The Electrical Structure Of The Mantle Beneath Tarim Area

The Tarim Basin is the most stable craton block located in the western part of the Chinese mainland.The block and its surrounding areas have experienced the opening,subduction,accretion,and multiple collisions of micro-blocks in the ancient Asian oceanic basin,and multiple periods of tectonic movement and Magma and mineralization.The Tianshan arc-shaped orogenic belt is formed on the northern edge,and the West Kunlun-Alkin arc-shaped orogenic belt is formed on the southern edge.These geological activities in the crustal area of the Tarim area are inevitable related to the nature and state of the deep mantle.However,the research on the properties of the mantle in the Tarim Basin so far mainly relies on the results of seismic imaging.In this paper,the geomagnetic sounding method,which is very sensitive to the conductivity of the mantle transition zone and the top of the lower mantle,is used to constrain the mantle properties in the Tarim area from the perspective of electrical properties.Geomagnetic sounding is a geophysical method that uses only the surface magnetic signal to study the electrical structure of the mantle.The period of the magnetic field signal can be as long as 100 days,and the effective measurement depth can reach about 1600 km underground.In this paper,a one-dimensional inversion of the geomagnetic sounding C-response obtained by the self-reference BIRRP data processing method is carried out.Affected by the duration and continuity of the data,the inversion results only give a one-dimensional conductivity model within 1200 km below the Tarim Basin.In the process of geomagnetic sounding inversion,the simulated annealing algorithm(Simulated Annealing,SA)is used,and the solution process is improved.The main improvement is reflected in the statistical analysis of the set of all high-quality solutions in the inversion process,and the expected value of the obtained solution as the approximate optimal solution of the model.The one-dimensional inversion test results of the synthetic data of geomagnetic sounding show that for the noise-free theoretical C-response,using the classic simulated annealing algorithm,the inversion can recover the optimal model with a very small difference from the real conductivity model.But when there is data noise,compared with the classic simulated annealing algorithm,the improved method can more effectively overcome the influence of data noise.When the noise level is lower than the acceptable level,reliable estimation results of multiple extremely similar model parameters can be obtained by inversion.One-dimensional simulated annealing inversion was performed on the actual data of the four geomagnetic stations HOQ,WEQ,WUS,and WMQ closest to the Tarim Basin.The results show that the overall conductivity of 410-900 km below WMQ station is relatively high;there are high-conductivity anomalies at 410-520 km and 660-900 km below WUS station;there are only high-conductivity anomalies below HOQ and WEQ stations at 660-900 km..Combining previous seismic imaging profiles and petrophysical experiment results,the analysis shows that the temperature of the upper mantle transition zone of WMQ and WUS stations is about 150 K higher than the global average temperature,and the temperature of the lower part of the mantle transition zone of WMQ station is higher than the global average temperature.The global average temperature is about 150 K,and the temperature at the top of the lower mantle at WMQ,WUS,WEQ,and HOQ stations is higher than the global average temperature by about150-300 K.Based on the calculated temperature information,we infer that there may be a high-temperature mantle plume below the Tarim Basin.Based on the temperature characteristics below the four stations,it is inferred that the hot upwelling in the mantle plume will accumulate at the top of the lower mantle blocked by the 660 km interface.The hot material penetrates the interface near the WMQ station and then accumulates at the 410 km interface.