Characteristics Of Chinese Continental Curie Point Isotherm

After 50 years of airborne geophysical survey, Chinese continent has achieved full coverage of airborne magnetic survey. The new series of aeromagnetic maps have been completely compiled in 2012. Based on the latest compilation of 1: 5 million “Chinese continental aeromagnetic ΔT reduction to the pole contour map”, using airborne geophysical data processing software GEOPROBE V2.0 which is independent research and development in our country, we estimated Chinese continental curie point depth by power spectrum method. During the calculation, the 1: 1 million separating aeromagnetic map data was used to grid sampling(distance 5 × 5km) and conducted a zoned reduction to the pole. Using a side length of 100 × 100 km “window” slide along the direction of the survey line and measurement points with 50 km overlapped each time. The calculation obtained 8004 Curie point depth values. Eventually, we completely compiled the Chinese continental Curie point isotherm map. The map fills the gap in the western Tibetan Plateau and first fully shows the characteristics of Chinese continental Curie point isotherm.By comparison with the results of previous studies, the Curie point depth can better reflect the tectonic characteristics of China. Uplift of Curie point isotherm usually reflects the tectonic unit boundaries and deep faults, while the depression zone usually indicates stable block. Studies have shown that Curie depth is 28-45 km inside the stable blocks, with the main features of depression, such as the Tarim Basin, the Yangtze Basin area, North China basin area, Junggar Basin, Qaidam Basin, Songliao Basin, Erlian Basin, Bayan- Wuwei- Chaoshui basin, Pearl River Estuary- southeast Joan basin, Hoh Xil- Bayan Har Depression area. The Curie depth of North China basin area show big difference with Tarim blocks and Yangtze block, relatively shallow and change dramatically, which may be related to the North China Craton suffered a complex post-transformation, resulting in the asthenosphere upwelling and lithospheric thinning. Hoh Xil- Bayan Har block is a NWW trending giant depression area lie within western Tibetan Plauteau, because the development of a large area of the Triassic sedimentary strata and less magmatism, its Triassic sediment thickness of up to about 10 km. The stable blocks coincidently have Moho uplift and Curie point isotherm depression. Conversely, the Curie point isotherm of active orogenic belt is characterized by uplift with the depth of 18-26 km, such as the Northeast mountain area, Northwest mountain area, Qinling- Dabie Mountain area, West Kunlun- Tibet-Sanjiang- Kangdian area, the southeast coastal areas and so on. This reflects the geothermal gradient difference caused by tectonic and magmatic activity. Curie point isotherm usually shallower than Moho, but sometimes it will be embedded in the upper mantle, indicating that the top of upper mantle is magnetic in some areas. For example, the Curie point isotherm of North China block would extend to the upper mantle 1-6 km in Kalaqinqi – Suixian geoscience transects.Combined with tectonic features, the Curie point isotherm is subdivided. First – order unit use the seven tectonic units: Tianshan- Xingmeng orogenic system, Tibet-Sanjiang orogenic system, Wuyi- Yunkai orogenic system, Qin – Qi- Kun orogenic system, North China continental block domain, Tarim continental block domain and the Yangtze continental block domain. Second – order and third – order units are named by feature of Curie point isotherm. Tianshan- Xingmeng orogenic system, Tibet- Sanjiang orogenic system, Wuyi- Yunkai orogenic system and Qin – Qi- Kun orogenic system are charaterizaed by uplift of Curie point isotherm. However, some local stable blocks in orogenic system are depression, as Junggar – Tuha, Songliao and Jiamusi block in Tianshan- Xingmeng orogenic system; the Qaidam block in Qin- Qi- Kun orogenic system; Bayan Har block in Tibet- Sanjiang orogenic system. North China continental block domain, Tarim continental block domain and the Yangtze continental block domain are represented by depression.816 heat flow data are collected to study the relationship between Curie point isotherm and geothermal gradient and heat flow. The results show that there is a nonlinear relationship between the depth and heat flow, however, when the depth is greater than 30 km, the heat flow are less than 100 m W/m2. This indicates that the area with deep Curie depth always have low activity of heat flow. Also, with the increase of heat flow, the high values gather to the east coast of China, southern Tibet-Sanjiang region, Qinling- Dabie area, Liaodong where are characterized by uplift of Curie point isotherm. These areas lie within the boundary of different tectonic unit are usually stress-concentrated. Through comparative study of geothermal resources and Curie depth, we found large and medium-sized geothermal fields mainly lie in southern Tibet, western Yunnan, Kangdian, southeast coast, Fenwei graben, Yanshan, Changbaishan. These places are mostly located in Curie point isotherm uplift or gradient zone where show intense tectonic activity. The collection of developed geothermal resources indicates that 67.4% of them are distributed in Curie point isotherm uplift or gradient zone with the depth of 18-27 km. Therefore, the Curie depth can be used to geothermal resources prospecting. By study the typical geothermal field, the results suggest geothermal resources have a very close relation with the Mesozoic-Cenozoic magmatic rocks which control the distribution of geothermal resources as a heat source. Combined Curie depth, magmatic rock and structure, nine prospective areas of geothermal resources are delineated in Chinese continent.Curie point isotherm can reflect the thermal anomaly caused by the plate boundaries and active deep faults as the thermaldynamic surface of the crust. Many scholars have written that the Curie point isotherm uplift and gradient is of high ranking site of the earthquake. We collected AD 1970 to AD 2014 the official earthquake catalog on the China Earthquake Networks Center(CENC), and extracted the semsic data with magnitude higher than four(Ms≥4) to study the relationship between Curie depth and earthquark. The results show that the earthquake epicenter are distributed mainly in transition zone of “hot and cold” block or “cold – cold” block, which are represented by Curie point isotherm gradient zones or “uplift in depression”. With the increased strength of the earthquake, the epicenter of the earthquake are contracted in the Tibetan Plateau where is the highest and widest Curie point isotherm uplift area in China. This may indicate that crustal heat activity is very intense in this area. Statistics show that the earthquake epicenter mainly in the uplift of Curie point isotherm and transition zone, these areas are concentrated more than 60% of the earthquake, depression area is relatively stable, low seismic activity. Active faults in the Tibetan Plateau and circum Pacific of eastern China are well developed, and often several groups of faults crossed. The feature of Curie depth is uplift and gradient. The remaining active faults distributed in boundaries of stable blocks, such as the peripheral edge of Junggar block, Tarim block, Yangtze block and Ordos block. These areas are characterized by a gradient zone. In peak ground acceleration map, the activity of Chinese continent is devided to two parts by the boundary of Helan Mountain- Liupan Mountain- Longmen Mountain – Wumeng Mountain. The east of China is more active than the west. The high value area of peak ground acceleration is mostly distributed in the west. The areas with peak ground acceleration greater than 0.3 g are mainly distributed in the uplift and the gradient zone of Curie point isotherm.