94 research outputs found
Exhumation, crustal deformation, and thermal structure of the Nepal Himalaya derived from the inversion of thermochronological and thermobarometric data and modeling of the topography
Two endâmember kinematic models of crustal shortening across the Himalaya are
currently debated: one assumes localized thrusting along a single major thrust fault, the
Main Himalayan Thrust (MHT) with nonuniform underplating due to duplexing, and the
other advocates for outâofâsequence (OOS) thrusting in addition to thrusting along the
MHT and underplating. We assess these two models based on the modeling of
thermochronological, thermometric, and thermobarometric data from the central Nepal
Himalaya. We complement a data set compiled from the literature with 114 ^(40)Ar/^(39)Ar,
10 apatite fission track, and 5 zircon (UâTh)/He thermochronological data. The data are
predicted using a thermokinematic model (PECUBE), and the model parameters are
constrained using an inverse approach based on the Neighborhood Algorithm. The model
parameters include geometric characteristics as well as overthrusting rates, radiogenic heat
production in the High Himalayan Crystalline (HHC) sequence, the age of initiation of
the duplex or of out-of-sequence thrusting. Both models can provide a satisfactory fit to the
inverted data. However, the model with out-of-sequence thrusting implies an unrealistic
convergence rate â„30 mm yr^(â1). The out-of-sequence thrust model can be adjusted to fit the
convergence rate and the thermochronological data if the Main Central Thrust zone is
assigned a constant geometry and a dip angle of about 30° and a slip rate of <1 mm yr^(â1). In
the duplex model, the 20 mm yr^(â1) convergence rate is partitioned between an overthrusting
rate of 5.8 ± 1.4 mm yr^(â1) and an underthrusting rate of 14.2 ± 1.8 mm yr^(â1). Modern rock
uplift rates are estimated to increase from about 0.9 ± 0.31 mm yr^(â1) in the Lesser Himalaya to
3.0 ± 0.9 mm yr^(â1) at the front of the high range, 86 ± 13 km from the Main Frontal Thrust.
The effective friction coefficient is estimated to be 0.07 or smaller, and the radiogenic
heat production of HHC units is estimated to be 2.2 ± 0.1 ”Wm^(â3). The midcrustal
duplex initiated at 9.8 ± 1.7 Ma, leading to an increase of uplift rate at front of the High
Himalaya from 0.9 ± 0.31 to 3.05 ± 0.9 mm yr^(â1). We also run 3-D models by coupling
PECUBE with a landscape evolution model (CASCADE). This modeling shows that the
effect of the evolving topography can explain a fraction of the scatter observed in the data but
not all of it, suggesting that lateral variations of the kinematics of crustal deformation and
exhumation are likely. It has been argued that the steep physiographic transition at the foot of
the Greater Himalayan Sequence indicates OOS thrusting, but our results demonstrate
that the best fit duplex model derived from the thermochronological and thermobarometric
data reproduces the present morphology of the Nepal Himalaya equally well
Convergence rate across the Nepal Himalaya and interseismic coupling on the Main Himalayan Thrust: Implications for seismic hazard
We document geodetic strain across the Nepal Himalaya using GPS times series from 30 stations in Nepal and southern Tibet, in addition to previously published campaign GPS points and leveling data and determine the pattern of interseismic coupling on the Main Himalayan Thrust fault (MHT). The noise on the daily GPS positions is modeled as a combination of white and colored noise, in order to infer secular velocities at the stations with consistent uncertainties. We then locate the pole of rotation of the Indian plate in the ITRF 2005 reference frame at longitude = â 1.34° ± 3.31°, latitude = 51.4° ± 0.3° with an angular velocity of Ω = 0.5029 ± 0.0072°/Myr. The pattern of coupling on the MHT is computed on a fault dipping 10° to the north and whose strike roughly follows the arcuate shape of the Himalaya. The model indicates that the MHT is locked from the surface to a distance of approximately 100 km down dip, corresponding to a depth of 15 to 20 km. In map view, the transition zone between the locked portion of the MHT and the portion which is creeping at the long term slip rate seems to be at the most a few tens of kilometers wide and coincides with the belt of midcrustal microseismicity underneath the Himalaya. According to a previous study based on thermokinematic modeling of thermochronological and thermobarometric data, this transition seems to happen in a zone where the temperature reaches 350°C. The convergence between India and South Tibet proceeds at a rate of 17.8 ± 0.5 mm/yr in central and eastern Nepal and 20.5 ± 1 mm/yr in western Nepal. The moment deficit due to locking of the MHT in the interseismic period accrues at a rate of 6.6 ± 0.4 Ă 10^(19) Nm/yr on the MHT underneath Nepal. For comparison, the moment released by the seismicity over the past 500 years, including 14 M_W â„ 7 earthquakes with moment magnitudes up to 8.5, amounts to only 0.9 Ă 10^(19) Nm/yr, indicating a large deficit of seismic slip over that period or very infrequent large slow slip events. No large slow slip event has been observed however over the 20 years covered by geodetic measurements in the Nepal Himalaya. We discuss the magnitude and return period of M > 8 earthquakes required to balance the long term slip budget on the MHT
Seismic swarms in Tricastin, lower RhĂŽne Valley (France): review of historical and instrumental seismicity and models
New perspectives in studying active faults in metropolitan France: the âActive faults Franceâ (FACT/ATS) research axis from the Resif-Epos consortium
Track E Implementation Science, Health Systems and Economics
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138412/1/jia218443.pd
Contribution of Maternal Antiretroviral Therapy and Breastfeeding to 24-Month Survival in Human Immunodeficiency Virus-Exposed Uninfected Children: An Individual Pooled Analysis of African and Asian Studies
Background:
Increasing numbers of HIV-infected pregnant women receive antiretroviral therapy (ART) to prevent mother-to-child transmission (PMTCT). Studies suggested that HIV-exposed uninfected (HEU) children face higher mortality than HIV-unexposed children, but evidence mostly relates to the pre-ART era, breastfeeding of limited duration and considerable maternal mortality. Maternal ART and prolonged breastfeeding under cover of ART may improve survival, although this has not been reliably quantified.
Methods:
Individual data on 19,219 HEU children from 21 PMTCT trials/cohorts undertaken 1995-2015 in Africa and Asia were pooled and the association between 24-month mortality and maternal/infant factors quantified using random-effects Cox proportional hazards models accounting for between-study heterogeneity. Adjusted attributable fractions of risks computed using the predict function in the R package "frailtypack" estimate the relative contribution of risk factors to overall mortality in HEU children.
Results:
Cumulative incidence of death was 5.5% (95%CI: 5.1-5.9) by age 24 months. Low birth weight (LBW<2500g, adjusted Hazard Ratio (aHR: 2.9), no breastfeeding (aHR: 2.5) and maternal death (aHR: 11.1) were significantly associated with increased mortality. Maternal ART (aHR: 0.5) was significantly associated with lower mortality. At population level, LBW accounted for 16.2% of child deaths by 24 months, never breastfeeding for 10.8%, mother not receiving ART for 45.6%, and maternal death for 4.3%; these factors combined explained 63.6% of deaths by age 24 months.
Conclusion:
Survival of HEU children could be substantially improved if public health strategies provided all mothers living with HIV with ART and supported optimal infant feeding and care for LBW neonates
Evidence for Late Miocene retrograde monazite in the lesser Himalaya of Far-Western Nepal
DĂ©formation de l'Himalaya du NĂ©pal
La microsismicitĂ© nĂ©palaise prĂ©sente de fortes variations latĂ©rales contrĂŽlĂ©es par la topographie. Ce contrĂŽle nous amĂšne Ă estimer une valeur de la contrainte rĂ©gionale puis les variations de contraintes de Coulomb prĂ©sentes en profondeur. Le modĂšle de dĂ©formation intersismique qui en est dĂ©duit suggĂšre qu'au premier ordre le grand axe de l'ellipsoĂŻde de dĂ©formation tourne avec l'arc prĂ©sentant des azimuts similaires aux glissements induits par les sĂ©ismes et semblables Ă ceux des linĂ©ations du moyen pays himalayen. Mais, en quels termes le modĂšle du cycle sismique peut ĂȘtre extrapolĂ© pour fabriquer le long terme? Pour rĂ©pondre Ă cette question, nous nous sommes focalisĂ©s sur l'Ă©tude du moyen pays himalayen, large domaine accrĂ©tĂ© Ă la chaĂźne. Cette rĂ©gion a atteint des tempĂ©ratures comprises entre moins de 300ÊżC et 550ÊżC. Le gradient de tempĂ©rature apparent inverse mis en Ă©vidence est trĂšs Ă©levĂ©, compris entre 20 et 50ÊżC/km, impliquant tout le moyen pays supĂ©rieur. L'histoire de son exhumation, tout comme son existence mĂȘme, sont incompatibles avec une extrapolation pure et simple du modĂšle cinĂ©matique HolocĂšne. La rampe mi-crustale du systĂšme chevauchant himalayen doit migrer, permettant ainsi une accrĂ©tion par sous-placage ductile. Les variations latĂ©rales du systĂšme sont reliĂ©es aux volumes de moyen pays sous-plaquĂ©s mais aussi Ă l'entretien de la localisation de la fenĂȘtre d'accrĂ©tion. De nouvelles contraintes gĂ©o-thermochronologiques nous permettent d'Ă©voquer un modĂšle de dĂ©formation long terme. Celui-ci prĂ©sente un chevauchement de milieu chaud sur milieu froid associĂ© Ă de l'advection des isothermes par Ă©rosion, des dĂ©formations de foot et hangingwall et du sous-placage, engendrant un diachronisme d'exhumation et un cisaillement suffisant pour expliquer les forts gradients inverses de tempĂ©rature. Ce modĂšle d'Ă©volution Ă dĂ©formation de foot/hangingwall et sous placage permet de rĂ©concilier les approches aux diffĂ©rentes Ă©chelles abordĂ©es.Lateral variations of nepalese microseismicity are controlled by the topography. This control allows us to determine a regional stress field and calculate the Coulomb stress variations at depth. Our modeling suggests that the azimuth of horizontal shortening varies along the arc with the azimuths of the seismic slip and the lesser himalayan lineations. But, can we build the himalayas by extrapolating in the past present kinematics of the deformation? To address this question we have studied the lesser Himalayas accreted to the Himalayan range. Their finite thermal structure shows peak temperatures ranging between 300 and 550ÊżC describing strong inverse temperature gradients from 20 to 50ÊżC/km. Their existence and location cannot be suitable with the Holocene kinematic model but suggest that the midcrustal ramp of the main Himalayan Thrust might migrate allowing a ductile underplating. The lateral variations of the lesser Himalayas geometry can be therefore linked to the evolution of the accretionnary window. New geo-thermochronological data showing exhumation diachronisms add strong constraints to a long term deformation model. This model presents a thrusting of hot on cold medium associated with isotherm advections by erosion, deformation of foot and hangingwall and underplating, shearing, leading to the observed thermal structure and timing of exhumation. The kinematics of this theoretical accretionnary model involving underplating of the lesser himalayas is suitable with the short term models reconciling both scale descriptions.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF
Un sĂ©isme de magnitude 9 est-il possible le long de lâarc himalayen ?
National audienceLa chaĂźne himalayenne, situĂ©e sur la bordure sud du plateau tibĂ©tain, est sans doute la manifestation la plus spectaculaire de la collision entre lâInde et lâEurasie. La vitesse de convergence actuelle, dâenviron 40 mm/an entre ces deux plaques, est accommodĂ©e pour moitiĂ© au travers de lâHimalaya, le long dâune grande faille chevauchante communĂ©ment appelĂ©e le MHT (pour Main Himalayan Thrust)
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