Preface Purnachandra Rao, N.; Bürgmann, Roland; Mugnier, Jean-Louis ...
Journal of Asian earth sciences,
12/2017, Letnik:
133, Številka:
C
Journal Article
Recenzirano
The 25 April 2015 Gorkha, Nepal earthquake of magnitude 7.8 devastated parts of Nepal and northern India with a death toll of more than 8000. Here, several aftershocks of moderate size were recorded, ...including one of M7.2 on 12 May 2015 on the eastern periphery of the main rupture.
A geomorphological map was constructed, based on satellite imagery of the recent geomorphological evolution of the delta of the Seman and the Vjosa rivers, Albania. Results of the analysis of data ...collected from unpublished geological profiles carried out by Russian and Albanian geologists indicate that concentrations of heavy minerals are clearly correlated with palaeo-shorelines, particularly in six boreholes drilled in the vicinity of the ancient city of Apollonia (7th century BC–7th century AD). Study was mainly based on sedimentological, pollen and diatom analysis. About twenty samples of wood or peat debris have been
14C dated (AMS), all between 3050
±
50 BP and 340
±
50 BP. Based on this evidence, a new palaeogeographic reconstruction of the shoreline over the last 3500 years is proposed. Two-thirds of the deltaic progradation occurred over the last 500 years. As a hypothesis, climatic fluctuations such as the Little Ice Age and socioeconomic fluctuations could play a major role in the acceleration of the process, in spite of an important neotectonic subsidence effect.
ABSTRACT
40Ar–39Ar dating of detrital white micas, petrography and heavy mineral analysis and whole‐rock geochemistry has been applied to three time‐equivalent sections through the Siwalik Group ...molasse in SW Nepal Tinau Khola section (12–6 Ma), Surai Khola section (12–1 Ma) and Karnali section (16–5 Ma). 40Ar–39Ar ages from 1415 single detrital white micas show a peak of ages between 20 and 15 Ma for all the three sections, corresponding to the period of most extensive exhumation of the Greater Himalaya. Lag times of less than 5 Myr persist until 10 Ma, indicating Greater Himalayan exhumation rates of up to 2.6 mm year−1, using one‐dimensional thermal modelling. There are few micas younger than 12 Ma, no lag times of less than 6 Myr after 10 Ma and whole‐rock geochemistry and petrography show a significant provenance change at 12 Ma indicating erosion from the Lesser Himalaya at this time. These changes suggest a switch in the dynamics of the orogen that took place during the 12–10 Ma period whereby most strain began to be accommodated by structures within the Lesser Himalaya as opposed to the Greater Himalaya. Consistent data from all three Siwalik sections suggest a lateral continuity in tectonic evolution for the central Himalayas.
Cold climate carbonates can be used as paleoclimatic proxies. The mineralogy and isotopic composition of subglacially precipitated carbonate crusts (SPCCs) provide insights into the subglacial ...conditions and processes occurring at the meltwater-basement rock interface of glaciers. This study documents such crusts discovered on the lee side of a gneissic roche moutonnée at the terminus of the Bossons glacier in the Mont Blanc Massif area (France). The geological context and mineralogical investigations suggest that the Ca used for the precipitation of large crystals of radial fibrous sparite observed in these crusts originated from subglacial chemical weathering of Ca-bearing minerals of the local bedrock (plagioclase and amphibole). Measurements of the carbon and oxygen isotope compositions in the crusts indicate precipitation at, or near to, equilibrium with the basal meltwater under open system conditions during refreezing processes. The homogeneous and low carbonate δ13C values (ca. −11.3‰) imply a large contribution of soil organic carbon to the Bossons subglacial meltwater carbon reservoir at the time of deposition. In addition, organic remains trapped within the SPCCs give an age of deposition around 6,500 years cal BP suggesting that the Mid-Holocene climatic and pedological optima are archived in the Bossons glacier carbonate crusts.
The postseismic deformation consecutive to the April 25, 2015 Gorkha earthquake (Mw 7.9) is estimated in this paper based on a cGNSS network installed prior to the earthquake and supplemented by 6 ...cGNSS stations installed after the main shock. Postseismic displacement are obtained from daily time series corrected for interseismic deformation and seasonal variations. The maximum postseismic displacement is found north of the rupture area, where locally it reached 100 mm between the date of the earthquake and late 2016. The postseismic deformation affects the northern part of the rupture area but not the southern part, along the southern part of the Main Himalayan Thrust (MHT). Three hypotheses for the mechanisms controlling postseismic deformation are tested through numerical simulations of the postseismic time series: (i) viscous relaxation, (ii) afterslip, or (iii) a combination of these two mechanisms. We can exclude postseismic deformation controlled by viscous relaxation of a thick deformation zone along the northern and lower flat of the MHT. However, it is impossible to discriminate between postseismic deformation controlled by either afterslip along the MHT (northern part of the rupture zone, crustal ramp, and lower flat of the MHT) or a combination of afterslip along the MHT (northern part of the rupture zone, crustal ramp) and viscous relaxation controlled by a thin (∼3–4 km thick) low-viscosity body centered on the lower flat of the MHT. The occurrence of afterslip along the northern part of the upper flat of the MHT and its longitudinal variations have been established thanks to the densification of GNSS network by our team presented in this paper.
Within many fold and thrust belts, preservation of bed thickness is generally assumed during deformation. A three-dimensional (3D) reconstruction method is proposed from outcrop structural data. This ...technique accounts for along-strike thickness variations. By a series of 3D spatial projections of dip and strike data, the thickness of strata from any point at the ground surface to reference horizons on stratigraphic columns can be calculated across-strike and projected along the poles of the associated strata, providing sets of dots in space, which represent folded horizons.
The technique is applied to the Baisahi antiform, located on the frontal Subhimalayan thrust. An along-strike deformation gradient is evidenced by two-dimensional cross-section balancing, which shows a passive-roof duplex structure. The 3D geometry of the roof is reconstructed, allowing other approaches such as unfolding to be done. The residual strain field induced by restoration shows distortion features, which are interpreted as internal strain due to the propagation of the fault.
Deciphering the complex interplays between climate, uplift and erosion is not straightforward and estimating present-day erosion rates can provide useful insights. Glaciers are thought to be powerful ...erosional agents, but most published 'glacial' erosion rates combine periglacial, subglacial and proglacial erosion processes. Within a glaciated catchment, sediments found in subglacial streams originate either from glacial erosion of substratum or from the rock walls above the glacier that contribute to the supraglacial load. Terrestrial cosmogenic nuclides (TCN) are produced by interactions between cosmic ray particles and element targets at the surface of the Earth, but their concentration becomes negligible under 15 m of ice. Measuring TCN concentrations in quartz sand sampled in subglacial streams and in supraglacial channels is statistically compliant with stochastic processes (e.g. rockfalls) and may be used to discriminate subglacial and periglacial erosion. Results for two subglacial streams of the Bossons glacier (Mont Blanc massif, France) show that the proportion of sediments originating from glacially eroded bedrock is not constant: it varies from 50% to 90% (n = 6). The difference between the two streams is probably linked to the presence or absence of supraglacial channels and sinkholes, which are common features of alpine glaciers. Therefore, most of the published mean catchment glacial erosion rates should not be directly interpreted as subglacial erosion rates. In the case of catchments with efficient periglacial erosion and particularly rockfalls, the proportion of sediments in the subglacial stream originating from the supraglacial load could be considerable and the subglacial erosion rate overestimated. Here, we estimate warm-based subglacial and periglacial erosion rates to be of the same order of magnitude: 0.39 plus or minus 0.33 and 0.29 plus or minus 0.17 mm a super(-1), respectively. Copyright copy 2015John Wiley & Sons, Ltd.
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