Buried Jurassic rift system in the middle segment of the northern Qaidam Basin: Implications for Mesozoic landforms of the northern Tibetan Plateau

ABSTRACT The Jurassic is one of the most important periods generating hydrocarbon source rocks and coals in Northwest China, and the Qaidam Basin is an important petroliferous and coal-forming basin. However, a series of explorations in the Qaidam Basin targeting Jurassic source rocks have failed over the past several years. These failures were mainly caused by missing the targeted Jurassic strata, suggesting that the current understanding of the Jurassic distribution in the Qaidam Basin is still limited. Therefore, this study conducts detailed analyses of the drilling wells and seismic data in the middle segment of the northern Qaidam Basin. The seismic profiles and borehole data indicate a rifting tectonic setting with discontinuous deposition and rapid facies transition during the Jurassic. A well-preserved buried rift valley proved in the Hongnan region reveals that the Jurassic deposits are mainly restricted in the narrow rift system. Therefore, further exploration targeting the Jurassic source rocks should focus on delineating the distribution of the Jurassic rift system in the Qaidam Basin, combined with the recovery of the complex Cenozoic tectonic overprinting. Graphic abstract


Introduction
After the formation of the Central Asian Orogenic Belt, considerable Jurassic sediments were widely distributed in extensional tectonic environment in Northwest China. Except the Ordos Basin (Liu, 1998;Liu & Yang, 2000;Liu et al. 2013), widespread extensional developed in the Tarim Basin, Junggar Basin and Qaidam Basin (Ritts et al. 1999Sobel 1999;Vincent and Allen 1999;Hendrix 2000;Ritts and Biffi 2000;Chen et al. 2003;Yang et al. 2004;Wang et al. 2005;Jolivet 2015). The Jurassic is one of the most important periods in terms of generating hydrocarbon source rocks and coals in Northwest China. Therefore, the Jurassic sediments in Northwest China can provide significant information on the geological evolution of Inner Asia as well as its hydrocarbon migration and accumulation, attracting widespread attention from both geologists and explorers.
The Qaidam Basin is located in the northern Tibetan Plateau, bounded by the Eastern Kunlun Mountains to the south, the Qilian Mountains to the northeast and the Altyn Mountains to the northwest (Métivier et al. 1998;Meyer et al. 1998;Jolivet et al. 2001Jolivet et al. , 2022Wang et al. 2006;Yin et al. 2007Yin et al. , 2008Kapp et al. 2011;Yuan et al. 2013;Wu et al. 2016Wu et al. , 2019Zuza et al. 2016Zuza et al. , 2018. Owing to its unique geographic location, the Qaidam Basin has become a key feature to explore the evolution of the northern Tibetan Plateau (Yin et al. 2008a(Yin et al. , 2008b, and numerous researchers have focused on its Mesozoic geological evolution. For example, Xia et al. (2001) proposed that the Qaidam Basin comprises a Jurassic foreland basin and a Cenozoic tectonic inversed extensional rifted basin. However, a series of researchers hold the opinion that the northern Qaidam Basin was in an extensional tectonic setting until the Early Cretaceous, developing a series of half-graben structures during the Jurassic (Di and Wang 1991;Chen et al. 2005;Lou et al. 2009). A third opinion is that the Qaidam Basin was in an extensional tectonic environment during the Early and Middle Jurassic, which changed to a contractional tectonic setting since the Late Jurassic or the Early Cretaceous Zeng et al. 2021;Liu et al. 2010;Wu et al. 2011;Tang et al. 2017;Cheng et al. 2019). At the same time, a series of detrital U-Pb zircon dating studies were conducted to analyse the source of the Jurassic deposits in the Qaidam Basin in detail, revealing complex routings of detrital materials (Yu et al. 2017cZhao et al. 2020aZhao et al. , 2020bQian et al. 2021;Zeng et al. 2021).
During the 1950s, petroleum exploration began in the northern Qaidam Basin, mainly targeting the Jurassic strata. However, the exploration of the Jurassic strata faced serious difficulties over the past several years. A series of drilling wells missed their targets, resulting in failed petroleum exploration. These drilling wells (yellow squares in Figure 2) include Dedong 1, Aibei 1 and Deye 1 Wells (No. 47,45,46 in Figure 2) in the Delingha Sub-Basin, Su 3 Well (No. 21 in Figure 2) in the Sugan Lake Basin, Teng 1 Well (No. 19 in Figure 2) in the Saishiteng region, Dongling 1 and Xing 1 Wells (No. 38,27 in Figure 2) in the Dahonggou region, and Lengbei 1 and 2 Wells (No. 10,9 in Figure 2) in the Lenghu region. These exploration failures were mainly caused by the following aspects. First, although thousands of wells have been drilled in the Qaidam Basin, so far, only approximately 30 wells have penetrated the top and bottom of the entire Jurassic strata (Supp . Table S1). Meanwhile, these wells are concentrated on only a few locations in the Qaidam Basin, making it difficult to comprehensively describe the whole picture of the Jurassic distribution. Second, the Silurian, Devonian, Carboniferous, Permian, and Triassic strata in the Qaidam Basin also show good stratification in their seismic tomography. It is easy to mistake these strata as the Jurassic strata, leading to misinterpretation of the seismic profiles.
In short, there remains a huge gap between the researchers' understanding and the actual Jurassic tectonic pattern and landforms of the Qaidam Basin. This gap has led to a widespread debate on the Mesozoic evolution of the Qaidam Basin and the recent failures of the exploration targeting the Jurassic hydrocarbon source rocks in the northern Qaidam Basin. Therefore, detailed work on the Mesozoic evolution of the Qaidam Basin needs to be conducted. This study addresses this problem through the interpretation of seismic profiles and boreholes in the middle segment of the northern Qaidam Basin.

Geological background
With an area of 120,000 km 2 , the Qaidam Basin is one of the most important petroliferous basins in Northwest China ( Figure 1). It has attracted a great deal of attention as a long-lived non-marine basin that includes a range of freshwater and saline lacustrine petroleum systems (Ritts et al. 1999;Hanson et al. 2001;Jin et al. 2002;Yang et al. 2003). Based on the lithological analyses of drilling samples (Yu et al. 2017a), palaeomagnetic data (Yu et al. 2014a(Yu et al. , 2014b, and calculations of effective elastic thickness (Braitenberg et al. 2003;Jordan and Watts 2005), the core part of the Qaidam Basin is thought to be of the Late Archaean to Early Proterozoic rigid basement the same as that in the Tarim Basin. Subsequently, the Precambrian-Silurian metamorphic rocks were overlain by Devonian-Cenozoic sedimentary strata (Huang et al. 1996;Lu et al. 2008;Yin et al. 2008aYin et al. , 2008bCheng et al. 2021).
By convention, the Xiaomeigou, Dameigou, Caishiling, and Hongshuigou Formations are assigned to the Jurassic strata, and the Quanyagou Formation is assigned to the Cretaceous. Detailed description of the Jurassic and the Cretaceous strata have been conducted in previous studies (e.g. Yang et al. 2003;Wu et al. 2011;Shu et al. 2017;Cheng et al. 2019;Zhao et al. 2020aZhao et al. , 2020bQian et al. 2021;Zeng et al. 2021). Previous geochemistry studies in the Qaidam Basin revealed a complex climate transition during the Jurassic (Wang et al. 2005Cao et al. 2008Cao et al. , 2012Jian et al. 2013;Qin et al. 2018;Guo et al. 2019;Hu et al. 2020;Liu et al. 2020Liu et al. , 2022Lu et al. 2020;Xie et al. 2021). The whole  Jin and Zhang (1998    Qaidam Basin transferred into an exhumation region during the Late Cretaceous, while the Upper Cretaceous is missing; then, the Qaidam Basin became endorheic and accumulated Cenozoic sediments with an average thickness of ~8 km (Yu et al. 2015a(Yu et al. , 2015bCheng et al. 2019;. Coal layers in the Jurassic strata are widely distributed in the Qaidam Basin, with many being exploited as a series of coalfields, such as the Aimunike South Coalfield (Figure 3 Figures 1 and 4). Detail Jurassic lithological descriptions and analyses have been conducted by previous study Wu et al. 2011;Shu et al. 2017;Tang et al. 2017;Cheng et al. 2019;Qian et al. 2021). According to the drilling well data from the Qinghai Oilfield, the Lower Jurassic Xiaomeigou Formation in Yuanqiu 1 Well mainly comprises light-grey sandy mudstone, muddy siltstone, and fine sandstone, as well as grey-black muddy siltstone with dark-black carbonaceous mudstone, coalbed and microconglomerate, grey-black mudstone and sandy siltstone, brownish-red mudstone, sandy mudstone, muddy siltstone, and brownish-grey sandstone. The lower part of the Middle Jurassic Dameigou Formation in Ku 1 Well is mainly psephitic sandstone, gravel-bearing inequigranular sandstone, and conglomerate with grey mudstone as well as sandy mudstones, which experienced differential weathering. Some lithological segments experienced serious weathering, and the weathered products are mainly grey-white kaolin. The upper part of the Middle Jurassic Dameigou Formation in Ku 1 Well mainly consists of grey, dark-grey mudstone, sandy mudstone, muddy siltstone, and siltstone with minor amounts of brown mudstone as well as sandy mudstones, brownishgrey psephitic sandstone, gravel-bearing inequigranular sandstone, and black carbonaceous shale. In Ku 1 Well, the Upper Jurassic Hongshuigou and Caishiling Formation are mainly brownish grey, tan, brownish red mudstone and sandy mudstone with brownish yellow grey and light grey muddy siltstone, as well as siltstone with a minor amount of brownish mudstone, sandy mudstone, muddy siltstone, brownish psephitic sandstone, gravel-bearing inequigranular sandstone and conglomerate.
The lower Cretaceous Quanyagou Formation in the Qaidam Basin is mainly purple red thick conglomerate mixed with pebbly sandstone or greenish grey feldspathic quartz sandstone, while the Qaidam Basin is lack of the upper Cretaceous (Wu et al. 2011). For example, the Cretaceous strata in the Ku 1 Well are mainly brownish grey mudstone, sandy mudstone, muddy siltstone, psephitic sandstone, gravel-bearing inequigranular sandstone, gravel-bearing fine sandstone and conglomerate, with a little brownish red mudstone, sandy mudstone and muddy siltstone.
Since the early Cenozoic, the Qaidam Basin was surrounded as an endorheic basin and accumulated thick Cenozoic strata (~8 km, Yin et al. 2008aYin et al. , 2008b;  Yu et al. 2015aYu et al. , 2015b. The Cenozoic strata in the Qaidam Basin are classified into the Palaeocene to early Eocene Lulehe Formation, the middle to late Eocene Xiaganchaigou Formation, the Oligocene Shangganchaigou Formation, the early to middle Miocene Xiayoushashan Formation, the late Miocene Shangyoushashan Formation, the Pliocene Shizigou Formation, the early Pleistocene Qigequan Formation and the middle to late Pleistocene and the Holocene (Yin et al. 2008a(Yin et al. , 2008bCheng et al. 2021). Meanwhile, the Qaidam Basin experienced intense Cenozoic compressional stress field during the India-Asian collision (e.g. Jolivet et al. 1999Jolivet et al. , 2001Jolivet et al. , 2003Jolivet et al. , 2022Yin et al. 2008a, b;Yuan et al. 2013;Wu et al. 2016Wu et al. , 2019Zuza et al. 2016Zuza et al. , 2018Cheng et al. 2021Cheng et al. , 2022Yu et al. , 2022.

Borehole data
Since 1954, thousands of wells have been drilled in the Qaidam Basin, of which more than 200 wells proved the existence of the Jurassic strata or not. In this study, we use all these wells and the outcrop data (Jin and Zhang 1998) to recover the distribution of the Jurassic strata in the Qaidam Basin. Among them, we focus on those wells that have failed to reach the targeted Jurassic strata over the past several years (Yellow squares in Figure 2), as well as representative wells that have confirmed that the Jurassic sedimentary thickness exceeds 1,000 m in recent years (Dark-blue squares in Figure 2). In addition, we conduct a statistical analysis on the 28 drilling wells that penetrate the top and bottom of the Jurassic (Supp .  Table S1).
At the same time, we select 10 representative wells to interpret the distribution of the Jurassic strata in the Qaidam Basin (Figures 2 and 5). These 10 wells can be divided into five groups: Niu 4 and Niu 3; Lengdong 3 and Lengdong 2; Su 3 and Sutan 1; Mabei 303 and Long 1; Dongling 1 and Yuanqiu 1 (Figures 2 and 5). Niu 4 and Niu 3 Wells are located to the north of the Eboliang Anticline, and the distance between two wells is only 2.6 km apart. Lengdong 3 and Lengdong 2 Wells are located to the east of the Lenghu Structure, and the distance between these two wells is only 3.9 km. Su 3 and Sutan 1 Wells are located in the centre of the Suganhu Basin, and the distance between the two wells is only 6.6 km. Mabei 1 and Long 1 Wells are located near Mahai, and they are only 11.1 km apart. Yuanqiu 1 and Dongling 1 Wells are located in the Dahonggou Anticline, and the distance between these two wells is 39.8 km.

Seismic data analysis
To further characterize the Jurassic palaeogeographic pattern of the Qaidam Basin, the detailed interpretation of seismic profiles was conducted. The selected seismic profiles focused on the middle segment of the northern Qaidam Basin. The Hong Shan Structure Zone (Figure 4), which is also called the Kuerleike Shan Structure Zone, is located between the Dachaidan Shan and Zongwulong Shan, placing it in a transition zone between the southwest-directed Dachaidan Thrust Zone (Yin et al. 2008a;Cheng et al. 2016) and the northeast-directed Olongbulak Thrust Zone ). These thrusts are thought to be active since the early Cenozoic (Yin et al. 2008a;Cheng et al. 2016;Yu et al. 2017b). It is connected to the Dachaidan Depression to the northwest and the Denan Depression and the Ounan Depression to the southeast. The Xiaochaidan Lake, which is located between the Lvlian Shan and Xitie Shan, lies to the west of the study area ( Figure 4). The Hongshan Structure Zone outcrops the most complete Mesozoic profile in the Qaidam Basin, so both the Dameigou Formation and the Xiaomeigou Formation were named at the Hong Shan Structure Zone.
Forty-four seismic profiles were used in this study, which were acquired in 1983,1985,2005,2006 and 2011 ( Figure 6). These seismic profiles are mainly oriented in the NWW-SEE, NEE-SWW, N-S and E-W directions ( Figure 6). We performed seismic data processing and interpretation with the KINGDOM software. In particular, six seismic profiles were selected to display the Mesozoic pattern of the Hongnan region in this study, covering the buried rift valley. The NWW-SEEstriking A-A' and B-B' and NEE-SWW-striking C-C' sections were selected to display the buried rift valley in the western part of the Hongnan region (Figure 7, Supp. Figures S1 and S2). The N-S-striking D-D' section was selected to reveal the Hongshan Front Fault (Supp. Figure S3), which strongly reformed the northern part of the buried rift valley. The NEE-SWW-striking E-E' and F-F' sections were selected to reveal the buried rift valley in the eastern part of the Hongnan region (Figure 10 and Supp. Figure S4).
In previous explorations, the Jurassic strata were thought to be continuously and extensively distributed in the northern Qaidam Basin (Feng et al. 2019), but recently years, some researchers find that this opinion is questionable (Cheng et al. 2019). Noticeably, over the past several years, a series of drilling wells targeting the Jurassic have failed. These unsuccessful explorations include Lengbei 1 and 2 Wells (No. 10 and 9, Figure 2 Figure 2) in the Saishiteng region. Many places that were believed to accumulate continuous Jurassic deposits but actually missed the Jurassic strata.
The selected five groups of adjacent drilling wells can more intuitively reflect the rapid change in the distribution of the Jurassic series over a short distance ( Figure 5). The distance between Niu 3 and Niu 4 Wells is only 2.6 km ( Figure 2); the former displays a total thickness of 867 m of the Jurassic sediments, whereas the latter missed the Jurassic. The distance between Lengdong 3 and Lengdong 2 Wells is only 3.9 km ( Figure 2); the former has Jurassic sediments with a thickness of 337 m, whereas the latter missed the Jurassic. The distance between Sutan 1 and Su 3 Wells is only 6.6 km ( Figure 2). The latter missed the Jurassic strata, whereas the former has Jurassic sediments with a thickness of more than 1,200 m. The distance between Mabei 303 and Long 1 Wells is only 11.1 km ( Figure 2); the former missed the Jurassic, whereas 1,202 m of Jurassic sediments accumulated in the latter. Both Dongling 1 and Yuanqiu 1 Wells are located in the instead of being continuous and gradual (Feng et al. 2019).
Four drilling wells have been conducted in the region covering seismic profiles displayed in this study, and they are Hongshancan 1, 2, Ku 1 and Hongshan 1 Wells  ( Figure 6). Hongshancan 1 and Ku 1 Wells are located in the Hongnan Region, while Hongshancan 2 and Hongshan 1 Wells are in the Hongshan Structure Zone ( Figure 6). In the Hongnan region, both Hongshancan 1 and Ku 1 Wells accepted ~200 m Quaternary (Figure 7). The Quaternary strata directly covered the Palaeogene strata (the Xiaganchaigou and Luhehe Formations) (Figure 7). Underlying the Palaeogene strata, there are Cretaceous and Jurassic strata. The stratigraphic sequence is normal in the Hongnan region. The penetrating strata in the Hongshan Structure Zone in the drilling wells such as Hongshancan 2 and Hongshan 1 Wells are similar to those in the Hongnan Region, except the missing of the Quaternary strata. However, there are many strata repetitions and abnormal sequences in the Hongshan Structure Zone, and several thrusts have been encountered in Hongshancan 2 and Hongshan 1 Wells (Figure 7). It reveals that the Hongnan Region is relatively steady during the Cenozoic, while the Hongshan Structure Zone is strongly reformed by the Cenozoic tectonics. Meanwhile, we display detailed well log of the Mesozoic strata in Hongshan 1 Well (Figure 8). The Jurassic strata in Hongshan 1 Well are comprised of complex interbedding of grey white, grey silty mudstone, muddy siltstone, siltstone, grey black, black, grey and dark grey carbonaceous mudstone, mudstone, sandy mudstone and coal layers, grey and grey white pebbly fine sandstone, pebbly siltstone, psephitic sandstone, coarse sandstone medium-sandstone and fine sandstone (Figure 8). In this well log, rapid change in the sedimentary colour and sedimentary grain size show frequency sedimentary facies transition, indicating a narrow valley environment ( Figure 8). Thus, the Jurassic strata in the Qaidam Basin are characterized by coal layers and rapid sedimentary facies transition (e.g. Ritts et al. 1999;Ritts and Biffi 2000;Wu et al. 2011;Tang et al. 2017;Shu et al. 2017;Cheng et al. 2019;Zhao et al. 2020aZhao et al. , 2020bQian et al. 2021).

Interpretation of seismic profiles
According to the interpretation of seismic profile A-A', a buried rift valley can be clearly identified in the Hongnan region (Figure 9). The northern boundary of the rift valley is the Hongnan NW Fault, and the southern boundary is the Hongnan SW Fault (Figures 6 and 9). These two faults are typical high angle normal faults. The width of the rift valley is approximately 10 km along the section direction. The rift valley is filled with the Mesozoic strata and buried by the Palaeogene sediments, which is dominated by the Lulehe and Xiaganchaigou Formation. According to the data from Ku 1 and Hongshancan 1 Wells (Figures 2 and 7), thin Quaternary strata (~200 m) cover the Palaeogene strata.
The structural pattern displayed in seismic profile B-B' (Supp. Figure S1) is similar to that in seismic profile A-A', except that the structure to the SEE direction has disappeared. In this section, only the southern boundary of the buried rift can be identified, and the northern boundary falls out of the scope of the figure. The Hongnan SW Fault, the southern boundary of the buried rift, is nearly vertical. The rift valley is filled by the Mesozoic strata. Palaeogene strata, including the Lulehe and Xiaganchaigou Formation, overlap on the rift valley. According to the data from Ku 1 and Hongshancan 1 Wells (Figure 2), thin Quaternary strata (~200 m) cover the Palaeogene strata.
Seismic profile C-C' (Supp. Figure S2) is perpendicular to seismic sections A-A' and B-B'. The nearvertical Hongnan SW Fault is the southern boundary of the buried rift valley, which is filled by the Mesozoic strata and overlapped by the Palaeocene strata. Inside the southern boundary, a high-angle normal fault and a low-angle thrust influencing the accumulation of the Mesozoic sediments can also be clearly identified. South of the Hongnan SW Fault, a north-directed thrust cuts the Palaeogene strata, so this fault is thought to be active after the Eocene. The Hongshan Front Fault, a south-directed low-angle thrust, can be identified at the right end of the section (Supp. Figure S2). According to the data from the Ku 1 and Hongshancan 1 Wells (Figure 2), thin Quaternary strata (~200 m) cover the Palaeogene strata.
Seismic profile D-D' mainly reveals the Hongshan Front Fault System (Supp. Figure S3), which is dominated by a series of south-directed thrusts, with several accompanying north-directed thrusts. The main thrust of the Hongshan Front Fault is a low-angle thrust. The buried rift system was significantly inverted by the Hongshan Front Fault System during the Late Cenozoic. The remarkable Hongshan Syncline produced by the Hongshan Front Fault System on the geological map ( Figure 4) can be clearly identified in this section.
Seismic profile E-E' reveals the buried rift system in the eastern segment (Supp. Figure S4). Similar to the western segment, the rift valley is bounded by two highangle normal faults, the Hongnan NE and SE faults. The width of the rift valley is approximately 10 km along the section direction. The buried rift valley is filled by the Mesozoic strata and covered by the Palaeogene sediments, which are dominated by the Lulehe and Xiaganchaigou Formations. The south-directed Hongshan Front Fault strongly inverted the buried rift valley at its northern boundary.
The structural pattern displayed in seismic profile F-F' is similar to that in seismic profile E-E' (Figure 10). The buried rift valley is controlled by high-angle normal faults and reformed by the Hongshan Front Fault during the Cenozoic. The Hongshan Front Fault is much steeper in the eastern segment than that in other segments. Unlike seismic profile E-E', bedrocks, rather than the Jurassic strata, directly thrust over the buried rift valley, which implies that the extent of the Jurassic strata in the upside of the Hongshan Front Fault is also limited.
In short, a buried rift system has been clearly identified in the middle segment of the northern Qaidam Basin (Figures 4 and 6). The narrow valley is controlled by high-angle normal faults. The average depth of the buried rift valley is generally over 2 km, and its maximum width is approximately 10 km. The rift valley is filled by Mesozoic strata, and covered by Cenozoic strata. Later on, the buried rift system was significantly reformed by compressional tectonics during the Cenozoic. According to these 44 seismic profiles, a typical buried rift valley can be identified on the map (Figures 4 and 6). The width of the rift valley is approximately 10 km, displaying as a narrow rift valley system. The middle segment of the northern boundary of the rift valley was destroyed by the late south-directed thrusting of the Hongshan Front Fault. We named this buried rift system as the Hongnan Rift System.

Implications for the Mesozoic landforms of the Qaidam basin
Before identifying the Mesozoic tectonic attribute of the Qaidam Basin and the exploration of Jurassic coal and petroleum resources, a key issue should be resolved: clarifying the distribution of the Mesozoic strata in the Qaidam Basin. As an endorheic basin, the Qaidam Basin naturally accumulated thick Cenozoic sediments (~8 km), leading to the subsidence of the Cenozoic sedimentary floor (Yu et al. 2015a(Yu et al. , 2015bGuo 2019, 2021;. Meanwhile, the Qaidam Basin experienced intense and complex Cenozoic tectonic activity (Jolivet et al. 1999(Jolivet et al. , 2003(Jolivet et al. , 2022Yin et al. 2008a, b;Cheng et al. 2016Cheng et al. , 2017aCheng et al. , 2017bCheng et al. , 2018Cheng et al. , 2019Cheng et al. , 2021Cheng et al. , 2022Wu et al. 2019;Yu et al. , 2022. The thick Cenozoic cover and complex Cenozoic tectonics exacerbate the difficulties in exploring the Jurassic strata in the Qaidam Basin. Due to the misidentification of the Jurassic strata from seismic data and the tendency to exaggerate their estimations, the previous isopach maps of the Jurassic strata in the Qaidam Basin (e.g. Feng et al. 2019) are highly suspected. According to borehole and seismic data, the thicknesses of the Mesozoic strata in the Qaidam Basin vary significantly (Figures 5,9 and 10). For example, the Jurassic thicknesses at the outcrops along the Altyn Tagh Fault  and in the boreholes of Long 5 and Chaxin 1 Wells are all over 2,000 m, whereas a large number of wells in the Qaidam Basin are completely devoid of the Jurassic strata (Figures 2 and 11). These findings support the opinion that the Jurassic deposits in the Qaidam Basin were positioned in small basins (e.g. Allen and Vincent 1997;Ritts and Biffi 2000;Sobel et al. 2001). Combining with the latest and widespread drilling well data, field data and previous geological maps (Jin and Zhang 1998), the extent of the Jurassic prototype basin was redrawn in this study (Figure 11), based on more than 200 drilling wells and outcrop data. This new map proposed that the distribution of the Jurassic sediments in the Qaidam Basin should be discontinuous and mainly restricted in the narrow valley system (Ritts and Biffi 2000;Sobel et al. 2001;Cheng et al. 2019), and it disagrees with the traditional opinion that the Jurassic sediments are widespread and continuous in the Qaidam Basin (e.g. Feng et al. 2019).
According to the detailed interpretation of seismic profiles and analyses of the drilling well data in the middle segment of the northern Qaidam Basin, we identified a well-preserved buried rift valley to the south of the Hongshan Structure Zone (Figures 6  and 9). The Jurassic sediments are restricted to the buried rift system. The Hongnan Rift System clearly displayed the landforms of the Qaidam Basin during the Jurassic, reasonably explaining the rapid changes in the distribution of Jurassic deposits in the Qaidam Basin. The sedimentary boundaries controlled by high-angle faults also indicate that transitional interpolation method is not suitable for drawing the Jurassic residual thickness maps of the Qaidam Basin. Meanwhile, the sedimentary source analyses of the Qaidam Basin also support a rift valley tectonic setting during the Jurassic. The Jurassic sources in the Qaidam Basin have been thoroughly analysed in a series of previous studies (Yu et al. 2017cCheng et al. 2019;Zhao et al. 2020aZhao et al. , 2020bQian et al. 2021;Zeng et al. 2021). The detrital zircon dating results show that the Oulongbulak Block adjacent to the Hongnan Rift System is an important source for the sediments in the Hongnan Rift (Yu et al. 2017c;Qian et al. 2021). At the same time, a detrital zircon U-Pb age peak of ~250 Ma, that can be only derived from the Eastern Kunlun Mountains and Ela Shan, also exists in many Jurassic samples from the northern Qaidam Basin (Bush et al., 2016;Wang et al. 2017;Cheng et al. 2017aCheng et al. , 2019Yu et al. 2017c;Lu et al. 2019;Wu et al. 2019). The complex and mixed sources from both proximal and remote sources in the Jurassic strata of the northern Qaidam Basin also favour a narrow basin during the Jurassic (Ritts and Biffi 2000;Sobel et al. 2001;Cheng et al. 2019). Meanwhile, four Jurassic provenance systems of the Hongshan and Huobuxun sags are recognized through heavy mineral assemblages: the Kuerleike-Zongwulong, Zongwulong-Oulongbuluke, Xitie, and Aimunike-Dadakenwula mountains provenance systems, also supporting rapid geomorphic changes and  Jurassic deposits outside the rift valleys are limited, and the overall landscape outside the rift regions appears as erosion highlands ( Figure 12). As a result, the Jurassic deposits in the Qaidam Basin display a discontinuous distribution and rapid facies transition rather than a wide and gradual distribution (Figure 12). This palaeogeographic pattern strongly supports the Northwest China experienced an extensional tectonic environment during the Jurassic (Ritts et al. 1999;Sobel 1999;Hendrix 2000;Ritts and Biffi 2000;Chen et al. 2003;Wang et al. 2005;Jolivet 2015). Later on, thick Cenozoic sediments covered the rift valley, and intense Cenozoic tectonic activity (Jolivet et al. 1999(Jolivet et al. , 2003(Jolivet et al. , 2022Yin et al. 2008aYin et al. , 2008bYuan et al. 2013;Wu et al. 2016Wu et al. , 2019Zuza et al. 2016Zuza et al. , 2018Cheng et al. 2021;Yu et al. 2022) further reformed the buried rift valley.

Implications for Mesozoic petroleum exploration in northern Tibetan Plateau
Previously, the Qaidam Basin was thought to be with continuous and extensive sediments (e.g. Feng et al., 2019). However, according to borehole and seismic data, during the Mesozoic, the Qaidam Basin displayed as a narrow rift system with discontinuous deposition and rapid facies transition, rather than a wide and broad extension system. Therefore, when conducting future petroleum exploration for the Mesozoic source rocks in the Qaidam Basin, using drilling well and seismic data to carefully delineate the Jurassic distribution can effectively improve the success ratio of Jurassic exploration. Indeed, the distribution of the existing wells drilled into the km-thick Jurassic strata provides a direction for further exploration of the Jurassic source rocks. The wells drilled into the km-thick Jurassic are mainly concentrated in the Lenghu, Suganhu, Yuqia, Hongshan, and Ganchaigou regions. Therefore, further exploration targeting the Jurassic source rocks should focus on these areas. Certainly, there may be other rifts under the thick Cenozoic covers in the Qaidam Basin. As the Cenozoic tectonic activity significantly reformed the buried rift system in the Qaidam Basin (Jolivet et al. 1999(Jolivet et al. , 2003Yin et al. 2008aYin et al. , 2008bYuan et al. 2013;Wu et al. 2016Wu et al. , 2019Zuza et al. 2016Zuza et al. , 2018Cheng et al. 2021;Yu et al. 2022), searching for other Jurassic rift valleys is relatively difficult. This work should be carefully considered in combination with the detailed recovery of the Cenozoic tectonics.

Conclusion
Based on seismic and drilling data, the Qaidam Basin was generally in a rifting tectonic setting during the Jurassic.
The current identified Jurassic rift valleys are mainly located at the northern margin of the Qaidam Basin and along the Altyn Tagh Fault, and the distribution of Jurassic sediments is characterized by discontinuous deposition and rapid facies transition. The wellpreserved buried rift in the middle segment of the northern Qaidam Basin clearly indicates that the Jurassic sediments are restricted in a narrow valley. Thus, these deposits can change from being absent to having considerable thickness over a short distance. Therefore, for further exploration of the Jurassic source rocks in the Qaidam Basin, carefully clarifying the planar distribution of the Jurassic rift valley should be a key issue. Only in this way can the exploration targets be effectively delineated and the frequency exploration failures be lowered.