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Nine point mineral–whole rock Sm–Nd isochron corresponds to an age of 140 ± 32 Ma with an initial 143Nd/144Nd of 0.513835 ± 0.000053 (ENd t = + 7.4). This age is consistent with the precise Early Cretaceous age of Hauterivian (132 ± 2 to 127 ± 1.6 Ma) to Aptian (121 ± 1.4 to 112 ±1.1 Ma) for the overlying volcano-sedimentary (radiolarian bearing chert) sequences based on well-preserved radiolarian fossils (Kojima, S., Ahmad, T., Tanaka, T., Bagati, T.N., Mishra, M., Kumar, R. Islam, R., Khanna, P.P., 2001. Early Cretaceous radiolarians from the Indus suture zone, Ladakh, northern India. In: News of Osaka Micropaleontologists (NOM), Spec. Vol., 12, 257–270.) and cooling ages of 110–130 Ma based on 39Ar/40Ar for Nidar–Spontang ophiolitic rocks (Mahéo, G., Berttrand, H., Guillot, S., Villa, I. M., Keller, F., Capiez, P., 2004. The South Ladakh Ophiolites (NW Himalaya, India): an intra-oceanic tholeiitic arc origin with implications for the closure of the Neo-Tethys. Chem. Geol., 203, 273–303.). As these gabbroic and volcanic rocks are interpreted to be arc related, the new Sm–Nd age data may indicate that intra-ocean subduction in the Neo-Tethyan ocean may have started much before 140 ± 32 Ma as this date is interpreted as the age of crystallization of the arc magma. Present and published age data on the arc magmatic rocks from the Indus suture zone may collectively indicate episodic magmatism with increasing maturity of the arc from more basic (during ~ 140 ± 32 Ma) when the arc was immature through intermediate (andesitic/granodioritic) at ~ 100 Ma to more felsic (rhyolitic/dioritic) magmatism at ~ 50–45 Ma, when the Indian and the Asian plates collided. 相似文献
A long-term (1948 to 2012) trend of precipitation (annual, pre-monsoon, monsoon, and post-monsoon seasons) in Bangladesh was analyzed in different regions using both parametric and nonparametric approaches. Moreover, the possible teleconnections of precipitation (annual and monsoon) variability with El Niño/Southern Oscillation (ENSO) episode and Indian Ocean Dipole (IOD) were investigated using both average and individual (both positive and negative) values of ENSO index and IOD. Our findings suggested that for annual precipitation, a significant increasing monotonic trend was found in whole Bangladesh (4.87 mm/year), its western region (5.82 mm/year) including Rangpur (9.41 mm/year) and Khulna (4.95 mm/year), and Sylhet (10.12 mm/year) and Barisal (6.94 mm/year) from eastern region. In pre-monsoon, only Rangpur (2.88 mm/year) showed significant increasing trend, while in monsoon, whole Bangladesh (3.04 mm/year), Sylhet (7.17 mm/year), and Barisal (6.94 mm/year) showed similar trend. In post-monsoon, there was no significant trend. Our results also revealed that the precipitation (annual or monsoon) of whole Bangladesh and almost all of the spatial regions did not show any significant correlation with ENSO events, whereas the average IOD values showed significant correlation only in monsoon precipitation of western region. The individual positive IODs showed significant correlation in whole Bangladesh, western region, and its two divisions (Rajshahi and Khulna). So, in the context of Bangladesh climate, IOD has the more teleconnection to precipitation than that of ENSO. Our findings indicate that the co-occurrence of ENSO and IOD events may suppress their influence on each other.
相似文献The present study has been a pioneering effort examining the role of an annual flood as a potent stimulus inducing changes in channel geomorphology of the Mayurakshi River, India. Twenty cross sections have been considered for the measurement of various hydro-geomorphic attributes of the river in both the pre- and post-flood conditions in 2018. The study sensed an escalating trend for channel width, width/depth ratio, and wetted perimeter while the reverse was also detected for average depth, maximum depth, cross-sectional area, and hydraulic radius. For example, the width/depth ratio recorded an increase of?~?11%, and the hydraulic radius depicted a decrease of?~?8%. Furthermore, channel asymmetry, bed asymmetry and bed relief index experienced a decrease after the flood. The sudden hydraulic impulse during monsoon flood as manifested in velocity, discharge, specific stream power, Reynolds number, Froude number increases the erosivity of the fluid. Besides the hydraulic factors, bank material (massive sandbank susceptible to hydraulic action and mixed bank constituted by alternate bands of sand and silt, and vulnerable to failure by piping action) brings substantial changes in channel morphology. Moreover, anthropogenic interventions such as sand mining are found to play a significant role in channel behaviour. The role of the multiple factors driving the morphological changes of the cross sections has been unpacked using canonical component analysis.
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