5.1 The results of potential data separation
Figure 4 shows the results of WMSD, and the depth was calculated using the radially averaged logarithm power spectrum. In the wavelet decompositions, the regional component of the 4th terms of wavelet decomposition corresponds to a depth of 55 km, the average depth to the Moho (Shen et al., 2016; Gao et al., 2020). Therefore, its regional component can be believed to be caused by Moho. The distribution characteristics correspond to the Moho undulation. Therefore, we can use the result to calculate the depth of Moho, and the sum of 1st, 2nd, 3rd, and 4th detail features of the wavelet decomposition can be used as the residual anomaly. The depths of 1st + 2nd, 3rd, and 4th wavelet decomposition are 5 km, 20 km, and 45 km.
Compared with the original gravity anomaly, the characteristics of positive and negative local field alternation are more prominent. The overall characteristics of low in the southwest and high in the northeast were removed, indicating that the influence of the fluctuation characteristics of the deep Moho is removed, and the crustal density structure can be obtained. In the interior of the basin, there are large areas of negative gravity anomalies. There are also apparent negative anomalies near the Wahongshan and Xinjie faults. Overall, there are numerous low-density anomaly field sources in the crust.
The gradient zone with sharp changes in NNW direction between 101 ° E and 102 ° E in the east of the working area appears to be caused by the Xinjie fault. The fault is ~ 20 km wide and has a prominent semi-flower structure (Zhang et al., 2020a). It is the western boundary of the Guide Basin and the boundary between the Guide and Gonghe Basins. It mainly exposes banded Indosinian granites formed by sliding melting carried by the fault (Zhang et al., 2007). Near the Xinjie fault, it is inferred that these hot springs correspond to this low-density gravity anomaly zone, and abundant hot springs are exposed nearby.
The NW trending gradient zone with abnormally sharp changes distributed in the west is caused by hidden faults such as the Guinan Nanshan fault, Wahongshan-Wenquan fault, and Wayuxiangka-Guinan fault. Intermittent strip or elliptical low-density gravity anomalies correspond to negative landforms such as intermountain basins and valleys. Generally, it is inferred that NW trending structures control the distribution of low-density bodies near the Gonghe Basin in the west, and the distribution of low-density bodies near the Guide basin in the east is controlled by NNW trending structures.
Similarly, we use the WMSD to separate the residual and regional anomalies of magnetic data. Firstly, residual results of the 1st, 2nd, 3rd, and 4th (Fig. 5) were obtained, and the equivalent depths were calculated using a radially averaged logarithm power spectrum. To compare with geological information, the depth of 1st + 2nd, 3rd, and 4th wavelet decomposition is 3 km, 9 km, and 15 km. Then the sum of the 1st, 2nd, 3rd, and 4th detail features of the wavelet decomposition can be used as the residual anomaly because the magnetism disappears under the Curie point depth. The residual magnetic anomaly obtained after removing the regional field can be regarded as the distribution of magnetic anomaly generated by the substances above the Curie point depth. The main distribution directions of magnetic anomaly strips are NNW and NW, which are consistent with the main distribution directions of gravity anomalies.
The main distribution direction in the western Gonghe Basin is NW, and NNW in the eastern Guide Basin. The weak magnetism is distributed in the Gonghe-Guide Basin. The monotonous and unvaried magnetic fields are generated by the overlying sedimentary rocks. Around the basin, positive and negative magnetic anomalies are distributed alternately in strips, which are close to the strike of main faults. The maximum value is 350 nT, which may be caused by granite intruding along the fault and its alteration zone.
The magnetic anomaly is more prominent in the boundary characteristics of the basin, and the basin is controlled by NNW and NW deep faults. This may be related to the Gonghe-Guide Basin being a faulted basin controlled by late-stage faults and magmatic intrusions. Thick sedimentary layers accumulated in the basin, showing low magnetic anomalies. The late tectonic movement allowed the granite magma to intrude along the NNW and NW faults. These granite intrusions and the surrounding hydrothermal alteration often show high magnetic anomalies. In addition, the positive anomalies scattered in the basin may be caused by concealed granite.
5.2 The inversion results
According to the results of WMSD, the field value variation characteristics at different depths were obtained, and the depth information was estimated. The average depth of the Moho surface is 55 km. Therefore, D4 is a regional field, and we have obtained the residual field of the crust by using the sum of the sum of 1st, 2nd, 3rd, and 4th, as shown in Fig. 6. Based on this information, the inversion of crustal structure density was calculated by using the ICDEXP algorithm.
During the inversion calculation, for the satellite gravity data, the maximum depth was set as 50 km, the structure index was 3, the number of iterations was 20, and the upper and lower density limits were 1 and − 1. The three-dimensional structure result and vertical slices diagram are shown in Fig. 7. The vertical slices AA’ is a profile located outside the basin. The densities of these slices are minor, and the negative data correspond to the Wahongshan fault.
The vertical slices BB’ and CC’ are located near Tanggemu and Gonghe, located in the west and east basin, respectively. The negative density data is prominent. In profile BB’, the two negative densities correspond to the west Gonghe Basin and the Wahongshan fault. The vertical slice CC’ goes through the Gonghe Basin and near the town of Guinan. Two negative densities are located near the town of Gonghe and Guinan, which may be caused by deep partial melting. In the three-dimensional diagram and vertical section, the depth range of the density anomaly is clearly displayed, concentrated at a depth ranging between 15 ~ 35km. Cross section DD’ passes through the town of Guide. The negative densities are also very obvious in this profile. The burial depth is 15 km ~ 35 km, corresponding to the Xinjie fault and deep partial melting near the town of Guide.
For display convenience, we intercepted the results of different depths (5, 10, 15, 20, 25, 30, 35 and 40 km), shown in Fig. 8. The inversion results of different depths varied between methods. The sedimentary layers at shallow depths (up to 5 km) have little density fluctuation, consistent with the previous conclusion that the Cenozoic sedimentary thickness is greater than 5km (Sun et al., 2011). The low velocity bodies from near surface to a depth of about 5–10 km are Quaternary loose or weakly diagenetic strata, and there is no strong density anomaly in the shallow part. At 10 km and on, the high- and low-density anomalies are patently staged by stage. In addition to the shallow sedimentary layer with little density fluctuation, the resolution of satellite gravity cannot obtain shallow details. The distribution of density anomalies changes significantly from 15 km to 35 km. At depths greater than 35 km, the difference between high and low density anomalies begins to decrease. This shows that the density anomalies are mainly concentrated in the range of 15 ~ 35km.
At a depth of 15km ~ 35km, the low-density anomaly is mainly distributed in the corresponding main fault zones, and the difference between high and low density anomalies is small. It is concentrated in the Wahongshan fault on the west side of the basin, the Xinjie Fault, and the Duohemao fault on the east side. The Xinjie fault corresponds well with known hot spots, so it can be inferred that the geothermal energy in this area is mainly concentrated in these low-density bodies and their vicinity.
At the depths > 35km, the density distribution characteristic changes significantly. At a depth of 40km near the Xinjie fault zone, there are apparent low-density anomalies, and the low-density anomalies at other locations tend to disappear. It is speculated that the undercut depth of the Xinjie fault zone changes from south to north. This phenomenon can explain why the temperature increases successively from south to north. Generally speaking, the faults on the west side of the Gonghe-Guide Basin extend deep, mainly consisting of large faults extending into the lower crust. The eastern side of the fault is shallow. Therefore, the basin basement is deep in the west and shallow in the east.
Similarly, based on the wavelet multi-scale decomposition method, Fig. 9 shows the residual magnetic anomaly, which is the magnetic material feature above the Curie point depth. During the inversion, for the aeromagnetic data, the maximum depth was set as 30km, the structure index was 3, the number of iterations was 20, and the upper and lower magnetization limits were 1 and − 1. The three-dimensional inversion results were obtained, as shown in Fig. 10. To compare with gravity data, slices were taken at the same locations as Fig. 7.
The magnetic variation characteristic from shallow to deep is prominent. In the AA’ profile, magnetism is strong because of the intense tectonic activity. However, it is negative at the Wahongshan fault. Similarly, in the BB’ section, there is a significant negative magnetic anomaly on the west side of the Gonghe Basin with a depth range of 6 ~ 24km. The magnetism slowly dissipates with depth. In the CC’ section, a strong positive magnetic body in the basin’s center extends to 24km and below. This is a high-density and high magnetic anomaly body relative to the surrounding partial melting. It is speculated that this is a hidden granite body. No partial melting has occurred. Near the towns of Chaka, Tanggemu, Gonghe, and Xinjie, small-scale high magnetic anomalies at shallow depths and low magnetic negative anomalies at deeper depths, indicat shallow granite bodies and deep partial melting areas. This is consistent with the gravity results and the depth range. The same characteristics occur in section DD’; the characteristics of the Guide area are very similar to Gonghe, both of which are shallow granite bodies with deep weak or negative magnetic anomaly characteristics, reflecting deep partial melting.
To facilitate analysis and display, slices at different depths were also made, as shown in Fig. 11. At 3km and shallower depths, there are no significant magnetic anomalies, indicating no apparent magnetic body in the shallow part dominated by sedimentation. At depths less than 3km, there is weak magnetism and no apparent magnetic anomaly. It is speculated that sedimentary rocks mainly cause these characteristics. Furthermore, the resolution of the shallow data also can be a factor.
In all the results at depths > 6km, there are scattered magnetic anomalies, mainly concentrated in the edge of the basin, faults, fold zones, and granite outcropping area in the western part of the basin. It is speculated that they are related to granite intrusions. The magnetic field inside the basin is consistent, and there is no obvious abnormal body. The magnetic bodies are mainly concentrated in the center of the basin.
At a depth of 6 km, an east-west elliptical positive magnetic anomaly appears in east Gonghe County. This location also corresponds with a negative gravity anomaly, speculated as the Qiabuqia dry hot rock mass. It has been confirmed that the Qiabuqia thermal rock mass is about 21 km long in the east-west direction and 14 km wide in the north-south direction. The plane is nearly elliptical and develops stably within the depth of 21km (Zhang et al., 2020a).
There is a positive magnetic anomaly with a length of ~ 150km in the east-west direction in the granite area near Tanggemu. This location is also a negative gravity anomaly. The magnetic anomaly disappears at a depth of 24km. It is inferred that it is a granite intrusion extending to 24km. The difference from the Qiabuqia dry thermal rock mass is that the intrusion extends deeper. However, this granite mass has no surface exposure and no thermal insulation cover. The intrusive body extends deeply, and the concealed body can be found nearby as a geothermal exploration target.
At a depth of 9 km, an NW bead-like positive magnetic anomaly is highlighted near the Wahongshan in the southwestern Wayuxiangka. There are dense EW and NW-W faults, numerous Permian and Triassic granites, and hot springs. There is also an apparent positive magnetic anomaly at a depth of 24km, indicating that this area is an excellent geothermal exploration area.
In general, the gradient zone of magnetic anomaly from 15 km to 24 km is gradually flattened, indicating that the existence of underground high temperatures leads to the disappearance or weakening of magnetism. It is speculated that high temperatures have demagnetized the material.
To further confirm and verify the correctness of the above interpretation, based on the inversion results, a 2D geologic model of CC’ profile is created to show the detailed tectonic framework (Fig. 12). In the basin, the experimental density and magnetic susceptibility of granites are 2.532–2.555 g/cm3 and 0.0456×10–3 ~13.847×10–3 SI separately (Zhao et al., 2020). The partial melting is widespread in in the upper-middle crust, where the density is set as 2.4–2.5 g/cm3. Correspondingly, middle and lower crust densities are set as 2.8 g/cm3 and 3 g/cm3, respectively. To fit the magnetic data, we set the magnetic susceptibility of granite as 0.1×10–3 SI for Ordovician granite and 0.05×10–3 SI for Indosimian granite. The geologic model of CC’ profile corresponds well with our inversion results, in which the partial melting is located deeper, and the granites are shallow.