Thermal and exhumation history of the Songnan Low Uplift, Qiongdongnan Basin: constraints from the apatite fission-track and zircon (U-Th)/He thermochronology
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Abstract:
Significant advancements have been made in the study of Mesozoic granite buried hills in the Songnan Low Uplift (SNLU) of the Qiongdongnan Basin. These findings indicate that the bedrock buried hills in this basin hold great potential for exploration. Borehole samples taken from the granite buried hills in the SNLU were analyzed using apatite fission track (AFT) and zircon (U-Th)/He data to unravel the thermal history of the basement rock. This information is crucial for understanding the processes of exhumation and alteration that occurred after its formation. Thermal modeling of a sample from the western bulge of the SNLU revealed a prolonged cooling event from the late Mesozoic to the Oligocene period (~80−23.8 Ma), followed by a heating stage from the Miocene epoch until the present (~23.8 Ma to present). In contrast, the sample from the eastern bulge experienced a more complex thermal history. It underwent two cooling stages during the late Mesozoic to late Eocene period (~80−36.4 Ma) and the late Oligocene period (~30−23.8 Ma), interspersed with two heating phases during the late Eocene to early Oligocene period (~36.4−30 Ma) and the Miocene epoch to recent times (~23.8−0 Ma), respectively. The differences in exhumation histories between the western and eastern bulges during the late Eocene to Oligocene period in the SNLU can likely be attributed to differences in fault activity. Unlike typical passive continental margin basins, the SNLU has experienced accelerated subsidence after the rifting phase, which began around 5.2 Ma ago. The possible mechanism for this abnormal post-rifting subsidence may be the decay or movement of the deep thermal source and the rapid cooling of the asthenosphere. Long-term and multi-episodic cooling and exhumation processes play a key role in the alteration of bedrock and contribute to the formation of reservoirs. On the other hand, rapid post-rifting subsidence (sedimentation) promotes the formation of cap rocks.
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Key words:
- granite buried hills /
- (U-Th)/He dating /
- fission-track dating /
- exhumation /
- Songnan Low Uplift
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1. Introduction
Granite buried hill oil and gas reservoirs have emerged as a novel area of interest in the field of oil and gas exploration. Notable discoveries of large granite bedrock oil and gas fields have been made in various countries such as Vietnam, Libya, Venezuela, and India (Pan et al., 2007), thereby opening up new avenues for exploration in buried hill formations. Consequently, geologists both domestically and internationally have increasingly focused their attention on bedrock oil and gas reservoirs (Ma et al., 2006). Notably, significant progress has been made in the exploration of buried-hill oil and gas reservoirs in Chinese waters, particularly in the Bohai Sea region (Deng, 2015; Hu et al., 2020; Xu et al., 2019a). In recent years, the drilling of high-quality gas reservoirs has been successful in the deltaic sandstone and granitic basement buried hill within the Y8-1 structure of the Songnan Low Uplift (SNLU) in the Qiongdongnan Basin (QDNB). Subsequently, thick and high-quality gas reservoirs with substantial gas flow rates have been discovered in the Mesozoic basement of the Y8-3 structure (Shi et al., 2019; Zhang et al., 2019). These findings indicate promising prospects for exploration in the basement burial-hill area of the SNLU and mark the beginning of a new era in natural gas exploration in the northern South China Sea (SCS).
The formation of large oil and gas fields is heavily dependent on favorable reservoir conditions. In the case of granite, which is inherently compact, it requires subsequent modifications to become an effective reservoir. Therefore, it is crucial to comprehend the evolutionary process of buried granite hills following their formation. Previous research suggests that the buried hills in the SNLU were formed during the Mesozoic era (Mi et al., 2023) and have undergone long-term uplift, weathering, and denudation (Tang et al., 2017; Xu et al., 2019b). However, due to limited data availability, the specific details regarding their post-formation uplift and exhumation history remain unclear, impeding our understanding of the exhumation weathering process of these buried hills.
Low-temperature thermochronological methods, such as zircon and apatite fission track (AFT) and (U-Th)/He dating, have proven to be effective tools for constraining shallow crustal processes (Chang et al., 2017, 2018; Ehlers and Farley, 2003; Gallagher et al., 1998; Qiu et al., 2010, 2014; Sehrt et al., 2017; Stockli, 2005; Tu et al., 2021). In this study, we present, for the first time, zircon (U-Th)/He and apatite fission track data of the basement granite in different tectonic units of the SNLU. Based on these data, we decipher the thermal and exhumation processes of the buried hills during the late Mesozoic-Cenozoic period and further discuss the underlying genetic mechanism. This research is significant as it enhances our understanding of the development period and modification conditions of the buried hill reservoir in the study area, providing valuable geological theory support for future exploration endeavors.
2. Geologic setting
The SCS is a large marginal basin in the western Pacific region that formed in a complex tectonic setting influenced by the Pacific, Indo-Australian, and Eurasian plates (Cullen et al., 2010). The initial rifting in the SCS likely began in the late Cretaceous to early Paleocene, when the stress field changed from compression to extension (Clift and Lin, 2001; Franke et al., 2014). Following multiple phases of rifting (Clift and Lin, 2001; Ru and Pigott, 1986), the continent broke apart and the SCS basin started to open (Briais et al., 1993; Li et al., 2014). The exact timing of seafloor spreading in the SCS is still debated, but it is generally believed to have started around 33 Ma ago and ended around 15.5 Ma ago (Li et al., 2014, 2015). Along the northern margin of the SCS, several sedimentary basins formed, including the Pearl River Mouth Basin (PRMB) and the QDNB (Fig. 1a).
Figure 1. Regional geological outline of the Qiongdongnan Basin (QDNB) (a), and the basin tectonic units (b). Fault distribution and observation points in b are from Zhou et al. (2019).The QDNB, located on the northwestern continental margin of the SCS, is a rift basin that developed on the Pre-Cenozoic basement. It is bounded by Hainan Island to the north, the Yinggehai Basin to the west, and the PRMB to the east (Fig. 1a). The basin can be divided into five tectonic belts from north to south: the Northern Depression, Central Uplift, Central Depression, Southern Uplift, and Southern Depression (Fig. 1b). Throughout the Cenozoic era, the basin underwent three stages: syn-rifting, post-rifting thermal subsidence, and post-rifting accelerated subsidence (Fig. 2) (Clift and Sun, 2006; Ren et al., 2014; Shi et al., 2017; Zhao et al., 2013). Sequence stratigraphy has been established based on seismic horizons and drilling well data, including the Lingtou Formation (Eocene), Yacheng Formation (lower Oligocene), Lingshui Formation (upper Oligocene), Sanya Formation (lower Miocene), Meishan Formation (middle Miocene), Huangliu Formation (upper Miocene), Yinggehai Formation (Pliocene), and Ledong Formation (Quaternary) (Cheng et al., 2021) (Fig. 2).
Figure 2. Comprehensive stratigraphic column of the Qiongdongnan Basin modified from Ji et al. (2021), Ren et al. (2022), and Wang et al. (2015).The study area, known as the SNLU, is located in the Central Depression of the QDNB, adjacent to the Lingshui Sag, Songnan-Baodao Sag, and Changchang Sag. The low uplift, which generally trends in the east-west direction, has been affected by secondary faults and can be further divided into the north bulge, west bulge, and east bulge (see Fig. 1b). In the study area, the Eocene Lingtou Formation is generally absent, and the Yacheng and Lingshui Formations are partially missing (Zhou et al., 2019).
3. Sample description
Two cutting samples were collected from the hydrocarbon exploration boreholes Q1 and Q12, which were drilled by the China National Offshore Oil Company. Borehole Q1 was situated on the western protrusion, while Borehole Q12 was situated on the eastern protrusion of the SNLU (refer to Fig. 1 for locations and Table 1 for sample details).
Table 1. Sample informationSample U-Pb age/Ma Burial temperature/℃ Lithology Overlying strata Q1 228.9 ± 1.0 ~ 63 quartaz monzonite Sanya Formation Q12 270.0 ± 1.2 ~ 75 quartaz monzonite Yacheng Formation | Show Table
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The major and trace element compositions of the two samples have been previously reported by Mi et al. (2023). Sample Q1 exhibit contents of SiO2 (63.43 wt%), Al2O3 (12.67 wt%), MgO (0.47 wt%), Fe2O3 (1.82 wt%), and CaO (13.06 wt%). Sample Q12 exhibit contents of SiO2 (64.56 wt%), Al2O3 (15.39 wt%), MgO (1.38 wt%), Fe2O3 (4.92 wt%), and CaO (4.76 wt%). These compositions place both samples within the quartz monzonite fields in the Total alkali silica classification diagram (Fig. 3a) and classify them as the high K calc-alkaline series (Fig. 3b). The plot of Ga/Al vs. Ce categorizes the samples into the I- and S-type field (Fig. 3c). Furthermore, the A/NK versus A/CNK plot (Fig. 3d) and the Rb versus Th plot (Fig. 3e) confirm their classification as I-type samples.
Figure 3. Sample geochemical features. a. SiO2 vs. K2O+Na2O (Middlemost, 1994); b. SiO2 vs. K2O scheme (Rickwood, 1989 ); c. 10 000 Ga/Al vs. Ce plot (Whalen et al., 1987); d. A/CNK vs. A/NK diagram (Maniar and Piccoli, 1989); e. Rb vs. Y plot (Chappell, 1999); and f. (Y+Nb) vs. Rb plot (Pearce et al., 1984).Normalized to chondrite, the quartz monzonites in this study show enrichment of light rare earth elements (LREE) and negative anomalies of Eu (Fig. 4a). When normalized to the primitive mantle, the samples exhibit positive anomalies in K and Pb, and negative anomalies in Nb, Ta, P, Zr, and Ti, (Fig. 4b). The Rb vs. Y + Nb plot (Fig. 3f) indicates that the samples fall into the volcanic arc field, as described by Pearce et al. (1984). Additionally, the rocks exhibit well-defined negative anomalies of Ta, Nb, and Ti (Fig. 4b), presenting the characteristics of subduction-related magmas, as defined by Sajona et al. (1993). In conclusion, the samples in this study display characteristics consistent with I-type affinity and were likely formed in a volcanic arc environment, possibly associated with the subduction of the Paleo-Pacific plate (Mi et al., 2023).
Figure 4. Chondrite-normalized REE patterns (a) and primitive mantle normalized trace element patterns (b) of the basement samples.4. Analytical methods
Zircon and apatite grains separation were performed at the Chengxin Geological Services Co. LTD, in Langfang, Hebei Province, China, following the standard heavy mineral separation techniques.
4.1 Apatite fission track
Apatite fission track (AFT) analysis was conducted at the State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration. Apatite fission track ages were obtained using the LA-ICP-MS method and calculated by the zeta calibration method (Hasebe et al., 2004; Pang et al., 2017). NIST612 was used as an external standard to measure the signal intensity and Durango apatite (31.4 Ma ± 0.5 Ma) was chosen as the age-calibration standard. Spontaneous fission tracks were etched in 5.5 N HNO3 at 21℃ for 20 s.
χ2 test was performed to assess the homogeneity of AFT ages. When P(χ2) > 5%, the single-grain AFT ages are assumed to belong to the same age population and relate to the same thermal event, then the central age is adopted as the AFT age of the sample. Otherwise, a P(χ2) of < 5% indicates a heterogeneous age distribution, with the resulting age being mixed.
4.2 Zircon (U-Th)/He
For zircon (U-Th)/He (ZHe) dating, zircon grains were picked after the recommendations of Farley (2002). Single zircon grains were loaded into Nb micro-tubes, then in-vacuum degassed by laser at ~1250℃ and analyzed for 4He by isotope dilution on noble-gas mass-spectrometer using the Helium extraction line in the State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration. Degassed zircon was dissolved and analyzed for U and Th on ICP-MS according to the procedures of Evans et al. (2005). Standard α-ejection correction and volume estimates were made using measured grain dimensions and assuming an orthorhombic prism geometry with pyramid terminations according to Ketcham et al. (2011). More details in method information can be found in Li et al. (2017).
5. Results
5.1 AFT results
The AFT dating results are shown in Table 2 and Fig. 5. All the samples pass the χ2 test, demonstrating that all the single-grain ages form a single population. Sample Q1 yielded a central age of 69.2 Ma ± 2.6 Ma. Confined track length distributions are unimodal with the MTL (mean confined track length) of (12.26 ± 0.28) μm. Sample Q12 presented a central age of 60.1 Ma ± 3.4 Ma. The confined track length distribution is unimodal with the MTL of 11.79 μm and a standard deviation of 1.26 μm. The short MTL values imply the thermal annealing of fission tracks.
Table 2. Apatite fission-track dataSample Nc Ns $\rho_{\rm{s}} $ /(105cm−2) 238U /10−6 P($\chi $2)/% Central age (Ma ± 1$\sigma $) NL MTL (μm ± 1$\sigma $) SD Dpar (μm ± SD) Q1 32 825 2.427 7.22 68 69.2 ± 2.6 24 12.26 ± 0.28 1.39 1.71 ± 0.23 Q12 33 367 3.22 10.49 64 60.1 ± 3.4 18 11.79 ± 0.29 1.26 1.51 ± 0.13 Nc: number of apatite crystals analyzed; Ns: total number of fission tracks counted; $\rho_{\rm{s}} $: spontaneous track density; P($\chi $2): chi-square probability that all single-crystal ages represent a single population of ages where degrees of freedom = Nc-1; NL: number of confined track lengths measured: MTL: Mean confined track length; SD: standard deviation; Dpar: mean track etch pit diameter parallel to the crystallographic c-axis; Apatite-Zeta NIST610 = 1 940 ± 50. | Show TableDownLoad:
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Figure 5. Radial plots of apatite fission-track (left) and confined track length histograms (right). Central ages are calculated using RadialPlotter (Vermeesch, 2009). MTL-mean track length, SD-standard deviation, NL-number of spontaneous tracks.5.2 ZHe results
Seven single grains from the two samples were analyzed by ZHe dating (Table 3). Three zircon grains from Sample Q1 yielded ZHe ages of 486.1 Ma ± 11.6 Ma (Q1-1), 248.5 Ma ± 5.8 Ma (Q1-2), and 103.7 Ma ± 6.5 Ma (Q1-3). Q1-1 and Q1-2 were excluded because the ZHe ages were older than the granitoid emplacement age (228.9 Ma ± 1.0 Ma) (Mi et al., 2023). Q1-3 was not considered in the following thermal history modeling because single grain ZHe age is not convincing. For Sample Q12, excluding the outlier of 152.7 Ma ± 2.64 Ma (Q12-4), other three zircon grains from the sample yielded ZHe ages of 45.3 Ma ± 1.0 Ma to 73.9 Ma ± 1.3 Ma, with a mean of 61.4 Ma ± 1.3 Ma.
Table 3. Zircon (U-Th)/He dataSample 238U /10−6 ±1$\sigma $ /10−6 232Th /10−6 ±1$\sigma $/10−6 He (ncc) ±1$\sigma $ /ncc Unc. age/Ma ±1$\sigma $/Ma Rs/μm FT Cor. age/Ma ±1$\sigma $/Ma Q1-1 100.5 2.4 41.7 1.0 21.454 1 0.258 5 368.5 8.8 46.7 0.758 486.1 11.6 Q1-2 97.7 2.3 34.0 0.9 16.197 5 0.160 5 205.1 4.8 51.5 0.825 248.5 5.8 Q1-3 222.4 5.0 56.0 1.2 22.400 5 0.221 5 83.4 1.9 57.4 0.804 103.7 2.4 Q12-1 2013.2 44.6 490.6 11.2 26.036 3 0.280 8 47.2 1.1 40.7 0.727 65.0 1.5 Q12-2 1711.5 36.5 487.5 12.3 12.010 9 0.132 8 31.2 0.7 34.5 0.689 45.3 1.0 Q12-3 715.1 11.5 174.3 2.3 12.912 5 0.130 8 57.3 1.0 38.5 0.775 73.9 1.3 Q12-4 334.17 5.02 79.89 0.96 11.347 8 0.114 9 116.3 2.01 37.0 0.762 152.7 2.64 Rs: sphere equivalent radius of hexagonal crystal; FT: alpha ejection correction factor. | Show TableDownLoad:
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6. Thermal history reconstruction
6.1 Modeling approach
HeFty (Ketcham et al., 2018) was used to further investigate the thermal evolution of basement samples of the SNLU. ZHe data were modeled using the He diffusion kinetic parameters from Guenthner et al. (2013). Alpha-particle stopping distances and α-ejection age corrections were conducted after the method in Ketcham et al. (2011). Apatite fission track ages and projected fission track lengths with Dpar as a kinetic parameter are included in the annealing model of Ketcham et al. (2007).
We follow a model strategy that was described in Tang et al. (2019). Besides the AFT and ZHe results achieved in this work, the following border conditions for the modeling were also assumed: (1) initial early constraint, placed above Tc of highest-T system or population modeled, at least 1.5 times of the oldest age; (2) the paleo-surface temperature when the samples deposited assumed to be (15 ± 10)℃; and (3) the present-day burial temperature.
6.2 Modeling results
All modeled paths presented in Fig. 6 resulted in high goodness of fit (GOF > 0.86) between the measured input data and modeled output data.
Figure 6. Modeling results for Sample Q1 (a) from the western bulge of the Songnan Low Uplift and Sample Q12 (b) from the eastern bulge. Illustrated are the t-T paths on the left (a1 and b1) with the corresponding confined fission-track length frequency distribution (a2 and b2) and the ZHe diffusion profile (b3) on the right. The t-T paths on the left show different fits: green paths, acceptable fit (GOF ≥ 5%); pink paths, good fit (GOF ≥ 50%); black line, weighted mean path.Although the two samples are located next to each other, they have experienced different thermal histories (Fig. 6). Sample Q1 from the western bulge of the SNLU shows an overall pattern with a two-phase thermal history: (1) a protracted cooling from the late Cretaceous to the early Miocene, and (2) a subsequent heating stage until the recent times (Fig. 6a1). Comparatively, Sample Q12 from the eastern bulge of the SNLU has undergone a more complex thermal evolution, with an overall pattern of four stages: (1) a major late Cretaceous-late Eocene cooling, (2) heating during the period of late Eocene-early Oligocene, (3) a minor cooling in the late Oligocene, and (4) a reheating from the early Miocene till present (Fig. 6b1).
7. Discussions
Constrained by the AFT and ZHe data, thermal history modeling results, as the weighted mean t-T paths presented in Fig. 7, indicate that samples from the western and eastern bulge of the SNLU have experienced differential cooling and corresponding exhumation during the late Eocene to Oligocene, followed by heating from the early Miocene to recent times, which was characterized with a rapid heating phase since the early Pliocene.
Figure 7. Comparative presentation of weighted mean paths from thermal models. The dashed line is the weighted mean thermal history for Sample Q12 from the east bulge, and the solid line is the weighted mean thermal history for Sample Q1 from the west bulge.7.1 Origin of the differential exhumation on the west and east bulge of the Songnan Low Uplift
Fault activity plays a crucial role in the formation and evolution of basins (Xie et al., 2007). In the QDNB, basement faults are predominantly oriented in the NE-SW direction (Fig. 1). Among these faults, No.2 and No.11 are the primary controlling factors influencing the tectonic evolution of the SNLU (Zhou et al., 2019). During the late Eocene to early Oligocene, the No.11 fault exhibited higher activity intensity compared to the No.2 fault (Fig. 8). Spatially, the No.11 fault displayed stronger activity in the west and weaker activity in the east (Fig. 8b), resulting in a corresponding high-to-low gradient in the SNLU with an overall eastward dip. On the other hand, the No.2 fault exhibited clear segmentation and can be divided into the Songnan section and Baodao section, both with similar activity rates (Fig. 8a) (Zhou et al., 2019). Consequently, while the western bulge experienced uplift and denudation (cooling) between approximately 36 Ma and 30 Ma, the eastern bulge underwent heating due to sedimentation of the Yacheng Formation.
Figure 8. Activity rate of the main controlling faults in the Songnan Low Uplift during the Eocene-early Oligocene and late Oligocene. a. No.2 fault, b. No.11 fault (Zhou et al., 2019). For locations of the observation points see Fig. 1b.During the late Oligocene, the activity intensity of the middle part of the No.2 fault significantly increased, reaching up to 550 m/Ma (Fig. 8a). As a result, the eastern bulge, located on the hanging wall of the No.2 fault, became warped and uplifted. Meanwhile, the No.11 fault entered an inheriting activity stage, with the activity center gradually shifting eastward (Fig. 8b). Consequently, the western bulge continued to uplift, and the activity of the No.11 fault influenced the eastern bulge, accelerating its uplift (Zhou et al., 2019). Consequently, both the western and eastern bulges of the SNLU experienced cooling and exhumation between approximately 30 Ma and 23.8 Ma (Fig. 7). In summary, differential fault activities resulted in the differential exhumation of the western and eastern bulges of the SNLU during the late Eocene to Oligocene.
7.2 Post-rifting accelerated subsidence and its origin
During the early Miocene, the process of rifting weakened and large-scale faulting ceased. Subsequently, the QDN (name of the region) entered a post-rifting stage (Xie et al., 2007; Zhao et al., 2013). Based on the modeled temperature-time (t-T) paths, the temperature gradually increased from approximately 12.6−30.8℃ to a range of 31.3−54.9℃ between ~ 22−5.2 Ma. From around 5.2 Ma onwards, the temperature rapidly increased to its present state (Fig. 6). This rapid heating is commonly attributed to enhanced sedimentation caused by accelerated subsidence during the post-rifting stage. This is supported by the simultaneous occurrence of rapid sedimentation (Cheng et al., 2021; Zhao et al., 2013, 2015) and anomalous subsidence in the QDNB region, as observed in previous studies (Li et al., 2012; Mao et al., 2015; Shi et al., 2017; Xie et al., 2006; Yuan et al., 2008; Zhao et al., 2018; 2013). Assuming a geothermal gradient of 31.9℃/km (Yuan et al., 2009), the thermal models suggest a subsidence rate of approximately 37−47 m/Ma during the thermal subsiding stage, but as high as 119−152 m/Ma during the accelerated subsiding stage. These subsidence rates align with values obtained through seismic interpretation (Zhao et al., 2013).
The observed features in the QDNB region deviate from the expected exponential decay pattern of thermal subsidence after rifting, as observed in the typical Atlantic passive margin (McKenzie, 1978; Steckler and Watts, 1978). While the phenomenon is well-established and widely accepted, the underlying tectonic mechanism remains a subject of debate. Several hypotheses have been proposed to explain the rapid subsidence observed in the QDNB region following rifting. These include the possibility of a new rifting episode and polarity change of the Red River fault (Yuan et al., 2008), dynamic topography (Xie et al., 2006), and lower crustal flow (Lei et al., 2013; Zhao et al., 2013).
It has been suggested that further crustal thinning resulting from a new rifting episode could lead to rapid post-rifting subsidence. However, faulting analyses indicate that there has been minimal faulting since the late Miocene (Zhao et al., 2013), making a new rifting episode unlikely. Another proposal suggests that the dextral strike-slip motion of the Red River fault in the late Miocene could be responsible for the rapid post-rifting subsidence (Yuan et al., 2008). While this fault motion may explain the rapid subsidence observed in the western QDNB region, it fails to account for the larger magnitude of subsidence observed in the eastern QDNB region (Shi et al., 2017; Yang et al., 2015). In theory, dynamic topography is a possible mechanism to cause abnormal post-rifting subsidence. However, the calculation of the dynamic topography model involves numerous uncertain parameters, leading to controversy regarding the predicted amount of abnormal subsidence (Xie et al., 2006). The lower crustal flow model, as suggested by Morley and Westaway (2006), contradicts gravitational buoyancy forces and is deemed unlikely to be valid (Allen et al., 2004). This model posits that lower crustal material would flow from the basin area (low pressure) to the sediment source area (high pressure). However, the uniformity in the whole crust stretch, as indicated by the thinning factors of the entire crust and the upper crust, suggests that lower crustal flow is improbable (Shi et al., 2017; Yang et al., 2015).
It is important to note that all the aforementioned hypotheses solely focus on the mechanism associated with the rapid post-rifting subsidence episode. However, observations in the QDNB reveal not only additional post-rifting thermal subsidence but also a deficit in syn-rifting tectonic subsidence (Shi et al., 2017). Therefore, we favor the mechanism proposed by Shi et al. (2017), which suggests that the rapid post-rifting subsidence may be linked to the decay of a deep thermal anomaly and the swift cooling of the asthenosphere. Evidence of the deep thermal anomaly is supported by magmatic intrusions identified through drilling and seismic data in the QDNB (Lu et al., 2011; Tang et al., 2013), as well as the presence of continuous low-velocity bodies beneath the adjacent areas of Hainan Island, as revealed by tomography (Lei et al., 2009).
8. Conclusions
This study presents the first dataset of AFT and ZHe thermochronology for the SNLU in the QDNB, located in the northern SCS. The findings of this study can be summarized as follows:
(1) The granite buried hills in the SNLU underwent long-term cooling and subsequent uplift after their formation. However, differential exhumation occurred in the western and eastern bulges during the late Eocene to Oligocene period.
(2) During the late Eocene to early Oligocene, the west bulge experienced uplift and denudation, while the east bulge was subjected to deposition and burial. Subsequently, both the west and east bulges uplifted and eroded together during the Late Oligocene. The differential exhumation evolution was primarily influenced by the varying activity of faults.
(3) In contrast to typical passive continental margin basins, both the west and east bulges underwent an accelerated post-rifting subsidence stage from approximately 5.2 Ma ago to the present. This phenomenon is likely attributed to the decay of a deep thermal anomaly and rapid cooling of the asthenosphere.
Acknowledgements: We are grateful to Jianzhang Pang and Ying Wang for their help during the analyses of the samples. We give our sincerest thanks to three anonymous reviewers for their constructive comments that improved the manuscript greatly. -
Figure 1. Regional geological outline of the Qiongdongnan Basin (QDNB) (a), and the basin tectonic units (b). Fault distribution and observation points in b are from Zhou et al. (2019).
Figure 2. Comprehensive stratigraphic column of the Qiongdongnan Basin modified from Ji et al. (2021), Ren et al. (2022), and Wang et al. (2015).
Figure 3. Sample geochemical features. a. SiO2 vs. K2O+Na2O (Middlemost, 1994); b. SiO2 vs. K2O scheme (Rickwood, 1989 ); c. 10 000 Ga/Al vs. Ce plot (Whalen et al., 1987); d. A/CNK vs. A/NK diagram (Maniar and Piccoli, 1989); e. Rb vs. Y plot (Chappell, 1999); and f. (Y+Nb) vs. Rb plot (Pearce et al., 1984).
Figure 4. Chondrite-normalized REE patterns (a) and primitive mantle normalized trace element patterns (b) of the basement samples.
Figure 5. Radial plots of apatite fission-track (left) and confined track length histograms (right). Central ages are calculated using RadialPlotter (Vermeesch, 2009). MTL-mean track length, SD-standard deviation, NL-number of spontaneous tracks.
Figure 6. Modeling results for Sample Q1 (a) from the western bulge of the Songnan Low Uplift and Sample Q12 (b) from the eastern bulge. Illustrated are the t-T paths on the left (a1 and b1) with the corresponding confined fission-track length frequency distribution (a2 and b2) and the ZHe diffusion profile (b3) on the right. The t-T paths on the left show different fits: green paths, acceptable fit (GOF ≥ 5%); pink paths, good fit (GOF ≥ 50%); black line, weighted mean path.
Figure 7. Comparative presentation of weighted mean paths from thermal models. The dashed line is the weighted mean thermal history for Sample Q12 from the east bulge, and the solid line is the weighted mean thermal history for Sample Q1 from the west bulge.
Figure 8. Activity rate of the main controlling faults in the Songnan Low Uplift during the Eocene-early Oligocene and late Oligocene. a. No.2 fault, b. No.11 fault (Zhou et al., 2019). For locations of the observation points see Fig. 1b.
Table 1. Sample information
Sample U-Pb age/Ma Burial temperature/℃ Lithology Overlying strata Q1 228.9 ± 1.0 ~ 63 quartaz monzonite Sanya Formation Q12 270.0 ± 1.2 ~ 75 quartaz monzonite Yacheng Formation 
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Table 2. Apatite fission-track data
Sample Nc Ns $\rho_{\rm{s}} $ /(105cm−2) 238U /10−6 P($\chi $2)/% Central age (Ma ± 1$\sigma $) NL MTL (μm ± 1$\sigma $) SD Dpar (μm ± SD) Q1 32 825 2.427 7.22 68 69.2 ± 2.6 24 12.26 ± 0.28 1.39 1.71 ± 0.23 Q12 33 367 3.22 10.49 64 60.1 ± 3.4 18 11.79 ± 0.29 1.26 1.51 ± 0.13 Nc: number of apatite crystals analyzed; Ns: total number of fission tracks counted; $\rho_{\rm{s}} $: spontaneous track density; P($\chi $2): chi-square probability that all single-crystal ages represent a single population of ages where degrees of freedom = Nc-1; NL: number of confined track lengths measured: MTL: Mean confined track length; SD: standard deviation; Dpar: mean track etch pit diameter parallel to the crystallographic c-axis; Apatite-Zeta NIST610 = 1 940 ± 50.
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Table 3. Zircon (U-Th)/He data
Sample 238U /10−6 ±1$\sigma $ /10−6 232Th /10−6 ±1$\sigma $/10−6 He (ncc) ±1$\sigma $ /ncc Unc. age/Ma ±1$\sigma $/Ma Rs/μm FT Cor. age/Ma ±1$\sigma $/Ma Q1-1 100.5 2.4 41.7 1.0 21.454 1 0.258 5 368.5 8.8 46.7 0.758 486.1 11.6 Q1-2 97.7 2.3 34.0 0.9 16.197 5 0.160 5 205.1 4.8 51.5 0.825 248.5 5.8 Q1-3 222.4 5.0 56.0 1.2 22.400 5 0.221 5 83.4 1.9 57.4 0.804 103.7 2.4 Q12-1 2013.2 44.6 490.6 11.2 26.036 3 0.280 8 47.2 1.1 40.7 0.727 65.0 1.5 Q12-2 1711.5 36.5 487.5 12.3 12.010 9 0.132 8 31.2 0.7 34.5 0.689 45.3 1.0 Q12-3 715.1 11.5 174.3 2.3 12.912 5 0.130 8 57.3 1.0 38.5 0.775 73.9 1.3 Q12-4 334.17 5.02 79.89 0.96 11.347 8 0.114 9 116.3 2.01 37.0 0.762 152.7 2.64 Rs: sphere equivalent radius of hexagonal crystal; FT: alpha ejection correction factor.
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