL-haiti.2011.pdf
DOI : 10.1029/2011GL049799

Éditeur(s) : HAL CCSD Geological Society of London
Résumé : The Zagros Simply Folded Belt (ZSFB) is an active fold-and-thrust belt resulting from the still continuing continental collision between the Arabian plate and the Iranian plate, which probably started in the Oligocene. The present-day shortening (N25°) is well documented by focal mechanisms of earthquakes and global positioning system (GPS) surveys. We propose in this study a comparison of published palaeostress markers, including magnetic fabric, brittle deformation and calcite twinning data. In addition, we describe the magnetic fabric from Palaeocene carbonates (10 sites) and Mio-Pliocene clastic deposits (15 sites). The magnetic fabrics are intermediate, with magnetic foliation parallel to the bedding, and a magnetic lineation mostly at right angles to the shortening direction. This suggests that the magnetic fabric retains the record of an early layer-parallel shortening (LPS) that occurred prior to folding. The record of LPS allows the identification of originally oblique folds such as the Mand Fold, which have developed in front of the Kazerun Fault. The shape parameter of the magnetic fabric indicates a weak strain compatible with the development of detachment folds in the ZSFB. The palaeostress datasets, covering the Palaeocene to Pleistocene time interval, support several folding episodes accompanied by a counter-clockwise rotation of the stress field direction. The Palaeocene carbonates in the ZSFB record a N47 LPS during early to middle Miocene detachment folding in the High Zagros Belt (HZB). The Mio-Pliocene clastic deposits recorded a N38 LPS prior to and during detachment folding within the ZSFB at the end of the Miocene-Pliocene. Similarly, fault slip and calcite twin data from the ZSFB also support a counter-clockwise rotation from NE to N20 between the pre-folding stage and the late rejuvenation of folds. This counter-clockwise trend of palaeostress data agrees with fault slip data from the HZB. During the late stage of folding in the ZSFB, the Plio-Quaternary palaeostress trends are consistently parallel to the present-day shortening direction.
Tectonic and stratigraphic evolution of Zagros and Makran during the Mesozoic-Cenozoic

hal-00545036
https://hal.archives-ouvertes.fr/hal-00545036
DOI : 10.1144/SP330.6

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : We use InSAR to measure deformation and kinematics of the Mw = 4.9 Borujerd (2005/05/03) and Mw = 6.1 Chalan-Chulan (2006/03/31) earthquakes that occurred in the Zagros fold-and-thrust belt. The focal mechanism of the 2006 event is consistent with right lateral strike-slip motion and the event ruptured the Dorud-Borujerd segment of the Main Recent Fault. An Envisat interferogram spanning the 2006 event shows peak ground deformation of 9 cm in the satellite line-of-sight along a 10 km long fault portion. The interferogram spanning the 2005 earthquake is rather related to atmospheric artefact than to ground deformation. Dislocation models of the 2006 Chalan-Chulan event indicate dextral slip amounting to a maximum of 90 cm at a depth of 4 km. The predicted vertical displacements are in good agreement with differential levelling data. The 2006 event filled only a small part of the seismic gap located between large M = 7 events that occurred in 1909 and 1957.
ISSN: 0954-4879

hal-00411313
https://hal.archives-ouvertes.fr/hal-00411313
DOI : 10.1111/j.1365-3121.2008.00797.x

Éditeur(s) : HAL CCSD
Résumé : 1 p.
International audience
The 12th May 2008 Wenchuan earthquake in the Longmen Shan occurred on a large thrust fault largely inherited from an Indosinian structure itself probably controlled by an older structural heritage of the South China block continental margin. Within the whole northeast Tibet region, such a structural inheritance has had a major impact on the Tertiary deformation. It appears of primary importance to assess the pre-Tertiary tectonic evolution of the main blocks involved to understand the actual deformation in the eastern edge of Tibet. Over the past decades, the Proterozoic to Cenozoic tectonic, metamorphic and geochronologic history of the Longmen Shan and Songpan Garzê area have been largely studied. We present a synthesis of the tectonic evolution of the Songpan Garzê fold and thrust belt from Triassic to present. The Songpan-Garzê belt was formed during closure of a wide oceanic basin ?lled with a thick (5 to 15 km) sequence of Triassic ?yschoid sediments. Closure of the basin due to Triassic subduction involved strong shortening, intense folding and faulting of the Triassic series. A large-scale décollement, that presently outcrops along the eastern boundary of the belt (Danba area), allowed the growth of a wide and thick accretionary wedge. It develops in the Paleozoic and Triassic series and separates the accretionary prism from an autochthonous crystalline basement which shares many similarities with the basement of the Yangtze Craton (0.7-0.9 Ga). To the north and northwest, below the thickened Triassic series of the belt, the composition (oceanic or continental) of the basement remains unknown. During the Indosinian orogeny the emplacement of orogenic granites (220 - 150 Ma) was associated to crustal thickening. The isotopic composition of granitoids shows that their magma source were predominantly derived from melting of the proterozoic basement with varying degrees of sedimentary material and negligible mantle source contribution. In the Danba area, the décollement outcrops in a large tertiary antiform with a NNW-SSE axis. It has been exhumed too in the hanging wall of the NE-SW faults of the Tertiary Longmen-Shan belt that marks the present day transition from the Tibetan plateau to the Sichuan basin. These faults have episodically absorbed signi?cant shortening since the Late Triassic. The amount and precise timing of post-triassic deformation are dif?cult to constrain especially because of the dif?culty to isolate the tertiary thermochronological signal from the protracted late Triassic - Cretaceous thermal history. Nonetheless it is generally accepted that Jurassic - Cretaceous tectonism did not modi?ed the general Triassic architecture of eastern Tibet contrarily to the Tertiary deformation. The long-term cooling histories obtained on Mesozoic granites and on the metamorphic series of the Danba dome are very similar showing a very slow and regular cooling during Jurassic andCretaceous, con?rming the absence of major tectonic event between c.a. 150 and 30 Ma. Low temperature thermochronology data indicate that ?nal exhumation and cooling occurred in theTertiary with an acceleration between 10 and 5 Ma along the major tectonic structures. Within the Longmen Shan range, a total denudation of 7 to 10 km is estimated for the late Cenozoic period. Similar amounts of late Tertiary denudation have been estimated along an east-west section across the Xianshuihe fault
Asian climate and tectonics seminar
Utrecht, Netherlands

insu-00610298
https://hal-insu.archives-ouvertes.fr/insu-00610298
DOI : 10.1016/j.jseaes.2010.03.008

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
The Main Himalayan Thrust (MHT) is the source of great earthquakes that have been documented along the range. Its geometry is a key parameter that influences accommodation of tectonic loading and earthquake magnitudes along the Himalayan Arc. Although seismic images are available for both the western and the central part of the range, this geometry remains poorly constrained for the Bhutanese Himalayas. Here we address this issue using a 10Be cosmogenic nuclides denudation transect across western Bhutan. We observe a wide low denudation rate domain between 50 km and 110 km from the front followed by a strong northward increase. Using a joint inversion of denudation rates, GPS data, and Holocene uplift rates, we interpret this pattern as a consequence of a flat-ramp transition along the MHT. Compared to central Nepal and Sikkim, this location of the ramp suggests a wider décollement, with implications for greater seismogenic potential of the MHT in western Bhutan.
ISSN: 0094-8276

Droits : info:eu-repo/semantics/OpenAccess
hal-01174088
https://hal.archives-ouvertes.fr/hal-01174088
https://hal.archives-ouvertes.fr/hal-01174088v2/document
https://hal.archives-ouvertes.fr/hal-01174088/file/lerouxmalloufGRL2015.pdf
DOI : 10.1002/2015GL063767

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
The transition zone at the downdip end of seismic coupling along subduction interfaces is often the site of megathrust earthquake nucleation and concentrated postseismic afterslip, as well as the focus site of episodic tremor and slip features. Exhumed remnants of the former Alpine subduction zone found in the Swiss Alps allow analyzing fluid and deformation processes near the transition zone region (30–40 km paleodepth). The Dent Blanche Thrust (DBT) is a lower blueschist-facies shear zone interpreted as a fossilized subduction interface where granitic mylonites overlie a metamorphosed accretionary wedge. We report field observations from the DBT region where multiple, several tens of meters thick foliated cataclastic networks are interlayered within the basal DBT mylonites. Petrological results and microstructural observations indicate that the various cataclasis events took place at near-peak metamorphic conditions (400–500°C, 1.1–1.3 GPa) during subduction of the Tethyan seafloor in Eocene times (42–48 Ma). Some of these networks exhibit mutual crosscutting relationships between mylonites, foliated cataclasites, and vein systems indicating mutual overprinting between brittle deformation and ductile creep. Whole-rock chemical compositions, in situ 40Ar-39Ar age data of recrystallized phengite, and Sr isotopic signatures reveal that DBT rocks also underwent multiple hydrofracturing and metasomatic events via the infiltration of fluids mainly derived from the oceanic metasediments underneath the DBT. From the rock fabrics, we infer strain rate fluctuations of several orders of magnitude beyond subduction strain rates (∼10−12 s−1) accompanied by fluctuation of supralithostatic and quasi-lithostatic fluid pressures (1 ≥ λ > 0.95). DBT brittle-plastic deformation switches highlight the diversity of deformation processes and fluid-rock interactions in the transition zone region of the subduction interface.
ISSN: 1525-2027

hal-01243526
https://hal.archives-ouvertes.fr/hal-01243526
DOI : 10.1002/2015GC005776