Gravity And Magnetic Anomalies In The Sichuan Basin,China:Implications For Deep Structures

Sichuan Basin, located in the northwestern area of the Upper Yangtze Block in the western South China Plate, is a superimposed basin with different basin types inherited or overlaid in different periods. Elucidating the basin’s deep tectonics including the basement structures and Moho is critical to understand the fundamental issues in geodynamics of this region.Based on the detailed analysis of magnetic and gravity data, together with seismological and geothermal data, we have decomposed the gravity and magnetic anomalies into four and three orders, respectively, with multi-scale wavelet decomposition. The apparent depths of causative sources were then assessed by logarithmic power spectrum analysis of each order. Low-order wavelet detail anomalies are used to study the basin’s basement structure such as major faults, the basement lithology, uplifts and depressions. The 4th order gravity approximation anomaly (A4G) was used for the inversion of Moho discontinuity using the Parker’s method.Nineteen major faults are identified and characterized after the characteristics of gravity gradient belts were enhanced mainly using the horizontal total gradient module and its first derivative. However, there is still some controversy over some faults’orientation or even their existence. Some faults (F1-F4 and F6) are clearly indicated by the gravity and magnetic anomalies that are supported by other geophysical and geological evidence. However, some faults cannot be marked precisely because there is not enough evidence (F5, F7-F8, F11, F13, F17-F19)or even to be determined regarding their existence (F9-F10, F12, F14-F16). The Parker interface inversion based on the residual gravity anomaly of Bouguer gravity data projected upward by 5 km and gravity data projected upward by 15 km reveals that the basement of the SB typically consists of 3 main uplifts and 10 depressions. Depths of most basement depressions are nearly 6.5 km with the deepest up to nearly 11 km near the Yaan area in the SB. While depths of most basement uplifts are about 6 km, the shallowest is about 3.5 km situated near the Dazu area in the SB.In addition, inversion based on the 4th order approximation of gravity anomalies shows that the depth of Moho is between 39.5 and 47 km and the uplift area is located in the southern part of the basin. The average depth of Moho is about 40-42 km and and the northern part is 44-46 km.Besides, the basin is divided into six tectonic units by the correlation coefficient R between the vertical derivative of the gravity field and reduction to pole transformation (RTP) of magnetic field after the GMA are extended upward to 10km. The six areas are Northwestern Guangyuan-Bazhong GMA(Ⅰ), Western Mianyang-Yaan GMA(Ⅱ), Northeastern Dianjiang-Fuling GMA(Ⅲ), Southeastern Chongqing-Luzhou GMA (Ⅳ), Central Dazhou-Yuezhi GMA(V), Southwestern Ziyang-Yibin GMA(VI). Three basement lithology anomalies can be discerned in SB.The basement stratum of the southeastern part:Banxi group and granites, the basement stratum of the center part:Kangding group, granites, intermediate-basic and basic igneous rocks, the basement stratum of the western part:Huangshihe group and basic igneous rocks.