The Mingyueshan, located on the Chongqing Municipality, lies between latitudes 29°28′–31°01′ N and longitudes 106°45′–107°56′ E. It is a 210-km typical eastern Sichuan Jura-type fold with narrow axis and asymmetrical limbs. The Yulin river and the Mingyue river pass through the south and north of the long anticline, respectively (Figure 1).

The study area is controlled by the semitropical climate, with distinct seasons. The average temperature is 16°C–18°C throughout the year. The rainfall is abundant and there is an obvious vertical zonation of rainfall, in which the average annual rainfall in the basin and river valley is about 998 mm, while the rainfall in the mountain area exceeds 1,100 mm (Yang et al., 2019). The river systems in the study area are well developed with a dendritic distribution.

The Eastern Sichuan fold-thrust belt (ESFTB) is situated in the eastern margin of the middle and upper Yangtze blocks in the South China Craton (Charvet, 2013; Lu et al., 2014). It had experienced Yanshan and Himalayas movement, then developed a serial of NNE–SSW trending asymmetric high-steep folds, one of which is Mingyueshan anticline. The Mingyueshan anticline is located in the southwest area of ESFTB, which is a narrow and long structural belt and subjected to the combined action of west-east compression and bottom-up uplift during the tectonic deformation. Therefore, the structural features and two limbs of the north, middle and south parts are presented differently (Figure 2).

The northern part of Mingyueshan was affected by the southward thrust and nappe of the South Dabashan mountains tectonic belt in the north of Mingyueshan, leading to the south-east bending of Mingyueshan fold belt and its continued northward expansion. The influence of the southward thrust and nappe of the South Dabashan mountains was getting more intense (Wen and Li, 2020). As a result, the limbs of the northern anticline are roughly symmetrical (Figure 2A and section a–a′). In the middle part, the lithology phase transition occurred in part of the area, the difference of lithology led to the concentration of stress and displacement, appearance of multiple rotation axis segments. The rock strata in the west limb of the middle part were inclined gently, while the rock strata in the east limb were steep (Figure 2B and section b–b′). In the southern part, affected by the thickness of the weak layer (detachment layer), the tectonic stress and displacement were more likely to transfer to the west along the detachment layer, resulting in the uplift of the anticlinal core. The difference in detachment layer thickness led to a significant difference in the transfer of tectonic deformation from east to west in the study area. The two limbs of the south part were roughly symmetrical (Figure 2C and section c–c′).

The soluble rocks in the anticline are only exposed in the core area, while the east and west limbs are clamped by the water-isolated sandstone and mudstone of the Upper Triassic Xujiahe Formation (T₃xj) (Chen et al., 2016; Szczygieł et al., 2018). The soluble rocks in the northern part are mainly limestone and dolomite in the Middle Triassic Leikoupo (T₂l) and the Lower Triassic Jialingjiang Formations (T₁j), with a small amount of gypsum dissolved breccia. The middle part has the same soluble rock lithology as the north part, which consists of limestone and dolomite. In addition to Leikoupo and Jialingjiang Formation, there are argillaceous limestone of the Upper Permian Changxing Formation (P₂c) and the Lower Triassic Feixianguan Formation (T₁f) in the anticline core of the southern soluble rocks (Figure 2). The carbonated rocks composition such as CaO, MgO and AIR (acid-insoluble residue) of water-bearing formation and two caves in the Mingyueshan area is shown in Figure 3.

Tóth (1962) and Tóth (1963) obtained the groundwater flow system in a basin of undulating topographic surface by analytical solution. Tóth’s study showed the nested groundwater systems of local, intermediate, and regional flow systems, discussed the geological force of groundwater flow (Tóth, 1999). In general, groundwater circulation is not entirely dependent on geological factors, but also controlled by natural geographical factors such as topographic potential energy and drainage system (Tóth, 2009; Liang et al., 2012; Liang et al., 2022). The difference of topographic potential energy is the main driving force of groundwater movement. Recharge areas with high terrain accumulate potential energy with recharge, while discharge area with low-lying terrain is difficult to accumulate potential energy. Therefore, topography usually controls the spatial distribution of topographic potential energy.

During the intermittent uplift of crust, the karst water circulation is closely related to the evolution of karst topography (Hill and Polyak, 2010). On the one hand, the evolution of karst topography is a process that changes with time, the alternate water cycle conditions control the evolution of karst topography. On the other hand, topographical control over hydrology can be manifested in a short period of time. The diversity and difference of karst landforms lead to different hydrological functions of karst systems, groundwater shows regional and local drainage characteristics. In order to obtain the relationship between karst topography and hydrological system clearly in the study area, transverse gully cut points and karst depressions were extracted automatically using digital elevation model (DEM) and geographic information system (GIS) data (Pavel et al., 2016; Wu et al., 2016; Meng et al., 2018).

Transverse gully cut points and karst depressions were extracted (Figure 4) and the elevation and development density of transverse gullies were calculated (Table 1) in each part of Mingyueshan. From north to south, the number of transverse gullies cut points increase, the depth of karst depression changes from shallow to deep, the development density and range of karst depressions also increase. In the area close to the fully-cut gully, groundwater circulation gradually adapts to the downward cutting of the fully-cut gully, the surface basically has no gully development, then the partly-cut gully loses the drainage ability. The terrain landform of the study area is obviously controlled by the geological structure and lithologic characteristic, the groundwater migrates mainly along the tectonic line from north to south to the Yulin river (the primary tributary of the Yangtze river), the lowest drainage base level. The dynamic conditions of groundwater are mainly controlled by the Yulin river which fully cuts through the anticline karst formation, the gullies, which partly cut into the anticline karst rock.

The depth of groundwater circulation is largely determined by the relative height of recharge and discharge zones (Stringfield et al., 1979). Under the unique background of the Jura-type folds in eastern Sichuan, the east-west direction transverse gullies that cut into anticline soluble rock play an essential role in discharging nearby karst water, the cutting length and depth determine the ability and control range of discharging groundwater. According to the degree to which the transverse river cuts through the soluble rock strata, it is divided into three types: fully-cut, partly-cut, and uncut gully. Their ability of discharging groundwater also ranks from strong to weak. The fully-cut and partly-cut gullies correspond to the deep and shallow circulation of groundwater respectively. A multilevel groundwater circulation system is formed under the control of combination of drainage base level with different cutting forms. At the dip end of the northern anticline, the Mingyue river valley, with an elevation of 420 m, was developed in the non-solvable rocks of the Xujiahe Formation and did not cut into the karst rocks of the anticline, therefore the Mingyue river valley should not be considered as an effective drainage area for karst water. Yanjing gully and Zhafang gully are the gullies that partly cut into the soluble rock, with the elevations of 488 m and 472 m respectively. Yulin river fully cut into the soluble rock with 185 m elevation and it is the regional drainage base level in the study area. Yanjin gully, Zhafang gully and Yulin river formed the local drainage base level of the water in the Mingyueshan and controlled the local karst landform features and groundwater circulation conditions. According to the spatial location of these drainage base levels, the anticline was divided into north, middle and south sections. The groundwater circulation characteristics of each section will be analyzed later.

The stable isotope values of the southern group samples are −46.40‰ to −40.70‰ for δ²H, and −7.59‰ to −6.48‰ for δ¹⁸O. The isotope values of the middle group varied between −50.40‰ and −47.70‰-for δ²H, from −7.92‰ to −7.69‰ for δ¹⁸O. The δ²H values of the northern group ranges from −58.10‰ to −49.20‰, δ¹⁸O ranges from −8.910‰ to −7.730‰ (Table 2). A binary scatter plots of δ²H and δ¹⁸O were drawn from 28 sets of isotopic data of water samples in the north, middle and south parts of the Mingyueshan (Figure 5). The samples are between the global meteoric water line (GMWL: δ²H = 8.14 × δ¹⁸O + 10.9; Craig 1961) and the local meteoric water line (LMWL: δ²H =7.85 × δ¹⁸O + 14.12; Wen 2017), indicating that there is a close relationship between the recharge source of the samples and atmospheric precipitation. It also reflects that the region is inland far from the steam source and at a high altitude, which is consistent with the actual situation in the study area.

In general, the terrain of Mingyue Mountain decreases from north to south, the average elevation of water samples in the northern and middle sections is generally higher than the southern section. The stable isotope is affected by the altitude, the southern section of Mingyueshan shows obvious subdivision compared with the northern and middle sections: the water samples in the south section are concentrated in the upper right of the scatter diagram, are distributed obviously denser in heavy isotopes δ¹⁸O and δ²H than those in the middle and north sections. The slope of stable isotope linear curves of the middle and northern group is parallel to that of LMWL and GMWL, which reflects wet climate and weak evaporation in the study area. However, the slope of isotope linear curve of the southern group is significantly less than that of LMWL and GMWL, showing positive δ¹⁸O deviation, which results from the water-rock interaction. The content of δ²H in rocks is very low, which is not enough to significantly affect the δ²H value in water, while the rocks are more enriched in δ¹⁸O than water (Pu, 2013). The long-term retention of karst water in the aquifer leads to the enhancement of water-rock interaction in the system, δ¹⁸O in the rock will transfer to the water, resulting in δ¹⁸O enrichment in the southern group. It is inferred that the south section groundwater has larger exchange capacity of material components than middle and north sections, and the groundwater circulation in the southern section is deeper than middle and north sections.

³H input concentration in precipitation curved graph and ³H output concentration curved graph from the PEM in study area were established (Figure 6). S01, S02 and S05 are all in the north section of the anticline. The tritium values of S02 and S05 springs are ranging from 5.58 to 6.1Tu. It is estimated that the groundwater runoff time is roughly 15–20 years and the formation time lies from year 1998 to 2003. S01 is a hot water hole involved in deep circulation and has a low ³H abundance less than 2. It should have been formed before the nuclear explosion in 1953 and has a long groundwater detention time. S08 is an ascending spring in the middle section of the anticline, the measured tritium value is 7.97Tu, which is presumed to have been formed around year 2007 and the groundwater circulation path is short and shallow. S24 is an outcropping spring in the south section of the anticline with 13.01TU tritium value, the groundwater retention age is relatively long, about 50 years. It is speculated that the groundwater of S24 migrates through the deep circulation system and is discharged to the inner spring mouth in the negative terrain. The estimation of the age of tritium retention in groundwater can provide a reliable basis for the classification of groundwater circulation patterns in Mingyueshan.

The north section is from Mingyue river to Yanjin gully, about 46 km long, with few transverse gullies, undeveloped surface water systems and shallow karst depressions. The main karst landforms on the surface are ridge-shallow troughs. The scale of karst development is weak, in which karst troughs of west limb are not obvious, while karst troughs of the east limb are more apparent. In the north section, due to the large area of non-soluble rock strata exposed by the overturning of the anticline, the penetrating valley of Mingyue river does not cut into the soluble rocks, as a result it does not act as an effective drainage area for the inland water of the anticline. Retention time of S02 and S05 spring are short, accompanied with weak water-rock reaction, corresponding to shallow circulation. While the sampling depth of S01 hot water borehole is deep, the groundwater ³H dating is old and the retention time is long. It is speculated that the overall flow of groundwater in this section is from north to south, the shallow groundwater is discharged through the deep transverse gullies of Yanjing gully, the deep groundwater migrates to the Yulin river. Under the control of the depth and drainage capacity of the deep transverse gullies, there are almost no other transverse gullies development in a certain range near this area. The groundwater in this section is mainly controlled by a single deep transverse gully, which is a unidirectional shallow circulation pattern (Figure 7).

The middle section is from Yanjin gully to Zhafang gully, about 70 km long. The karst landforms developed in this section are ridge-single troughs. The soluble rock strata of Leikoupo and Jialingjiang Formation formed valleys after dissolution, while the non-carbonate rocks on both limbs formed monocline ridge. The structural stress in the middle section is not distributed evenly, resulting in asymmetry between the two limbs of the anticline. To be more concrete, the Xujiahe Formation in the west limb stands up and goes inverted, with steepness occurring in parts of the area. The dip angle of east limb formation is gentle, forming a single plane structure. In this area, the outcrop of Xujiahe Formation is thin, which is vulnerable to the trace-back erosion of cross-cut gullies and cut into the core of the anticline. Rainfall is mainly discharged by lateral runoff from the west limb to the east limb gullies through slope flows, forming deep circulating karst water within a certain depth range below the drainage base level, and leading to longitudinal runoff to the Yulin river. Far away from the control area of fully cut valley, the non-soluble rock strata formation holding karst layer groups are cut open by transverse gullies, as the main drainage channel of groundwater in karst trough valleys. With the continuous tractive erosion of groundwater, the groundwater between adjacent gullies and valleys continuously attacks and merges with each other, leading to the increasing difference in the depth and shallow degree of the transverse gully development. In the development of a series of shallow transverse gullies, a deep cutting gully with a large control range appeared, which controls the discharge of groundwater in the trough valley together with the shallow transverse gullies. Isotope sampling data from the middle section indicate weak water-rock interaction and a short groundwater retention age. The groundwater in the middle section is jointly controlled by the two partly cut transverse gullies Yanjing gully and Zhafang gully, forming a bidirectional shallow circulation pattern (Figure 8).

The south section is from Zhafang gully to Yulin river, with a length about 54 km. The karst landforms, mainly ridge-double troughs and transverse gullies and valleys, are developed along both limbs of the anticline. The karst depressions are distributed roughly in a beaded shape, the bottom elevation of the depressions decreases from north to south. In this section, the rainfall in the trough valleys is discharged to the syncline area through the cross-cutting gullies on both sides. Meanwhile, due to the development of the surface karst negative terrain in this section, the atmospheric rainfall is rapidly introduced into the underground through the negative terrain, and is discharged along the east-west trough valleys of the anticline to the southern side of the Yulin river. Impacted by the tectonic stress extrusion in the south section, the Feixianguan Formation and Changxing Formation exposed in the core, controlled by the non-soluble rocks of the Feixianguan Formation, the hydraulic connection between the east and west limbs of the anticline is hindered, with a manifestation that the shallow groundwater is mainly discharged through partly cut transverse gullies. In a certain range away from the fully cut valley, where groundwater circulation has not adapted to the valley down-cutting, the valley has a weak influence on the transverse valleys, resulting in the strong development of transverse gullies and valleys and forming shallow depth of groundwater level. Groundwater, is discharged through the shallow transverse gullies, with a small amount of groundwater circulating deeply through the deep dissolution gap. For areas close to the Yulin river, under the control of crustal uplift, fully cut river valley continues to cut down, which controls the groundwater circulation to the deep, leading to a deep depth of groundwater level. When groundwater circulation is adapted to the down-cutting of fully cut valleys, there is basically no gully development on the surface, transverse gullies and valleys will lose the discharge ability. The stable isotopes of the water samples data in the southern section far away from the Yulin River basically fall on the LMWL, indicating that the groundwater is excreted by temporary runoff and belongs to shallow circulation. The stable isotopes of the water samples to the south showed a trend of δ¹⁸O enrichment, the ³H dating also showed that the groundwater retention time was long, which was a deep circulation area. It’s inferred that the fully-cut Yulin river valley and partly-cut Zhangfang gully together constitute a multilevel discharge base level of karst water, thus forming a unidirectional shallow-deep nested circulation system jointly controlled by fully cut river valley and partly cut gully (Figure 9).

Under the special conditions of the east Sichuan Basin, where a series of NNE–SSW trending Jura-type folds developed, we considered that the near east-west (EW) direction gullies determine the drainage of karst water largely. In this study, according to the degree of transverse gullies cutting through the soluble rock strata, it can be divided into three types: fully-cut, partly-cut, and uncut gully. Based on geomorphic and environmental isotopes analysis, the karst water circulation under the control of multiple drainage base levels and their combinations is discussed in typical Jura-type fold Mingyueshan. However, these discussions are essentially qualitative, some numerical simulations of groundwater flow can be considered in future work. The combination of numerical simulation and field data helps to determine the groundwater flow system quantitatively.