## 3 Results  
### Bouguer Gravity Anomalies at Different Depths  
Performing several wavelet decomposition and field source depth estimations for the Bouguer gravity anomaly of the study area, we conclude that the 7 th-order wavelet details correspond to a depth of 136 km. This depth is the maximum depth of the area between the lithosphere and the upper mantle of the Tibetan Plateau. Thus, to reveal the characteristics of the gravity field and infer the corresponding tectonic features at different depths in the study area, we performed a 7 th-order wavelet decomposition on the Bouguer gravity anomalies using a biorthogonal 3.1 wavelet (bior3.1) and then evaluated the field source depth using the radial power spectrum algorithm. Further, we established the spherical multiquadric (MQ) model [39] on each wavelet detail and calculated the total horizontal derivative (THD), which has a higher accuracy than the vertical derivative (VD) method when dealing with the potential field on grid points [40]. The wavelet decompositions of Bouguer gravity anomalies are shown in Figure 2, and the corresponding field source depths are listed in Table 1.  
The 1 st- to 5 th-order wavelet details clearly reflect the tectonic boundaries of the orogenic zone in Figure 2a-e. The Bouguer gravity anomalies of the Pamir Plateau, Tian Shan range and tectonic boundaries of the Tibetan Plateau are very complex. Positive and negative anomalies are alternately distributed and reflect evident features of the tectonic boundaries that separate the Himalayan-Tibetan orogenic zone from its surrounding structures. The complexity is related to the uneven density of the upper crust. As the depth  
Figure 1: Major active faults related to the Himalayan-Tibetan orogenic zone. The faults’ names are listed here. Altyn Tagh fault (ATF), Apalong fault (ApIF), Chaman fault (CF), Gaoligong fault (GlgF), Herat fault (HF), Haiyuan fault (HyF), Jiali fault (JIF), Talas-Fergana fault (TF), Karakakash fault (KKF), Karakorum fault (KF), Kashgar-Yecheng transfer system (KYS), Nujiang fault (NjF), Nantinghe fault (NthF), Sagaing fault (SF), Sarez-Karakul fault (SKF), Xianshuie fault (XshF); Burmese fold belt (BFB), Longmenshan thrust (LmsT), Main Central thrust (MCT), Main Frontal thrust (MFT), Pamir thrust system (PTS), Qilian Nanshan thrust (QNT), Southern Tian Shan thrust (STT); normal faults and extensional structures: Gurla-Mandhata detachment (GM), Sangri-Nacuo fault (SNF), Yadong-Gulu fault (YGF); Anyimaqen-Kunlun-Muztagh suture zone (AMS), Bangonghu-Nujiang suture zone (BNS), Jinshaiang suture zone (JS), Rushan-Pshart suture zone (RE), Shyok suture zone (SSZ), Yalu-Zangbo suture zone (YZYS). of the field source increases, under intense tectonic activity, widespread metamorphism occurs in the sedimentary structures. Therefore, the gradient belts of Bouguer gravity anomalies connect into larger pieces, and its complexity is reduced. At a depth of 17 km in Figure 2e, the anomalies show a regular distribution between positive values and negative values, which is obviously related to the spatial distributions of major active faults exposed at the Earth's surface. The elongated belts of high positive gravity anomalies are distributed along the Main Frontal thrust (MFT), Allyn Tagh fault (ATF) and Karakash fault (KKF) on the southern and northern margins of Tibet. The spatial distribution characteristics of Bouguer gravity anomalies change significantly when the depth increases. The negative anomalies in the bottom of the lithosphere are mostly distributed at the eastern and western ends of the Tibetan Plateau in Figure 2f, such as the MFT, KKF, ATF, southern segment of the Yadong-Gulu fault (YGF), Jiali fault (JIF), the Nujiang fault (NjF) and Burmese fold belt (BFB). Of these, the negative anomalies along YGF, southern BFB and NjF extend from the shallow crust to the deep crust. The volcanic rocks in this depth could not form such spatial patterns. It indicates that the negative anomalies mostly result from crustal stacking or folding of the Mohorovicic discontinuity.