Journal of Asian Earth Sciences: X最新文献

The Kargil Formation in the region of Ladakh (northern India) is known for its late Oligocene mammal fauna of both large mammals and rodents. New excavations in the area yielded a maxillary fragment of an insectivore with three premolars and two roots of a canine. The fossil record of the insectivores on the Indian subcontinent is as yet scanty. Based on the peculiar morphology of the last premolar, the Ladakh fossil could be identified as belonging to a new species of Erinaceinae, Ladakhechinus iugummontis n. gen. n. sp. The new find confirms the large diversity among hedgehogs in Asia during the Oligocene.

The Ladakh-Karakoram zone is a unique testing ground for understanding the geodynamic evolution of the Himalaya-Karakoram orogeny. Despite the accumulation of a large amount of strain energy that originated due to the India-Asia collision along the Karakoram fault, earthquakes of M ≥ 7.0 are considerably less in the Karakoram Fault Zone (LKZ) compared to the Himalayan seismic belt in the northwest Himalaya. The spectral analysis of microearthquakes data (1.9 < M_L < 4.3) recorded during 2009–2012 by 10 seismological stations of the LKZ reveals that the region produces low-stress drop earthquakes. Analysis of selected 51 earthquakes shows the seismic moment (Mo) within the range of 1.2 × 10¹²–4.3 × 10¹⁵ Nm with stress drop values varying from ∼ 0.06 to 64.36 bar. The low stress-drop earthquakes have significant implications in understanding seismogenesis. The earthquake cluster near the Tso-Morari Crystallines is associated with the brittle shear failure on the faults (e.g. Zildat, and Karzok Faults), and the low-stress drop of the earthquakes are explained by the partial stress drop model. The low effective fault strength due to fault weakening mechanisms in the studied segment of the Karakoram Fault zone promotes aseismic creeping patches producing low-stress drop earthquakes at the expense of generating large earthquakes. This study provides the scaling relations between important source parameters for the first time in the region which can serve as useful inputs for the assessment of earthquake hazards.

We investigate the neon isotopic composition and elemental abundance in eucrites and diogenites and compare them with the inventories from interiors of the terrestrial planets. The framework of this study is the utilization of isotopic data of eucrites and diogenites in order to constrain the trapped components in Vesta. The isotopic ratios ²⁰Ne/²²Ne and ²¹Ne/²²Ne in eucrites and diogenites are dominated by the cosmogenic component. We estimate the average abundances of ²⁰Ne trapped in diogenites as, 1.02 × 10^-8 cm³STP/g and in eucrites as, 8.02 × 10^-9 cm³STP/g, respectively. The concentrations of trapped ²⁰Ne in diogenites and eucrites are lower by an order compared to the chondrites, but similar to the interiors of the planets Earth and Mars. Preferential degassing of neon during accretionary and post-accretionary processes in Vesta and its subsequent loss to space is considered as a viable mechanism for the observed depletion in the eucrites and diogenites.

Iranian porphyry copper deposits (PCDs), resulted from the evolution of Neo–Tethys Ocean during the Mesozoic and Cenozoic, dominantly distributed in the five main tectono-magmatic belts that meanwhile the Kerman Porphyry Copper Belt (KPCB) is the most famous and important belt in the south of Iran while the Lut Block is the oldest and modern known porphyry belt in eastern Iran. PCDs of the KPCB (+Najmabad; group 1) hosting moderate to giant world-class deposits belong to the Oligocene–Miocene. On the other hand, PCDs of the Lut Block (except Najmabad; group 2) formed during the Eocene–Oligocene, are mostly subeconomic to barren. Despite the similarities in tectonic setting, host rock composition, hydrothermal alteration zones, and mineralization type, there are significant differences in geochemical characteristics (depression in MREE/HREE pattern (group 1) vs. horizontal trend (group 2), Sm/Yb greater than 2.4 (group 1) vs. < 3 (group 2)), melting conditions (Eu/Eu* = 0.8–1.2 (group 1) vs. 0.49–0.96 (group 2), La/Yb greater than 20.1 (group 1) vs. < 21 (group 2) and Sr/Y greater than 40.8 (group 1) vs. < 58.1 (group 2)), source composition (Sr/Y, Dy/Yb greater than 1.5 (group 1) vs. < 1.8 (group 2) and La/Yb ratios)), and magma generation depth (Dy/Yb and La/Yb ratios). The review of different geochemical processes shows that alongside the agents such as dehydration, magma generation depth, source composition, Eu anomaly, and oxygen fugacity, the metal source is the most prominent factor in the formation of porphyry type mineralization.

YTT eruption is viewed as a catastrophic event that resulted in deforestation, human bottlenecks and their migration across Southeast Asia and Africa. However, the question of whether the eruption had explicitly destroyed the vegetations, humans and animal populations in the Asia or, its impacts were localized and modest are still remain unanswered. To better comprehend the same, a systematic review of literature has thoroughly been carried out, specifically on the aspects of human inhabitation, vegetational scenario and animal remnants. The existing records suggest that humans were present in Africa long before the said eruption, however, their population had declined around 200 ka BP. It is also established that the humans began spreading out from the Africa in groups to other continents including India around 100 ka and witnessed dramatic growth between 60 and 80 ka. The vegetational scenario established on stable isotopes studies suggest existence of mixed C₃-C₄ vegetation in India around 100 ka which endured to sustain climatic fluctuations from Late Quaternary to Holocene period. The vertebrate remains indicate that the animal populations with genetic diversity existed in India between Late Pleistocene to Holocene. Existence of both diversified vegetation and animal remains plainly indicate that the living media was persistent and had experienced climatic changes long before and after the YTT eruption, concluding that the eruptive activity had insignificant impacts. Considering these summaries, the authors would like to point out that it would be a wrong attempt to correlate bottleneck of living populations with post-repercussions of YTT eruption as there is a long history of their endurance in various climatic conditions that occurred in the past and after the said eruption.

Inversion of Rayleigh wave dispersion curves is challenging due to its nonlinearity and multimodality. In this paper, a Simulated Annealing (SA) algorithm is applied to the nonlinear inversion of fundamental-mode Rayleigh wave group dispersion curves for shear and compressional wave velocities. In our approach, we invert thickness, velocities and densities and their vertical gradients of four layers, sediments, upper-crust, lower-crust and lithospheric mantle.

At first, to determine the efficiency and stability of the method, noise-free and noisy synthetic data sets are inverted. Results from the synthetic data demonstrate that the SA applied to the nonlinear inversion of surface wave data is interesting not only in terms of accuracy but also in terms of the convergence speed. In fact, the SA method is suitable for large-scale optimization problems, especially for those in which a desired global minimum is hidden among many local minima. In a second step, real data in SE Iran are inverted to examine the usage and robustness of the proposed approach on real surface wave data. Then, we applied 3D gravity modeling based on surface wave analysis results to obtain the density structure of each layer. The reason for using both types of data sets is that the gravity anomaly does not have a good vertical resolution and surface wave group velocities are more appropriate for placing layer limits at depth, but they are not very sensitive to density variations. Therefore, the use of gravity data increases the overall resolution of density distribution. Compared with the Shuffle Complex Evolution (SCE) method that was implemented in a previous study, we found out that the SA method is more stable and has less variability of model parameter values in successive tests. In the final step, we reapplied the SA method to invert the fundamental-mode Rayleigh wave group velocities based on the results of gravity modeling. Gravity results, such as thicknesses and densities were used to limit the search space in the SA method.

Our results show that the Moho depth across the Makran subduction zone is increasing from the Oman seafloor (24–30 km) and Makran forearc setting (34–42 km) to the Taftan-Bazman volcanic arc (47–49 km). Also, the results show high shear and compressional velocities under the Gulf of Oman, decreasing to the north of the Makran region. The density image shows an average crustal density with maximum values under the Gulf of Oman, decreasing northward to the Makran region.

The Kontum Massif, located in central Vietnam, played an essential role in the evolution of Indochina and its adjacent areas. The Van Canh granites, exposed throughout the Southern Kontum Massif, display SiO₂ contents ranging from 65.82 to 75.35 wt%, and total alkaline (Na₂O + K₂O) from 5.47 to 9.82 wt%, with A/CNK values between 0.97 and 1.08. The main rock-forming minerals of the Van Canh granites are quartz (25–30 vol%), K-feldspar (27–30 vol%), plagioclase (28–30 vol%), and biotite (3–5 vol%). The whole-rock chemical composition is characterized by Ba, Nb, Ce, P, and Ti- depletion, and the enrichment of Th, K, Pb, Nd, and Y. The ɛ_Hf(t) values of zircon grains range from −11.1 to −6.7, and the Hf model ages (T_DM2) are from 1.97 to 1.70 Ga. Their geochemical features are similar to the S-type granite, being derived from partial melting of Paleoproterozoic sources. The zircon Hf model ages and inherited zircon ages are evidence of Paleoproterozoic basement in the study region. LA-ICP-MS zircon U-Pb dating yields crystallization ages of 251–229 Ma, showing the existence of Triassic magmatism in the Kontum Massif. The Van Canh granites in the Kontum Massif, central Vietnam, provide significant evidence for the syn-/post-collision of North Vietnam-South China and Indochina blocks.

We propose a model-based Amplitude Variation with Offset (AVO) approach to address the assumption of weak elastic contrast in the linearized approximations of Aki-Richard equation. Existing approximations, especially for the Class 1 AVO response, which indicates the positive elastic contrast, deviate significantly from the Zoeppritz equation in presence of large elastic contrast. Results of the proposed approach show that it can minimize the deviation from the Zoeppritz equation, improve AVO attributes, and is capable of providing the desired attribute for characterizing a reservoir. The method starts with two matrices. One matrix is of the simulated rock properties termed as the rock-property-matrix and the other of the Zoeppritz AVO responses for those rock properties termed as the response-matrix. A model-based AVO equation or the basis-function-matrix is computed utilizing the rock-property-matrix and the response-matrix. The inverse of the basis-function-matrix is applied to the real data to get the AVO attributes from this approach. The conventional AVO (Aki-Richards) and model-based AVO attributes are compared in a class 1 AVO environment from the offshore east coast of India. The curvature attribute based on model-based AVO shows significant improvement. The synthetic-seismic correlation of the curvature attribute improves from −0.03 to 0.9 while synthetic-seismic correlation of the gradient attribute improves from 0.5 to 0.77. The improved AVO attribute volumes add significant values in the reservoir characterization.

The evolution of the Quaternary landforms in the Champawat area of the outer Kumaun Lesser Himalaya is reconstructed from the paleolake sections and the geometry of the landscape. Conjugate sets of normal faults are observed in both the bedrocks and Quaternary deposits at Banlek. The most prominent normal faults are those striking NE-SW and NW-SE and most of them are steeply dipping faults while some are vertical faults. Sediments akin to the lacustrine deposits are observed at four sites; while the contact between the bedrock and the lake sediments is observed at only one site in the peripheral side. The exposed sediments thicknesses vary from 8.30 m to 4.8 m and consist of an alternation of black carbonaceous mud and sandy horizons indicating different depositional regimes. The total thickness of the lake sediments appears to be more than 75 m as deduced from elevational differences between the exposed sites. From Site 1, two OSL samples collected from 2.5 m and 8 m from the present ground surface, were analyzed which give OSL ages of 16 ka and 17 ka, respectively; while one sample collected from Site 4 gives an OSL age of 13 ka. A total of 3653 lineaments were mapped encompassing 2,277.8 sq km from the eastern Kumaun Himalaya. Maximum of the lineaments trend ENE-WSW or almost E-W (∼46.5%); the major principal stress is deduced to be in NE-SW direction. Lake, mature topography and gentle stream gradient in the Champawat area are the result of highly weathered bedrocks and development of the normal faults.

Since the 2001 M_w 7.7 Bhuj earthquake in Gujarat, western India, a number of small-to-moderate earthquakes have occurred to the east of the Bhuj rupture, near the town of Bhachau. In this paper, we present an overview of seismic monitoring in the region, and analysis of a M_w 5.3 earthquake that occurred near Bhachau on 14 June 2020. Moment tensor analysis of regional broadband data reveals a reverse mechanism with a small strike-slip component, generally similar to that of the 2001 Bhuj earthquake. A total of 20 aftershocks with magnitudes ranging from 2.0 to 4.6 were recorded within the first 10 days of the sequence. After that, this region recorded 129 tremors of magnitude 2.0–4.2 until June 2021. We conclude that this sequence occurred on the South Wagad fault (SWF)-Kachchh Mainland fault (KMF) system, a complex zone comprising many separate mapped faults. A recent analysis of Global Positioning System (GPS) data reveals that the area is under broad compressive stress, with a maximum strain of 5 nano-strain/year. The occurrence of the 2020 Bhachau earthquake underscores concerns for continuing seismic hazard in the region.