1 Introduction

The ecosystems of plateaus are significantly impacted by the presence of rift lakes (Zhang et al. 1.1 Research gaps and motivation

Compared with the environmental aspects of the lake sediments of the Yunnan Plateau tectonic lakes, comparatively little attention has been given to the geophysical exploration of Yunnan lakes in SW China. Therefore, comprehensive geophysical research must be conducted on Lake Fuxian. The purpose of this study was to explore the research gaps present among the deepest freshwater lakes within the Yunnan Plateau through geophysical study. This research aims to fill the following significant research gaps;

  1. 1.

    The geophysical understanding of Lake Fuxian has yet to be explored. There is no published literature on the Lake Fuxian using the deep-water multichannel seismic exploration data. Additionally, the former understanding of the deep stratigraphic structure was limited mainly to the reference of the stratigraphic structure of the lake shoreline of Lake Fuxian.

  2. 2.

    Understanding sediment thickness is fundamental to exploring the crustal structure, sedimentary loading and long-term sedimentation rates of a basin. However, the exact thickness distributions within the southern, central and northern basins are still unknown.

  3. 3.

    The depositional settings of the Lake Fuxian basin need to be adequately addressed to describe the spatial and vertical distributions of the unknown stratigraphic sequences.

  4. 4.

    What are the controlling factors that influence the mechanism of crustal extension and the formation of horst-graben structures? Additionally, what are the effects of seismicity and lake-level variations on the sedimentation rates?

2 Geology of the study area

2.1 Topography

Lake Fuxian (102° 49ʹ–102° 57ʹ 7 E, 24° 21ʹ–24° 38ʹ 8 N) is a north‒south trending karst rift lake shaped like an inverse gourd and located in Yuxi city is 70 km (43 mi) away from Kunming (Dai et al. 2017) (Fig. 1b). Lake Fuxian is the deepest lake within the Yunnan Plateau region (Cui et al. 2008) and the third deepest freshwater oligotrophic lake in China (Zhang et al. 2021a, b). The lake has a storage capacity of 18.9 billion cubic meters. It is approximately 31.5 km long from north to south and narrow in the middle, with a width of approximately 11.5 km in the north and a width of approximately 3.2 km in the south. The water depth is deep in the north and shallow in the south, and the average water depth is approximately 89.6 m, with a maximum depth of 157.5 m (Fig. 1c) (Zeng and Wu 2009). The nearshore lake bottom on both sides is steep, but the slope becomes gentle below a water depth of 50 m and becomes flatter below 100 m. The runoff area of the basin is 1053 km2. It covers an area of 211 sq. km (81 sq. mi), and the maximum depth is 155 m (508 ft.), making it the third deepest freshwater oligotrophic lake in China (Zhang et al. 2021a, b). More than 100 rivers flow into Lake Fuxian, and the water is discharged outside from a single outlet (Zhang et al. 2021a, b).

2.2 Tectonics and structure

Lake Fuxian is located in the basin range area of the Yunnan-Guizhou Plateau in the southeastern Tibetan Plateau (Zhang et al. 2021a, b). The geotectonics of Lake Fuxian belong to the intersection of the Yangtze paraplatform, South China fold system and Lan**-Simao fold system (Bureau of Geology and Mineral Resources of Yunnan Province 1990). In terms of the geological structure, Lake Fuxian is located on the ** Sichuan-Yunnan Block of the Tibetan Plateau. J Asian Earth Sci 21(10):1085–1096" href="/article/10.1007/s40948-024-00777-7#ref-CR24" id="ref-link-section-d99000110e777">2003). The northern part is a simple fault and consists of Qiaojia to Menggu, which are approximately 50 km long. However, the middle to southern part has numerous complicated minor faults. The southern part comprises two parallel branch faults ~ 15 km apart, referred to as the east and west branches (Cheng et al. 2003; Li et al. 2019a, b; Cheng et al. 2021).

The **aojiang and Qujiang faults are developed in the southeast and southwest corners, respectively, whereas the Puduhe fault and Yujiang fault run through them in the west. The neotectonic movement in the study area is strong and includes rifts along faults, differential uplifts and downs of fault blocks, strike-slip faults and numerous seismic events (Wang et al. 2009). The fault cliffs on both sides of the lake are high-angle normal faults, and the secondary faults on both sides are distributed in a feather shape, with a horizontal offset of 2.0 km. The XJF has been active many times since the Neogene. A series of late Cenozoic basins, such as Dongchuan, Yanglin, Yiliang, Chengjiang, and Jiangchuan, and modern lake basins, such as Lake Yangzong, Lake Fuxian and Lake **ngyun, are developed along the fault zone. Since the Quaternary, the XJF has been characterized by a strong left-lateral strike-slip fault, and its western branch fault has shown noticeable tension and torsion (Nan**g Institute of Geography and Limnology 1990). The differential movement on both sides reaches 7.5 ± 3.8 mm/a, which is one of the significant signs of Holocene tectonic activity, with more than 10 earthquakes of M ≥ 6 occurring alongside the XJF seismic belt (Wang et al. 2009; Guo et al. 2021).

2.3 Stratigraphic settings

According to lithology classification, there are three main types of strata exposed around the Lake Fuxian basin (Bureau of Geology and Mineral Resources of Yunnan Province 1990):

  1. 1.

    Limestone and dolomite, with a distribution area of 159 km2;

  2. 2.

    Shales and conglomerates, with an area of 159 km2;

  3. 3.

    Basalt, with a distribution area of approximately 55 km2.

According to the regional geological data, sedimentary rocks are the most common in the study area, lacking Ordovician and Silurian rocks. The bedrock around Lake Fuxian is mainly Sinian, Cambrian, Devonian, Carboniferous, Permian, Triassic and Jurassic (Fig. 1d). The lithologies of the different formations during their periods are described below from older to younger strata as follows:

The Sinian (Zac, Zbn, Zbd, Zbdn) system is dominated by grayish purple-grained feldspar lithic sand, glacial conglomerate and dolomite. The Cambrian (C1q, C1c, C1l, C2d) strata are dominated by light black dolomite with intercalations of siltstone and shale. The Devonian rocks (D1, D2, D2h, D3) are dominated by dark gray dolomite, red sandy limestone and calcareous mudstone. The Carboniferous (C1y, C1dw, C1ds, C2w and C3m) strata consist of gray oolitic limestone, siltstone and black shale. The Permian rocks (P1d, P1q, P1m, P2β) comprise gray black basalt, yellow bauxite shale and coal seams. The Triassic (T2, T3p, T3g and T3s) system is dominated by sandstone and silty shale. The Jurassic (J1f, J2z, J2s, J2t) system is composed of yellowish white sandstone, reddish siltstone and calcareous mudstone. The Cretaceous (K) system is composed of sandstone, conglomerate and muddy siltstone. The Paleogene (E) strata are mainly composed of red conglomerate and calcareous mudstone. The overburden is composed of Neogene and Quaternary rocks. The Neogene (N) rocks are dominated by lake facies and swamp facies sedimentation, and their lithology is mainly composed of lignite, sandy gravel, and silt intercalated with sandy clay. The Quaternary sediments (Q1, Qall, and Qal) are dominated by lake sedimentation, and their lithology is composed of clay, fine sand, and sandy gravel (Nan**g Institute of Geography and Limnology 1990).

3 Data and methods

3.1 Acquisition and processing of seismic reflection profiles

To comprehend the ancient seismic records of Lake Fuxian, the Jiangsu Geological Exploration Technology Institute, Nan**g (JSGETI), undertook a multichannel seismic exploration survey on Lake Fuxian in collaboration with the Institute for Ecological Research and Pollution Control of Plateau Lakes, Yunnan University. The design, evaluation work, and processing of the survey were completed in two phases. Initially, the first six seismic lines (L1–L6) were collected during October 2018. In the second phase, nine additional seismic lines (L7–L14) were collected during October 2021. The total length of the completed survey and the survey orientations are shown in Table 1. A total distance of 154.3 km2 of digital, poststacked 2D seismic data was available for this study. The data were collected in such a way that dip lines such as Line-6 were orthogonal to the horst-graben system of the Lake Fuxian basin. These include 9 nearly WNW-ESE-oriented and 6 SSW-NNE-oriented seismic profiles.

Table 1 Summary of the dataset showing the orientations, coordinates and lengths of the seismic survey lines
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The procedure for acquiring and processing field information is as follows: the seismograph utilized in the study was tobtained from the GEO-SENSE LW 48 (Netherlands) channel seismic acquisition system. The source employed during the survey included two GS-1.2/250 (France) air compressors and the double gun combination of the Mini G air gun. To ensure the authenticity of the received signal, passband reception was utilized in this work. The technical parameters, incorporating the source and receiver parameters, are presented in Table 2. To process the data, Landmark Promax processing software was used to improve the signal-to-noise ratio of the seismic data. The seismic data utilized in the study are poststacked time-migrated seismic profiles stored in SEG-Y format.

Table 2 Summary of the acquisition and processing survey parameters
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3.2 Stratigraphic and paleobathymetric interpretation

The average velocities of each seismic profile were also available and were processed in HRS interpretation software to convert the time sections to depth sections. The depth sections were loaded in two software programs. The Fugro Jason™ Geoscience Workbench (JSW) and the Petrel™ E&P software suite from Schlumberger and CGG are used for further interpretation of horizons and fault markings. Initially, a basemap was established by utilizing the navigation and SEG-Y datasets to highlight the geometry and orientation (dip and strike) of the seismic profiles. Seismic sequence stratigraphy and seismic facies analysis are performed for detailed interpretation of paleoseismic records of the basin. Later, the interpretations were manually marked using Corel Draw graphics software to present a detailed interpretation of the depositional sequences, stratal terminations and fault markings.

3.3 Seismic sequence stratigraphy

The interpretation approach was concentrated on building a comprehensive stratigraphic framework by employing the methodology of seismic sequence stratigraphy for the Lake Fuxian basin, as employed for the lacustrine strata (Bohacs et al. 2000). Seismic stratigraphic analysis was performed utilizing the Petrel interpretation software. The principles of sequence stratigraphy were employed to map depositional sequences via the identification of stratal terminations on seismic profiles (Lyons et al. 2011). Several sequence boundaries are marked as horizons showing onlap patterns or truncation of reflections. Furthermore, these mapped horizons may show the characteristics of flooding surfaces marked by downlap patterns on seismic sections (McGlue et al. 2008).

4 Results

4.1 Seismic facies analysis

Several types of facies patterns were recognized on seismic sections in the basin. Several authors in the literature have recognized the seismic facies pattern on seismic reflection profiles of lakes at the regional level (Colman et al. 2003; Bohacs et al. 2000; Mcglue et al. 2008; Lyons et al. 2011). By analyzing and comparing the external geometry, amplitude, frequency, and continuity of the reflected wave groups of the seismic time profiles of each survey line, the effective reflected wave groups and their obvious anomalies, such as faults, were delineated. Moreover, the geophysical data results were combined with regional geology data to determine the stratigraphic structure, sedimentary characteristics, and faulted strata to provide detailed information for the study of the Lake Fuxian basin. We have presented an abridged form of seismic facies types, which are as follows:

Hemipelagic drape facies are composed of straight parallel reflections with high continuity and medium amplitude and form an onlap** pattern on the preexisting reflections of the lake floor (Fig. 2a). In addition, these facies usually overlie the steeply dip** pattern of clinoform sets. These facies include detrital clay and silt along with interbedded amounts of silica and organic matter (Johnson et al. 2002).

Fig. 2
figure 2

Categorization of basic seismic facies interpreted on the L6 survey line. The seismic reflection characteristics of the a hemipelagic drape facies, b progradational delta facies, c fan facies, d channel-levee fluvial facies, and e basin-fill lacustrine facies

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The progradational delta facies are composed of steeply dip** sigmoid-shaped reflections with low continuity and low amplitude (Fig. 2b). There are reflections toplap on the upper surfaces and downlap on the lower surfaces. These facies are composed of relatively coarse-grained gravels and sand (Butch 1996).

Fan facies are recognized by downdip reflections that are internally chaotic and wedge-shaped with planar basal surfaces (Soreghan et al. 1999). They usually comprise coarser mass-flow packages with low internal grading (Wells et al. 1999). These facies include slope fan facies, canyon fan facies, and delta-fan facies (Fig. 2c).

Channel-fill facies have U-shaped and convex bottom features on seismic sections (Anees et al. 2019) and are usually composed of medium-amplitude and medium-frequency reflections that might truncate adjacent to each other (Lyons et al. 2011). Channels are also characterized by fining-upward sequences and coarser gravity flow packages (Wells et al. 1999). Levees are often linked with channels, display a topographically higher location with respect to the channels, and exhibit high amplitude and transparent seismic features (Torrado et al. 2020) (Fig. 2d).

Basin-fill facies are associated with the depocenters of the basins and are composed of semicontinuous, high-frequency, and low- to high-amplitude internal reflections. Externally, these patterns are basal onlap** patterns (Wells et al. 1999) (Fig. 2e).

4.2 Seismic sequence stratigraphic analysis

We have presented a comprehensive analysis of the acquired seismic profile of each survey line in the Lake Fuxian basin. To analyze the depositional sequences, we subdivided the seismic reflections into a set of packages with identical external geometry (shape) and internal geometry (features and texture) that can be distinguished via sequence boundaries. Overall, we analyzed 14 different seismic facies types (SFs) and presented an abridged form of graphical presentation, shape, texture and reflection features, as well as their geological significance and interpretation, in Table 3. Initially, the seismic packages of depositional sequences were marked on the L6 dip line, which is the main survey line and represents the whole structural and stratigraphic setting of the lake basin (Fig. 3). Later, the depositional sequences were marked at the intersection of each survey line based on the regional geology and the corresponding reflection sequences. It is preliminarily inferred that the effective reflection sequences of the main chronostratigraphic interface can be divided into 6 horizons and 5 sequences. These horizons are marked as T0, with T1, T2, T3, T4, and Tg (youngest to oldest).

Table 3 A concise classification of seismic facies (SF) showing the graphical presentation, reflection features, and their geological significance and interpretation
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Fig. 3
figure 3

Uninterpreted (top) and interpreted (bottom) seismic sections showing stratigraphic sequences and associated variations in the L6 dip survey line

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4.2.1 Sequence-1 (Sq-1)

The topmost depositional package in the research region is Sq-1, which lies between the T0 and T1 horizons. The present-day sediment–water contact is near the top of Sq-1, which is marked by the T0 horizon. The deepest point of the T0 horizon is 171 m, which lies at the L10-2 survey line (Fig. 4). The thickness of Sq-1 varies from 16 m in the SSW region to 501 m in the NNE region. The maximum thickness of Sq-1 is 330 m, which lies at the L10-2 survey line near the NW region. The minimum thickness is 39 m, which lies toward the SSW region of the lake near the L1 survey line.

Fig. 4
figure 4

Uninterpreted (top) and interpreted (bottom) seismic sections of the L10-2 survey line showing the northern lake basin

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The seismic texture in Sq-1 shows parallel and wavy reflections in the middle and discontinuous reflections toward the margin that depict sheet-drape geometry. These reflections are related to the uniform deposition of silt and clays along with organic matter in the Holocene sediments. The parallel reflections at the top are marked by a toplap, whereas the downdip** reflections are mapped through the demarcation of onlaps and downlap patterns toward the NEE region of the basin, where the topography of the Lake Fuxian basin changes from a homoclinal ramp to distally steeper slope ramps (Fig. 3). The reflections of Sq-1 gradually terminate on tilted Sq-2 reflections, creating an unconformable contact at the L6 survey line near the 17,150 CDP.

Sq-1 is mostly characterized by high-amplitude, high-frequency, and parallel to subparallel continuous seismic reflections (SF-1). These facies are associated with the low-energy environment of lake-floor deposits. In contrast, toward the depocenter region, the reflections exhibit the characteristics of high-to-low amplitude, high frequency, and wavy parallel reflections (SF-2), which extend to a few kilometers. These facies are related to the folding of the strata and feature fine-grained deposition. The two types of facies that are dominant in Sq-1 are hemipelagic drape facies and channel-fill facies. The reflections of hemipelagic drape facies exhibit very low amplitudes and onlap to the existing reflections on the lake floor (T0) (Fig. 2a). However, the channel-fill facies in Sq-1 are mainly characterized by divergent-fill (SF-8) and complex-fill (SF-10) facies. The onlap pattern of the divergent-fill facies shows low-energy, shale-prone sediments in the graben-fill. However, the complex filling of the channel provides an indication of rapid variations in water and sediment provenance. In a general trend, the uppermost reflections of the channel-fill facies truncate onto the preexisting reflections of Sq-1.

4.2.2 Sequence 2 (Sq-2)

Sq-2 is situated beneath Sq-1 and lies amid the T1 and T2 horizons. Sq-2 cannot be tracked throughout the lake basin, and some reflections terminate on the reflection pattern of Sq-1. This termination is evident at the L2 survey line near 1922 and the 1312 CDP (Fig. 5). The structural thinning of stratigraphic Sq-2 is also evident at L10-2 toward 12,493 CDP (Fig. 4). Sq-2 is thinner than Sq-1. The maximum thickness of Sq-2 is 187 m, which lies at the L10-2 survey line near the NW region toward the depocenter of the basin. The minimum thickness of Sq-2 lies at L2, where the reflections terminate against the overlying gently dip** strata of Sq-1.

Fig. 5
figure 5

Uninterpreted (top) and interpreted (bottom) seismic sections of the L2 survey line showing the southern lake basin. The interpreted section shows lake sequences associated with seismic variations

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Within Sq-2, the parallel reflections of overlaying strata of Sq-1 are marked by toplap, whereas the downdip** prograding packages are traced via onlaps and downlap** patterns toward the SSW and NNE regions of the basin. The basal contact of Sq-2 displays high-angle reflections toward the margin of the seismic sections and shows wedge-shaped prograding fan facies reflections (Fig. 2b). These fan facies are semicontinuous and feature oblique tangential and sigmoidal texture textures (SF-4 and SF-5). However, the reflections are flat-layered but wavy in the middle region and show sheet-drape geometry (SF-2).

Further toward the NNE region of the lake basin, the reflections of Sq-2 change the morphology from wavy parallel to oblique parallel reflections, where the topography of the basin changes from sheet-drape (SF-2) to a distally prograding fan facies. These fan-shaped facies are associated with high-energy interbedded lake coarse-grained facies that are associated with sand and gravel, whereas the subparallel to wavy reflections within Sq-2 are associated with intercalations of silt and clays. The existence of irregular diffracted reflections is possibly associated with paleosemic activity during the Holocene.

4.2.3 Sequence 3 (Sq-3)

Sq-3 lies beneath Sq-2 between the T2 and T3 horizons. The reflections of Sq-3 terminate against the Sq-2 reflections. In some cases, Sq-3 reflections terminate against Sq-1 reflections, forming an unconformable contact. Hence, the reflections of Sq-3 cannot be tracked throughout the lake basin. These terminations on Sq-1 are evident at the L5 survey line near the 5063 CDP (Fig. 6). Sq-3 is thicker than Sq-2. The maximum thickness of Sq-3 is 272 m, which lies at the depocenter of the L10-2 survey line. Structural thinning of Sq-3 is evident toward the SSW region of the lake basin (Fig. 5). The minimum thickness of Sq-3 is 6 m, which is observed at the L2 survey line near the 1922 CDP. The basal contact of Sq-3 shows high-angle reflections terminated against the Sq-2 and Sq-1 horizons. The reflection characteristics of Sq-3 are highly variable. At several locations, the semicontinuous reflections of Sq-2 and Sq-3 form convex-bottom features and establish local unconformities via erosional truncations of the overlaying sequence reflections.

Fig. 6
figure 6

Uninterpreted (top) and interpreted (bottom) seismic sections of the L5 survey line showcase the presentation of the central lake basin. The interpreted section shows the interpretation of the lake sequences associated with seismic variations

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Toward the SSW region of the lake basin, there are numerous high-amplitude parabolic-shaped reflection patterns (Fig. 2d). These parabolic reflections formed the horst block toward the southern region of the lake basin. The convex-bottom features show the characteristics of lens-shaped, high-amplitude, semicontinuous, low-frequency and high-angle reflections on the seismic section and are associated with channels. The adjacent linked wedge-shaped, uplifted, and high-amplitude features show levees. The sediments associated with these parabolic features are coarse-grained in the channels and are overlain by fine-grained facies in the levees.

Sq-3 shows the characteristics of prograding clinoforms associated with SF-3 and Sf-4, as evident at the middle of the L5 strike line and L6 dip lines. Toward the NNE region of the lake basin, Sq-3 features slope-front deposits (SF-14), which are linked with foreset and bottomset beds (Fig. 5). These high-dip** reflections are characterized by onlaps and downlap patterns and exhibit low-amplitude, continuous, and high-frequency features. The morphology of the clinoforms varies among SF-3, SF-4 and SF-5. However, steeply dip** foresets are always terminate at the top of sequence-4.

The lenticular-shaped channel-fill facies present in Sq-1 have a deep impact on the lake sediments. The effects of these channel-fill facies are prominent within Sq-2 and Sq-3 and extend to the deeper sequences of the Lake Fuxian basin (Fig. 7a). These channel-fill facies are associated with coarse-grained sand and shale sediments.

Fig. 7
figure 7

a Presentation of different types of channel-fill facies toward the southern basin showing variations in sediment provenance and water flow. b Slum** and complex filling seismic reflections providing indications of paleosemic activity that occurred during the Cenozoic era. c Mound-shaped deposits associated with carbonate buildups. d Evidence of backstep** of the progradational delta toward the central basin of the lake. e Aggrading and prograding fans indicating highstand settings. f Clinoforms terminating against the seismic section toward the northern basin of Lake Fuxian, indicating the deepest and oldest permanent sedimentation

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4.2.4 Sequence 4 (Sq-4)

Sq-4 is located under Sq-3 between the T3 and T4 horizons. The reflections of Sq-4 terminate against the Sq-3 reflections. As a result, toward the margins of the lake basin, Sq-4 reflections cannot be traced across the whole lake basin at some survey points. These terminations may be observed near 2166 CDP on the L2 survey line (Fig. 5). The thickness of Sq-4 is highly variable. The maximum thickness of Sq-4 is 247 m at L10-2, which is less than the 272 m thickness of Sq3 at the same location. The thickness of Sq-4 is also lower at the L6 survey line. However, the thickness of Sq-4 is greater than that of Sq-3 at some locations on the L2 and L5 survey lines.

The reflections of Sq-4 have a wide range of features. The basal contact of Sq-4 features steeply dip** reflections, and the external geometry is mainly characterized mainly by mound-shaped and wedge-shaped facies. Toward the SSW region of the lake basin, there is a mound-shaped concave feature where reflections of Sq-3 are gently dip** and onlap** onto the mound-shaped reflections of Sq-4. This feature is evident on the L5 survey line near the 1823 CDP and was termed SF-12 in our study (Fig. 6). This texture was associated with velocity sag and is usually associated with carbonate-prone deposits in shelf-edge settings (Schlaf et al. 2005). The internal geometry of these reflections shows low amplitude, low continuity, and low frequency.

Numerous stacked oblique tangential (SF-4) and sigmoidal (SF-5) clinoforms also tend to appear within Sq-4 toward the northern basin of the lake. The depositional sequence is marked by a succession of well-built, northeast-trending prograding clinoforms toward the graben-fill distal region. Lens-shaped features also occur within Sq-4 that lie toward the WNW region of the lake basin and are associated with complex fill (SF-10) and chaotic-fill channel-shaped geometry (SF-11) (Fig. 7b). The internal texture of the reflections is chaotic and is associated with high-energy and rapid filling of sediments. These facies are the deeper versions of SF-8 at the L2 survey line, as described in the Sq-2 explanation (Fig. 7a).

At the center of the lake, there are some mound-shaped reflections that thicken the seismic structure. These mound features are associated with backstep** of the prograding delta facies (Fig. 7d, e) and represent flooding surfaces that are associated with retrogradation of the lake.

4.2.5 Sequence 5 (Sq-5)

Sq-5 is the oldest depositional sequence in the basin that can be tracked in the study area. Sq-5 is situated beneath Sq-4 and lies between the T4 and Tg horizons. The Tg horizon marks the top surface of the basement. There are continuous frequent irregular chaotic facies, which confirms the presence of basement below the Tg horizon. The reflections of Sq-5 can be tracked throughout the basin at every survey line. The thickness of Sq-5 is variable. The maximum possible thicknesses are 206 m at L10-2 and 310 m at the L8 survey line, toward the depocenter of the basin where there is a prominent graben structure at the regional level, creating additional accommodation for the lake sediments. At the L10-2 survey line, the reflections of Sq-5 show the internal geometry of moderate reflections and semicontinuous and high-frequency features and prograde until the end of the seismic sections, which indicates that the thicknesses of Sq-5 can further extend to ~ 50 m until the start of the basement (Fig. 7f).

The reflections of Sq-5 terminate and overlap with the reflections of Sq-4. This depositional sequence features a variety of facies patterns. The basal contact of the Sq-5 reflections is marked by downlap features. The reflections of Sq-5 show a V-shaped onlap-fill lenticular external geometry. The internal geometry of the reflections is characterized by moderate-to-low amplitude, low continuity, and low frequency (SF-9). The geological significance of SF-9 is interpreted as the low-energy filling of erosional channels. Sq-5 also shows local-chaotic reflections. The internal geometry of these reflections is chaotic and characterized by moderate-to-low amplitude, low continuity, and low frequency (SF-6). These chaotic reflections are sandwiched amid the smoothed oblique parallel reflections, which are possibly caused by paleoseismic activity and denote slump deposits.

Sq-5 shows frequent mound-shaped features that are evident at the L2 (1312 CDP) and L5 (1823 CDP) survey lines. These reflections are concave-shaped and are named SF-13 in our study. The external geometry is mound shaped, the texture is termed velocity pull-up, and the internal features exhibit low amplitude, semicontinuity, and low frequency. These mound-shaped facies are also prominent toward the SE region of the basin (Fig. 7c). The cause of the difference in the SF-13 facies is the combination of the high-velocity zone in Sq-4 against the low-velocity zone in Sq-5 (Trinchero 2000). These facies are associated with carbonate-prone deposits with multistage growth (Schlaf et al. 2005).

The wedge-shaped reflections within Sq-5 show a slope-front fill geometry with high-angle dip** parallel divergent reflections, and the internal features show high amplitude, continuity, and low frequency (SF-14). These reflections are linked with deep water fan facies and show clays and silts in a low-energy environment (Fig. 7d). These facies are also evident in the ESE region of the L2 survey line. The sheet-shaped reflections within Sq-5 are parallel to subparallel shapes and show high amplitude, high continuity, and high frequency. These reflections are characterized as basin-fill facies and are linked with a low-energy environment related to basin-floor deposits. There are thick-bedded basin-fill lacustrine facies at the depocenter of the lake basin, and the reflections show bidirectional onlap patterns that indicate the permanent and oldest sedimentation of the lowstand settings (Fig. 4).

4.2.6 Basement

Below Sq-5, there is a frequent presence of chaotic texture, which is associated with the basement (SF-7). The topmost part of the acoustic basement features the characteristics of frequent discontinuous reflections establishing a main unconformity toward the base of the sedimentary layer. The external geometry is irregular, with low coherence and no specific characteristics. However, the internal features show low amplitude and low continuity. These features confirm the presence of basement material. These poorly established reflections are probably associated with the Moho discontinuity because of poor-quality seismic responses in the deeper regions of the seismic sections. Several deep water multiples and diffraction patterns are also observed in the acoustic basement region.

5 Discussion

5.1 Evidence of lake-level variations by drilling cores from Sq-1

Several contemporary studies conducted on the Lake Fuxian basin have utilized drilling cores within the lake epilimnion zone (~ 3 m deep on the lake floor) to evaluate the effects of paleoclimate and grain-size characteristics (Wang et al. 2018; Li et al. 7d, 7e), which denote the backstep** prograding delta deposits. The wedge-shaped reflections indicate a moderate sediment supply and are associated with a rise in sea level, where fine-grained low-energy sediments were deposited by pooly sorted prograding slopes with poor sorting (Fig. 7b). The parabolic-shaped reflections indicate the presence of a high-energy fluvial environment in the form of channels and levees (Fig. 7d). The textures of these channel-filled reflections are onlap and indicate the presence of shale and sand deposition. The sedimentary thickness of the lake is highly variable in the middle and northern regions compared to that in the southern region. The sedimentation of Lake Fuxian was affected by tectonic activity (Li et al. 7a). Overall, these seismic reflections of Sq-1 are composed of parallel to subparallel, divergent and semicontinuous reflections, distinguishing them from the underlying seismic reflections that are representative of relatively deep waters of the Lake Fuxian basin. The drilled cores within Sq-1 suggest the presence of ooze, detrital clay, silt and intercalated of organic matter that was accumulated by suspension (Wang et al. 2018). The presence of these fine-grained sediments also indicates that sedimentation was controlled by suspension. Continuous straight reflections are present at the regional level throughout the basin within Sq-1, suggesting that prevalent deepwater settings occurred during most of the Sq-1 timescale. However, in a recent study, the authors suggested that the last rise in the lake level occurred at ~ 2.2 cal. Ka B. P and reached the modern lake surface (Li et al. 2019a, b).

5.4 Geological controlling factors influencing sedimentation in lake basins

Tectonic lakes often form as a consequence of rifting and strike-slip tectonics occurring in an extensional environment (Sladen 2012). Climate change has a greater and more lasting impact on the hydrology of rift-lake basins compared to basin geodynamics or volcanism, when considering shorter periods of geologic time within the African rift lakes (McGlue et al. 2008). Some of the main factors that play a crucial roles in controlling the evolution of great global lake sequences include analyzing the subsidence rate versus the accumulation rate, climate, sedimentary thickness, and geology of the rock types (Bohacs et al. 2000; Gierlowski-Kordesch and Kelts 2000; Sladen 2012). In another study conducted on the lake Malawi, the authors suggested that sediment supply and lake-level variations are relatively important factors in controlling lake sequences (Lyons et al. 2011). In addition, the slope curvature of the clinoforms also provided important information about the in lakes Switzerland (Adams et al. 2001).

Tectonic forces have had a profound impact on lake deposits in the Fuxian Basin, molding the climatic and geological features of successive lake sequences. The results of seismic patterns toward the southern edge of L3 show circular-shaped low amplitude chaotic reflections, providing evidence of intense seismic activity (Fig. 9a). In addition, a steep slope of around ~ 500 m transported the sediments to the deepest region under the influence of gravity triggered by strong tectonic influence. The chaotic reflections below the basement indicate a pretectonic regime. Above these reflections, subparallel low-energy lacustrine patterns are present that were deposited in the posttectonic regime. On the shallow surface, there are smooth seismic patterns occur, providing a evidence of stable tectonic regime. Toward the western side (WNW) of the southern basin on L2, the dominance of the SF-8, SF10, and SF-11 seismic facies provides sufficient evidence of folding and compression led by intense seismic activity (Fig. 5).

Fig. 9
figure 9

a Seismic patterns on L3 representing events of tectonic regimes within the lake basin. b A proposed lake basin model showing generalized stratigraphy. c, d, and e Provide insights into the total sediment thickness distribution toward the southern, central and northern lake basins, respectively

Full size image

Three seismic survey lines L1, L2, and L3 extending WNW-ESE, are present in the southern lake basin. L1 is present at the southern end and has the lowest sediment thickness, high fluctuations and low extension of faults. The precise total sedimentary thickness was calculated from the lake-floor horizon (T0) to the bedrock horizon (Tg). L2 and L3 have the maximum sediment thickness in the southern basin; however, L3 has a sediment thickness of ~ 450 m, which is greater than that of L2 (Fig. 9c). L3 nearly follows L2, where fault subsidence affects the ESE area and reflections are steeper than those in the WNW region. The wavy nature of the reflections toward the southern basin at L6 suggested folding and compression, thus, reducing the area of the lake basin.

L4 is situated in the center of the basin, at the narrowest point with an orientation of WNW-ESE, and it has a width of approximately 2.5 km. The maximum sedimentary thickness on L4 is ~ 350 m (Fig. 9d), and maximum denudation likely occurred where less sediment was transported and deposited, thus reducing the sedimentation column. The seismic lines in the northern basin show smoother reflections with good continuity, high frequency, and low-to-medium amplitude. There are fewer fluctuations and the faults are steeper with a maximum throw. The maximum length of some of these normal faults extends beyond ~ 800 m, which strongly indicates that Lake Fuxian is a graben lake formed by fault subsidence with normal parallel faulting and an overall crustal extensional regime. The maximum subsidence in the northern basin provided the maximum space for sediment accumulation, thus, maximizing the sedimentation column (Fig. 3). The faults are parallel and show an extensional regime. Overall, the L10-2 has a maximum possible thickness of 1238 m and lies toward the NNW region in the Lake Fuxian basin (Fig. 9e).

In summary, the main controlling factor within the Lake Fuxian Basin is tectonics related to the normal faulting, which is the main cause of rifting and crustal extension and the formation of horst and graben structures. Other reasons include weathering and erosion (denudation), fill and subsidence, which contribute to the infilling of basin depressions, sediment accumulation rates (SAR), which are related to lake-level variations, sediment thickness distribution, and climate.

5.5 Depositional architecture of the Lake Fuxian basin

Tectonic lakes are responsible for the formation of some of the deepest and largest lakes in the world. These lakes are formed due to robust tectonic activities such as folding and faulting (Johnson 1984). Lake Fuxian is also a tectonic lake on the Yunnan Plateau, and it is assumed that this lake formed due to tectonic activity during the late Pleistocene (Cui et al. 2008). Lake Fuxian is the deepest lake among the nine lakes within the Yunnan Plateau because tectonic activity has created crustal extension, and alternating series of horst and graben structures have formed. The results of the present study provide evidence that an extensional regime formed within the rift-lake basin formed due to fault subsidence. The lake basin comprises several parallel normal faults on both flexural margins of the lake (Figs. 2, 3). The structures that are dominant within Lake Fuxian are grabens, half-grabens, and horst structures. The source of the sediments was highlands, which were triggered by high seismic activity. Additionally, the clastic sediment depositional systems were considerably influenced by several geological events. Due to strong tectonic uplift, the western and eastern banks of Lake Fuxian are hilly, with steep slopes and extensive regions of reasonably dry ground (Dai et al. 2017). In addition, numerous karst landforms and limestone mountains have developed within the basin (Li et al. 2019a, b). Therefore, the sedimentation of the carbonates in the lake was quite abundant.

There are three main types of depositional landforms within the Lake Fuxian basin: lacustrine alluvial plains, alluvial fans and terraces. The Lake Fuxian basin is characterized by strong vertical movement, with fault block activity as a basic feature. Uplift and subsidence formed a vertical offset in adjacent blocks, resulting in terraces formation. The faults are also evident in these small blocks, giving rise to half-grabens. The regular parallel spacing contours on the bathymetry map (Fig. 1c) show the presence of a vertical offset that creates a trough-shaped basin termed the graben block in our study.

Several U-shaped and V-shaped erosional structures are also evident within the lake basin. These structures were properly described and named SF-8, SF-9, SF-10, and SF11 in our study. These features reveal the presence of channels that are associated with variations in sediment provenance and water flow. The main lithologies associated with these features are sand and shales. However, the mound features named SF-12 and SF-13 are associated with velocity variations and show the presence of carbonate deposits. The slope-front deposits and fan facies suggest deep-water slope fan facies and delta fan facies.

The following conclusions are drawn regarding the depositional settings of the Lake Fuxian basin (Fig. 9b);

  1. (a)

    The sediment source originated from highlands, which were triggered by high seismic activity. Different geological episodes significantly controlled the clastic sediment depositional system. From a regional geological perspective, seismic facies, sedimentary sequences, and tectonic events in the lake basin and its surroundings uniformly triggered highly intense seismic activity, which governed the large geological basins in the region.

  2. (b)

    Lake Fuxian is the largest intermountain basin and the deepest graben in the Yunnan Plateau region. The dominant sedimentary units were deposited in deep calcareous sediments and shallow alluvial as well as lacustrine sediments.

  3. (c)

    They consist of open to marginal lacustrine deposits, breccia deposits and fluvial deposits. The upper units (probably Quaternary and Neogene) feature sequences of fan delta deposits, interbedded marginal lacustrine deposits, and fluvial deposits, whereas thick coarse-grained fan-delta deposits are interfingered at the foot of the main relief with fluvial-lacustrine deposits.

  4. (d)

    The two distinct disconformable sedimentary units represent the pre-syngenetic tectonic activity in the basin. The low-slip margin fine sediment rock units are pre-tectonic, while the high-slip margin coarse sediments are syngeneic due to high tectonic activity.

6 Conclusions

The main conclusions are as follows:

  1. 1.

    Five seismic sequences were interpreted. The oldest sequence (Sq-5) at L8 shows that the clinoforms terminate against the seismic section and that the sedimentation of the sedimentary cover can extend beyond the section to ~ 50 m to ~ 100 m. The deeper sequences (Sq-5 and Sq-4) show deposition during the prograding late lowstand and early lowstand stages. The deepest clinoforms show that the lowest lake level of the lake basin was ~ 210 m during late lowstand settings. Sq-3 shows the transition phase of transgressive stages. Several mound-shaped reflections in Sq-4 and Sq3 provide evidence of backstep** deltaic sedimentation, suggesting littoral and postlittoral zonations. Sq-2 shows the deposition of sediments during late highstand stages, during which several flooding surfaces were deposited, resulting in further retrogradation. However, Sq-1 provides insights into the initial phases of hemipelagic deposition in a profundal paleoenvironment.

  2. 2.

    It is inferred that the coastline migrated nearly 1.5 km on each side horizontally in the southern basin, approximately 2 km toward the ESE region in the central basin, and approximately 19 km to the SSW region in the northern basin at its current location.

  3. 3.

    The sedimentary thickness of the basin is highly variable. The maximum thickness of the sedimentation toward the southern basin is ~ 303 m at L1, ~ 348 m toward the central basin at L4, and ~ 1238 m toward the northern basin at L10-2.

  4. 4.

    The lake sediments toward the southern basin underwent more tectonic activity since the disruption and reflection patterns were more chaotic and dispersed at L2. In general, tectonic activity played an important role in crustal extension and the formation of horst and graben structures within the lake basin.

  5. 5.

    The lake basin can be characterized into three main depositional patterns that incorporate alluvial delta plains, alluvial fans, and terraces. The U- and V-shaped erosional patterns show the fluvial nature of lake sediments that are linked with paleochannels and associated levees. However, a mound-shaped pattern is linked with backstop** progradational settings and velocity variations.

  6. 6.

    The major geological controlling factor within the Lake Fuxian basin is crustal tectonics that related to the parallel normal faulting, which causes rifting and crustal extension and the formation of horst and graben structures. Secondary factors include fill and subsidence, SAR in relation to lake-level variations and sediment thickness distribution.

7 Recommendations

The proposed study lays the foundation for the Lake Fuxian drilling project. Since this pioneer study to utilized multichannel seismic refection profiles, the precision of the interpretations among seismic sequences and core lithologies must be checked by actual drilling activity.