Elastic waves are affected by viscoelasticity during the propagation through the Earth, resulting in energy attenuation and phase distortion, in turn resulting in low seismic imaging accuracy. Therefore, viscoelasticity should be considered in seismic migration imaging. We propose a Q compensated multi-component elastic Gaussian beam migration (Q-EGBM) method to (1) separate the elastic-wave data into longitudinal (P) and transverse (S) waves to perform PP-wave and PS-wave imaging; (2) recover the amplitude loss caused by attenuation; (3) correct phase distortions caused by dispersion; (4) improve the resolution of migration imaging. In this paper, to accomplish (2), (3), and (4), we derive complex-valued traveltimes in viscoelastic media. The results of numerical experiments using a simple five-layer model and a sophisticated BP gas model show that the method presented here has significant advantages in recovering energy decay and correcting phase distortion, as well as significantly improving imaging resolution.
The South Mid-Atlantic Ridge (SMAR), the most typical slow-spreading ridge on the Earth, is a significant area to study the oceanic crust-mantle evolution. We used gravity data and the S-wave velocity model to derive a 3D density model of the upper-mantle beneath the South Atlantic Ocean to better understand the relationship between ridges, hotspots, and large igneous provinces (LIPs). Any density variations are related to thermal and compositional changes in the upper-mantle. The segment near the Agulhas-Falkland (AF) Fracture Zone (FZ) is characterized by low-density but inconspicuous low-velocity anomalies reflecting the depleted composition. Its composition is different from the northern part of the SMAR. A low-density layer that extends 400 km from the ridge to the Discovery hotspot and that disappears near the AF FZ, is also present. The higher degree of mantle melting beneath the segment near the AF FZ is most probably influenced by the residual plume materials from the Discovery hotspot. Deep low densities beneath the Rio Grande Rise (RGR) and the Walvis Ridge (WR) indicate a higher degree of melting and their formation is probably influenced by the Tristan da Cunha plume. When the RGR and the WR are driven away from the plume, their original hot materials cool down and present as cold and dense uppermost mantle deposits.
The northern margin of North China Block (NCB) is a world-class gold metallogenic province with >1000 t gold, where has undergone multiphase tecto-magmatic overprinting since the Archean. Yet the genesis of the mineralization is still in doubt due to poor geochronological constraints using methods such as Ar-Ar and Rb-Sr dating. The Zhongshangou Au-Te deposit is located in the central part of the NCB gold metallogenic province, where it is structurally controlled and hosted by an amphibolite quartz monzonite phase of the Shuiquangou Alkaline Complex (SAC). Hydrothermal alteration at the deposit is characterised by an intense K-silicate assemblage with variable amounts of Fe-Mn carbonate, which is commonly observed in alkaline plutons associated with Au-Te deposits. Pyrite and chalcopyrite associated with the Au-Te mineralization contain very low common Os and yield model ages ranging from ca. 384 to 373 Ma, with a precise Re-Os isochron age of 379 ± 5 Ma (2σ, MSWD = 0.92). Analyses of molybdenite associated with the Au-Te mineralization yield precise model ages ranging from ca. 379 to 375 Ma, with a Re-Os isochron age of 379 ± 4 Ma (2σ, MSWD = 0.3). The Re-Os geochronology thus provides a reliable constraint of ca. 379 Ma for the mineralization, which is coeval with the syenite in the SAC. The Re-Os geochronology of the pyrite and chalcopyrite also yield a very low initial 187Os/188Os value of 0.22 ± 0.11, which indicates a significant source from the mantle material. Our evidence coupled with previous published data in the literature show that the Zhongshangou deposit is an alkaline pluton-hosted epithermal system related to the emplacement of the SAC in an extensional setting. The Re-Os geochronology using the low-level highly radiogenic sulfides has constrained the timing of the gold metallogeny at the northern margin of NCB and has opened new perspectives for gold exploration in the metallogenic province.
The Curie point depth (CPD) has been established to enhance the determination of magnetic sources average bottom depth. In this study, the CPD and heat flow regime of the Eratosthenes Seamount in the Eastern Mediterranean were investigated in order to understand the relationship between them. When CPD and heat flow values are determined together, we can understand the types and processes of geothermal resources formation. CPD and heat flow were obtained by applying the spectral analysis method to magnetic data, which was obtained from the Earth Magnetic Anomaly Grid (EMAG2). The result shows that CPD is approximately 22 km across the Seamount and is approximately 9 km in the northern part of the Eratosthenes Seamount near Cyprus. The heat flow regime manifested from CPD is about 95 mW/m2 in the investigated region, where the average thermal conductivity value (k) is considered to be 2.5 W/m °C. The heat flow regime is characterized by an increase in the northward part of the Eratosthenes Seamount and decreases towards the south.
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