Timing of the initiation and duration of the Cretaceous extensional regime in South-east China: Constraints from growth strata in terrigenous basins

Abstract

There has been no consensus yet regarding the precise initial timing and duration of the late Mesozoic extensional tectonics in the South-eastern China Block. This work focusses on the growth strata of the Early Cretaceous red beds in the South-eastern China Block to determine the late Mesozoic tectonics and the precise timing of the initiation and duration of extensional tectonics in this area. Field observation of several terrigenous basins shows that the dip angles of the Cretaceous red beds have varied from moderate to gentle from basin edges to interiors (or centres). The visible and estimated thickness within a single bed increases slightly downwards from the upper to the lower part. These characteristics indicate that the sedimentary area of these beds has undergone an extensional process with expansion and deepening of the sedimentary basins. Rotation of the border surfaces (limbs) and downward warping of the hanging walls or retreat of the footwalls of listric normal faults causes three types of extensional growth (or syntectonic) strata in the deposits of different basins. Dating of the volcanic rocks related to the growth beds reveals that the sedimentary basins were enlarged and deepened when the Early Cretaceous strata were deposited in the South-eastern China Block from ca 140 to 137 Ma. Regionally, under the influence of Palaeo-Pacific plate rollback since ca 140 Ma, the South-eastern China Block stress field has led to lithospheric uplift and pull-apart structures near the surface, causing the half-graben basins to receive sedimentation. Although the extensional event was interrupted by a short compressional event during 120 to 105 Ma, with the oceanward retreat of the trench, the area of extension gradually enlarged and rejuvenated south-eastwards until the end of the Cretaceous. This Cretaceous extension event of the South-eastern China Block must belong to a worldwide geological event with global significance.

INTRODUCTION

The late Mesozoic extensional tectonics in South China are subject to a tectonic shift from an E–W-trending Palaeo-Tethys structural domain to an NE–SW-trending tectonic system across the north-western Pacific (Li, 2000; Shu & Zhou, 2002; Mao et al., 2002; Yu, X.Q. et al., 2010; Wang et al., 2015). After a regional compression event at approximately the transition from the Jurassic to the Cretaceous (Li, J.H. et al., 2012, 2020; Ji et al., 2017a; Wang et al., 2018; Suo et al., 2019; Yang et al., 2021; Xu et al., 2022), the South-eastern China Block (SECB), the focus of this article, underwent intracontinental extension, which mainly occurred during the Cretaceous (Shu et al., 2007, 2009; Li, J.H. et al., 2012a, 2013a, 2014a, 2014b, 2020, 2022; Li, Z. et al., 2014c; Ji et al., 2017b; Huang et al., 2019; Suo et al., 2019; Yang et al., 2012, 2021). Based on a number of recent studies reporting on magmatic petrological, geochronological and geochemical data, there is agreement that the subduction of the Palaeo-Pacific plate played a key role in shaping the tectonic architecture of South-eastern China (Li, 2000; Zhou & Li, 2000; Shu et al., 2009; Li et al., 2010; Li, J.H. et al., 2014a; Li, S.Z. et al., 2019a; Xu et al., 2016a; Liu et al., 2017; Suo et al., 2019). Thus, these tectonics may be ascribed to a NW–SE extension caused by the subduction of the Palaeo-Pacific plate (Gilder et al., 1991; Zhou & Li, 2000; Lin et al., 2000; Li & Li, 2007; Mercier et al., 2007; Shu et al., 2009; Wei et al., 2014), although the Cretaceous tectonics were previously recognised and attributed to a NE–SW strike-slip regime (Xu et al., 1987; Gilder et al., 1996; Li, J.W. et al., 2001a; Zhu et al., 2005). For the initial timing of the extension, some researchers have indicated that the extensional tectonic regime of South China began in the (Mid) Late Jurassic and lasted throughout the entire Cretaceous (Li, X.H. et al., 2003a; Wei et al., 2016; Zhou et al., 2004; and references therein). Moreover, several researchers have suggested that the extension occurred between 110 and 90 Ma (Wong et al., 2009; Jiang et al., 2013; Zhou et al., 2015; Liu et al., 2016; Pan et al., 2018; Yang, J.B. et al., 2018a), while others have stated that the extension began at ca 135 Ma (Li, J.H. et al., 2013a; Liu et al., 2014; Gu et al., 2017; Ji et al., 2017a, 2018; Yang, Y. et al., 2018b; Zhang et al., 2018; Huang et al., 2019). Thus, the tectonic regime in the late Mesozoic and the precise initiation time (Early or Late Cretaceous) of extensional tectonics remain vigorously debated. Worldwide, this issue is also a subject of controversy (like the Early Cretaceous extension in north-east Brazil, Sénant & Popoff, 1991; mid-Cretaceous in east-central Alaska, Pavlis et al., 1993; Cretaceous in New Zealand, Baker & Seward, 1996; mid-Cretaceous in the western Pyrenees, Bodego & Agirrezabala, 2013; Late Cretaceous in Chilean main range, Muñoz et al., 2018; Cretaceous in the North-western Andes, Zapata et al., 2019). In addition, consensus is lacking as to the duration of this extensional regime in SECB (Li, J.H. et al., 2014; Chen, C.H. et al., 2016; Ji et al., 2017b).

Extensional structures in the SECB that formed during the late Mesozoic manifested as widespread extensional basins or half-grabens (e.g., the Hengyang Basin in Hunan, the Jinhua–Quzhou Basin in Zhejiang, the Xinjiang and Yongfeng–Chongren basins in Jiangxi, and the Huangshan Basin in southern Anhui; Gilder et al., 1991; Li, J.H. et al., 2012a; Shu et al., 2007, 2009), extensional dome structures (e.g., the Wugongshan, Lushan and Hengshan domes; Lin et al., 2000; Shen et al., 2008; Shu et al., 1998), and numerous A-type granitic and bimodal volcanic rocks (e.g., the Xiangshan, Gan–Hang Belt, and south-east coastal area; Li, 2000; Liu, Q. et al., 2012b; Wong et al., 2009; Yang et al., 2012; Zhou & Li, 2000; Zhou et al., 2006). To date, most previous studies have focussed on igneous rocks with only a few researching the basin fill of red basins and/or half-grabens to determine the tectonic evolution of South China during the late Mesozoic (Shu et al., 2007, 2009; Wang et al., 2011; Li, J.H. et al., 2014b; Chen, L.Q. et al., 2016b; Xu, X.B. et al., 2016b; Lin & Wei, 2020; Meng et al., 2022). In fact, syntectonic growth strata in depositional basins provide a direct link to tectonic deformation in different geological stages (Hamblin & Rus, 1989; Vergés et al., 2002; Liu et al., 2018; Zhang et al., 2020; Lin & Wei, 2020). It is widely accepted that the ages of growth strata can be used to define the timing of deformation because growth, or syntectonic, strata are stratigraphic intervals deposited during deformation (Gawthorpe & Hardy, 2002; Vergés et al., 2002; Pochat et al., 2009; Ahmadi et al., 2013; Santolaria et al., 2015). Thus, the growth strata in red basins or half-grabens are important in ascertaining the tectonic regime and time of extension in the SECB (Li, H. et al., 2012b; Wong et al., 2009; and references therein). As the influence of depositional morphology and base–level changes on growth strata geometries has received limited attention, this paper presents some comprehensive case studies on the growth strata in several basins in the SECB to determine the formation age, duration of extension and tectonic background, and then provide evidence for the occurrence of lithospheric extension in the SECB since the late Mesozoic.

GEOLOGICAL SETTING

Formed by the Neoproterozoic amalgamation of the Yangtze and Cathaysia blocks, South China is bounded to the north by the North China Craton and to the west and south-west by the West China and Indochina Blocks, respectively (Figure 1; Li et al., 2002; Li, X.H.(Xianhua) et al., 2009a; Zhao & Cawood, 2012; Zhang et al., 2013; Zhao et al., 2016; Cawood et al., 2013, 2018; Yao et al., 2019). South China underwent continental rifting after amalgamation, forming the Nanhua Basin at ca 820 Ma (Wang & Li, 2003; Li, Z.X. et al., 2003b). The tectonic evolution of South China hereafter involved extensive crustal reworking (emphasised by orogenic events) in the Ordovician to Silurian, Triassic, and Jurassic to Cretaceous (Shu et al., 2008, 2014, 2015; Wang et al., 2007, 2013; Chu et al., 2012; Li et al., 2016; Li, J.H. et al., 2017; Li, L.M. et al., 2018). And finally, within the ENE-directed late Mesozoic strata, the sedimentary and tectonic characteristics of South China were shaped (Yu et al., 2003, 2020; Ji et al., 2018; Chu et al., 2019; Li et al., 2020). In South-eastern China, controlled by the Palaeo-Pacific plate subduction, these late Mesozoic arc-related sedimentation–deformation–metamorphic–magmatic events are dominant in the coastal region of the SECB (Li, 2000; Zhou & Li, 2000; Shu et al., 2007, 2009; Li, J.H. et al., 2014a, 2022; Xu, D.R. et al., 2016a; Suo et al., 2019).

FIGURE 1. Geological sketch map showing the Early Cretaceous (145–120 Ma) basins and volcanic and plutonic rocks of South China. The boundary between the Yangtze and Cathaysia blocks is from Ji et al. (2017).

According to Allen's genetic classification and tectonic settings of basins (Allen & Allen, 1990), Shu et al. (2009) identified two types of basins in the SECB: postorogenic basins (T₃–J₁) and intracontinental extensional basins. The latter formed in an extensional setting and is always bordered by normal faults. These basins are widely distributed throughout the SECB. Structurally, this type of basin possesses two basin patterns, namely, graben basins and half-graben, or faulted-depression basins (Shu et al., 2007, 2009). Only a few faulted-depression basins translate into depression basins in the last stage of the basin process. According to the regional geological survey (Table S1), some single composite basins involve these graben and half-graben basin types. The half-graben basins are elongated depressions between normal faults and were generated during the Middle Jurassic followed by a bimodal volcanic eruption. The half-graben basins have formed since the Late Jurassic or Early Cretaceous due to intracontinental extension caused by subduction of the Palaeo-Pacific plate (Shu et al., 2009). Beginning in the Early Cretaceous, the half-graben basins were basically occupied by red beds with rhyolitic lavas and volcanoclastic rocks, and in the Late Cretaceous–Palaeogene, they were entirely filled by red-coloured terrestrial material (Jiangxi BGMR, 1987; Zhejiang BGMR, 1996; 1997; Yu et al., 2003; Shu et al., 2007, 2009). Regionally, these red beds gradually rejuvenated south-eastwards. The sedimentary strata accumulated in a variety of depositional environments: fluvial fan, fan-delta, lake, braided river, meandering, alluvial plain (Zhu et al., 2012; Li, X.H.(Xianghui) et al., 2013d) and aeolian (Jiang et al., 2008). Pedogenic (Li, X.H.(Xianghui) et al., 2009, 2013d) conditions with variable clastic lithologies, carbonates and evaporites comprise more than two dozen formations. The strata in these terrigenous basins are thick, up to ca 10,000 m, and interbedded volcanic rocks are abundant. These rocks can provide precise geochronological constraints on the late Mesozoic terrestrial strata and thus have the potential to document a complete evolutionary history of sedimentation and volcanism that is closely related to tectonism.

METHODOLOGY

The following methods were used to investigate the growth strata and their characteristics. (1) Several basins containing Cretaceous strata were selected by referring to regional geological maps of South China at a 1/200,000 scale (an area within 1° longitude and 40′ latitude) (Table S1). Because plantations and settlements occupy the largest basin area, only those sections along newly built highways and similar places could be observed. (2) Large-scale to small-scale outcrop observations, including strata attitude measurements, were followed by conventional sedimentological analysis. (3) Line drawing was performed on panoramic photographs to investigate the proximal to distal variations in bedding attitude and geometry.

The most prominent feature of the growth strata, in a subsidence centre near the normal fault in the hanging wall, is that within each bed, the thickness increases slightly downwards towards the lower part. To view these features clearly and realistically, the studied terrigenous basins should be easily distinguishable and laterally observable, that is, these basins should display a good degree of exposure. Moreover, the sedimentary characteristics in these basins have been locally documented in several previous outcrop-based studies (Yu et al., 2003; Li, J.H. et al., 2014b; Shu et al., 2007, 2009; Tang et al., 2016; Xu, X.B. et al., 2016b).

BASIN FILLING CHARACTERISTICS AND GROWTH STRATA IN LATE MESOZOIC BASINS

There are nearly a thousand Late Mesozoic basins in the SECB (Table S1). As mentioned above, only those sections along newly built highways and such like can be observed. Several basins e.g. the Huangshan Basin in Anhui, the Jinhua–Quzhou Basin in Zhejiang, Ji’an–Taihe, Xinjiang and Yongfeng–Chongren basins in Jiangxi, Jianning and Taining basins in western Fujian were selected to investigate the growth strata. These basins all developed Cretaceous terrestrial sediments (Figure 2) and can be used to make stratigraphic correlations and identify growth strata features.

FIGURE 2. Sedimentary columns of the Cretaceous basins in SECB (stratigraphic units see text; column locations see Figure 1). (A) The Huangshan Basin in Anhui (this study); (B) The Jinhua–Quzhou Basin in Zhejiang (Wu et al., 2015); (C) The Yongfeng–Chongren Basin (Tang et al., 2014a), (D) The Xinjiang Basin (Wang et al., 2019), (E) The Ji'an–Taihe Basin in Jiangxi (Jiangxi BGMR, 1997); (F) The Jianning and (G) Taining Basins in western Fujian (Ren & Zhang, 2021).

Sedimentary characteristics in the Huangshan Basin, southern Anhui Province

The Huangshan Basin is located in southern Anhui Province (Figure 3). There are Jurassic and Cretaceous sediments deposited within this terrigenous half-graben basin. The bottom quartzose conglomerate bed, including covered mudstone layers and a thin coal interbed, belongs to the Early Jurassic Yuetang Formation (J₁y), unconformably covering the Neoproterozoic basement series. The Middle Jurassic Hongqin Formation (J₂h) mainly consists of purple and yellow medium-grained to fine-grained sandstones, which generally onlap the basement rocks. Above the Hongqin Formation, the Bingqiu Formation (J₃b) is composed of fluvial conglomerate and sandstone. The Late Jurassic–Early Cretaceous Shiling Formation (J₃-K₁s) represents an intermontane volcaniclastic formation that is mainly composed of rhyolite and rhyolitic tuff. The local thickness of the Yantang Formation (K₁y) is less than 100 m, and here consists of a fine grained clastic lacustrine carbonate of limited geographic extent that is rich in biological fossils. Figure 3 indicates that the Cretaceous depocentre migrated slightly to the north relative to the previous Jurassic depocentre. In addition, accompanied by enlargement of the sedimentation area, the Early Cretaceous Huizhou Formation (K₁h) unconformably covers various layers of older strata. Starting with the Huizhou Formation sedimentation, the basin accumulated an incomplete series from the lower molasse-like formation (K₁h) through the middle red bed formation (K₂q) and the upper molasse-like Xiaoyan formation (K₂x), which overlies the Jurassic series, with a basalt bed in the Xiaoyan Formation (K₂x).

FIGURE 3. Sedimentary characteristics of the Huangshan Basin, southern Anhui Province (for the location, see Figure 1). (A) Geological map of the Huangshan Basin. (B–D) Three sections showing the growth strata of the Huizhou Formation.

Regional geological investigation along several sections in the Huangshan Basin finds that the growth strata are developed in Cretaceous red beds (Figures 2A and 3). There are lateral variations in the attitudes of the red beds in the lower to middle parts of the Early Cretaceous Huizhou Formation (K₁h) and the late Cretaceous Xiaoyan Formation (K₁x). Several sections reveal that the dip angles of these red beds vary from moderate to gentle from basin edges to interiors (or centres), and the thickness within a single bed increases slightly downwards from the upper to the lower part.

Section A-A' (Figure 3B) shows that several measured dip angles of the lower part of the Huizhou Formation gradually vary from 49° to 26° towards the north, and that the individual bed thickness increases slightly towards the lower part of this bed (I₂ > I₁, photograph C in Figure 4). Observations along the basin margin reveal that many similar occurrences appear in the eastern and south-eastern areas of the Huangshan Basin. Section B-B' (Figure 3C) shows dip angles in the lower part of the Huizhou Formation that gradually vary from 44° to 26° towards the north and north-east. Section C-C' (Figure 3D) reveals that strata in outcrops have dip angles in the lower part of the Huizhou Formation that gradually vary from 23° to 8° towards the north.

FIGURE 4. (A) Cretaceous sedimentary system of the Huangshan Basin. (B) Growth strata in the Xiaoyan Formation (C) Growth strata in the Huizhou Formation.

To the north-east of the Huangshan Basin (Figure 3A), the sediments of the Jixi Basin also exhibit the growth strata of the Early Cretaceous Huizhou Formation. Along the slope of a new highway excavation, the inclination/orientation of the Lower Cretaceous Huizhou Formation (K₁h) gradually varies the dip angles from 46–40° to 25°–20° from basin edge (west) to near centre (east), showing the shape of one type (named type I) growth strata.

Except for the growth strata in the lower part of the Huizhou Formation in the Huangshan Basin, field observations and large-scale regional geological mapping shows negligible variations in the thickness of the Late Cretaceous Qiyunshan Formation (K₂q). Some individual beds of the lower Huizhou and the upper Xiaoyan formations exhibit slightly increasing thicknesses towards the lower part and approximate the shape of the extensional growth strata (Figure 4). The studied section is located west of the Huangshan Basin and is oriented from south to north. This section reveals that the dip angles of sediments of the Xiaoyan Formation (K₂x) gradually vary from 28° to 8° or nearly horizontal towards the north (Qiyunshan Mountain, Figure 4B), and individual bed thickness increases downwards to the lower part. Thus, there are two sets of type I growth strata developed in the Huangshan Basin.

Sedimentary characteristics in the Jinhua–Quzhou Basin, Zhejiang Province

The Jinhua–Quzhou Basin is located in western Zhejiang Province (Figure 1) along the Jiangshan–Shaoxing fault belt (also named the Gan-Hang rift belt, Li, J.H. et al., 2014b; Shu et al., 2017; Wang et al., 2018; Huang et al., 2019; Yang et al., 2021). The Early Cretaceous Qujiang Group (K₁Q) in this basin consists of the Zhongdai, Jinhua and Quxian formations (Figure 2B) in ascending stratigraphic order (Zhejiang BGM, 1996; Chen, 2000; Li, Y.X. et al., 2001b). These strata overlie the pre-Cambrian to late Palaeozoic series via an angular unconformity in the west and the Late Jurassic to Early Cretaceous strata (J₃-K₁, primarily the Moshishan Group volcanic suite) in the east.

The basement of the basin, the underlying J₃-K₁ volcanic suite, is identified here as the Moshishan Group (J₃-K₁M) and consists predominantly of grey ignimbrite and purple rhyolite. This rock series was cut by the normal faults that dominated basin opening, followed by sediment infilling, implying that volcanism predated some extension (Yang et al., 2021). Within the basin, the lower Zhongdai Formation (K₁z) is mainly distributed in the southern part (Figure 5) and consists of conglomerates, red sandstones and mudstones. The middle Jinhua Formation (K₂j) is distributed throughout the basin and consists of conglomerates, red sandstones and black shales with interbedded basalts (Luo & Yu, 2004). Along the southern margin of the Jinhua–Quzhou Basin, this group overlies the Zhongdai Formation with an angular unconformity of >30° (Figure 5B). The upper Quzhou Formation (K₂q) is distributed on the north side of the basin and is composed of red sandstones, conglomerates and mudstones (Zu et al., 2004; Yu, Y.W. et al., 2010b; Yu, J.H. 2012a).

FIGURE 5. Sedimentary characteristics of the Jinhua–Quzhou Basin, west Zhenjiang Province (for the locations see Figure 1). (A) Geological map of the Jinhua–Quzhou Basin. (B) Basin marginal fault and growth strata on the north-west side of the basin. (C) Growth strata on the south-west side of the basin. (D) Growth strata in the eastern part of the basin. (E) Group of faults in the eastern part of the basin.

In the northern Jinhua–Quzhou Basin, the Quxian Formation (K₂q) covers the Ordovician shales and limestones via a normal fault (Figure 5B). The red beds show that the dip angles of the lower part of the Quxian Formation gradually vary from 46° to 22° towards the south-east (the same dip direction as the normal fault), revealing another type (named type II) growth stratum. In the southern Jinhua–Quzhou Basin, the Jinhua Formation (K₂ j) exhibits type I growth strata caused by the outward migration of the border and clockwise rotation of the border line (Figure 5C), like those in Huangshan Basin. Three line-representative bed surfaces vary in their dip angles from 35° to 19° towards the north-west, all showing the type I growth strata morphotype. Figure 5C clearly shows that, within the same bed, there is a downwards trend towards increasing thickness from the higher end (I₁) to lower end (I₂), i.e., thickness I₂ > I₁. To the east of the Jinhua–Quzhou Basin (Figure 5D), the type I growth strata phenomenon in the Jinhua Formation (K₂ j) appears on a slope of the new highway. Moreover, a group of faults are found nearby (Figure 5E). The outcrop displays two reverse faults with large differences in the dip angles. Detailed surveys have found a reverse fault close to this set of faults in a south-east direction. This reverse fault tilted this set of faults (hanging wall). This phenomenon implies that the extension event continued until the compression during the Late Cretaceous. The synsedimentary fault (photograph in Figure 6) also indicates that the extension event occurred during the Late Cretaceous.

FIGURE 6. Three sections across the Jinhua–Quzhou Basin showing the blind growth strata based on geophysical prospecting results (gravity and electromagnetic geophysical tests) and verified by drilling holes. The map on the lower left shows the locations of the sections and geophysical lines.

Owing to poor exposure, the sedimentary characteristics in the basin centre, especially in the bottom of the basin, can be deduced only by drilling wells and geophysical methods (ZIGS, 2005). In Figure 6, three sections are shown to reflect the sedimentary characteristics deep within the basin centre. These sections are drawn based on geophysical prospecting results (gravity and electromagnetic geophysical tests) and verified by drilling holes (ZIGS, 2005). The hanging walls of the normal fault clearly present the shape of third type (named type III, a buried type) growth strata within the Zhongdai Formation and Jinhua Formation. The Quxian Formation has no obvious growth strata, perhaps due to the small scale of the sections.

Stratigraphic characteristics of the Xinjiang and Yongfeng–Chongren basins, Jiangxi Province

The Xinjiang and Yongfeng–Chongren basins are located to the south-west of the Jinhua–Quzhou Basin within the Gan-Hang Belt (Figure 1; Yu et al., 2006). The Lower Cretaceous strata are composed of the Daguding Formation (K₁d) and Ehuling Formation (K₁e, which consists predominantly of rhyodacite and porphyroclastic lava; Figure 2C,D). The Upper Cretaceous red beds of the Guifeng Group (K₂GF) can be further divided into three lithostratigraphic units, namely, the Hekou (K₂h), Tangbian (K₂t) and Lianhe (K₂l) formations respectively, progressing upwards (Figure 7; Jiangxi BGM, 1987; Li, Y.X. et al., 2001b). The Guifeng Group is unconformably underlain by the porphyroclastic lava of the Ehuling Formation (K₁e) and is overlain by Quaternary colluvial deposits. According to regional stratigraphic correlation, the Guifeng Group was deposited during the Coniacian to Maasstrichtian stages (Cao, 2013), ca 90–65 Ma (Chen, L.Q. et al., 2016b, 2017), generally as products of a desert depositional environment in an arid climate. The sandstone samples are purple–red in colour, fine-grained to medium-grained feldspathic quartz sandstones. Sand particles are mainly composed of quartz, feldspar, mica and rock fragments, which are subangular to subrounded and well sorted (Chen et al., 2017; Wang et al., 2019). The lower Guifeng Group contains several interbedded basalts (Figure 7B).

FIGURE 7. Stratigraphic characteristics of the Xinjiang and Yongfeng–Chongren basins, northern Jiangxi Province (for locations see Figure 1). (A) Geological map of the Xinjiang and Yongfeng–Chongren basins. (B) Section showing growth strata in the Yongfeng–Chongren Basin. (C) Section showing growth strata in the Xinjiang Basin.

The Xinjiang and Yongfeng–Chongren basins also contain growth strata. In Figure 7B,C, the type I growth strata appear on the south-eastern side of the basins, the dip angles of the lower beds vary from ca 35° to 22° north-west towards the basin centre.

Sedimentary characteristics in the Ji’an–Taihe Basin, Jiangxi Province

The Ji’an–Taihe Basin (Figure 8), which is close to the Yongfeng–Chongren Basin in the north-east (Figure 7A), is a Mesozoic fault basin developed during the Cretaceous and covers a total area of ca 4000 km² (Yu et al., 2005). The Early to Late Cretaceous strata comprise the Ganzhou Group (K_1-2G) and the Guifeng Group (K₂GF) in ascending order (Figure 2E, Jiangxi BGMR, 1997). Caledonian and Yanshanian granites were emplaced and crop out around the basin (Wang, 2015), and Cretaceous (143–66.3 Ma) basic igneous rocks sporadically crop out in the interior of the basin (Peng et al., 2004; Yu et al., 2005). The south-eastern part of the Taihe depression is a subsag to the south of the basin and occupies a total area of ca 1400 km². It is mainly controlled by the Ganjiang and Suichuan faults (Figure 8). The Early Cretaceous Maodian Formation (K₁m) and Late Cretaceous Zhoutian Formation (K₂ z) of the Ganzhou Group are composed of siltstone, mudstone and gypsum, along with thin-bedded sandstone and sandy conglomerate.

FIGURE 8. Sedimentary characteristics in the Ji’an–Taihe Basin, Jiangxi Province (for locations see Figure 1). (A) Geological map of the Ji’an–Taihe Basin. (B) Four 2D seismic survey sections in the basin, after Liu et al. (2022). (C) The deduced profile across the Ji'an–Taihe Basin shows the predicted growth strata.

According to a 2D seismic survey (Liu et al., 2022; Figure 8B) and drilling in the Taihe depression, the structural pattern and characteristics of the basin are summarised by means of refined relative-amplitude-preserved seismic processing and integrated interpretation. Liu et al. (2022) reported on the seismic profiles and structural attributes. Based upon previous research and geophysical interpretation, the deduced profile across the Ji’an–Taihe Basin can obviously predict the existence of type III growth strata within Cretaceous strata (Figure 8C).

Sedimentary characteristics in the Jianning and Taining basins, western Fujian Province

The Cretaceous lithostratigraphic units in the basins in western Fujian Province consist of the Lower Cretaceous Douling Group (K₁D, volcanic rocks), Bantou Formation (K₁b, terrestrial deposits), Upper Cretaceous Baiyashan Formation (K₂b, volcanic rocks), and Chishi Group (terrestrial deposits of three formations, namely, the Junkou, Shaxian and Chong’an formations, Figures 2F,G and 9, Fujian BGMR, 1997). The Jianning and Taining (local name Julan) Basin is located in western Fujian Province and formed during the Cretaceous after two-step downfaulted movements (Bi, 2016). The first downfaulted event occurred in the early stage of the Early Cretaceous and thus resulted in an extensional basin. The terrestrial Bantou Formation (K₁b) was formed later. After a short uplifting and erosion during the middle Early Cretaceous, the second downfaulted event occurred from the late Early Cretaceous to the Late Cretaceous and formed a half-graben basin (Bi, 2016).

FIGURE 9. Sedimentary characteristics in the Jianning and Taining Basin, western Fujian Province (for locations see Figure 1). (A) Geological map of the Jianning and Taining Basin (revised after Ren & Zhang, 2021). (B) Section A-A' shows the blind growth strata (Bi, 2016). (C) Section B-B' shows the surface growth strata (Ren & Zhang, 2021).

The terrestrial deposits of the Bantou Formation are underlain by the Douling Group and overlain by the Chishi Group, with the two groups separated by an angular unconformity (Fujian BGMR, 1997). The deposits mainly consist of greyish green and greyish black siltstones, tuffaceous sandstones and shales, with thin interbeds of fine sandstones, sandy conglomerates and small tuff deposits (Cao et al., 1990). After a volcanic event, the terrestrial sediments of the Junkou Formation of the Chishi Group were unconformably underlain by the Baiyashan Formation and conformably overlain by the Shaxian Formation. The sediments mainly consist of greyish, greyish yellow, and greyish green siltstones, along with calcareous siltstones and dark grey mudstones that were deposited in a fluvial environment. The upper Shaxian and Chong'an formations are a suite of terrestrial sediments. The Shaxian Formation is widely composed of a red clasolite series of fluvial–lacustrine facies, including mudstone, siltstone, calcium siltstone, lithic siltstone, sandstone, composite conglomerate and pebbled complex sandstone. The Chong’an Formation comprises thick sandy conglomerate, composite conglomerate and pebbled complex sandstone.

Geophysical survey and borehole drilling have been performed in this area since 2005 for coal and oil–gas exploration (Ren & Zhang, 2021), resulting in several geological profiles (Figure 9B). Obviously, section A of Figure 9 exhibits type III growth strata and section B shows the type I growth strata. These two type growth strata are all developed in the Late Cretaceous Chishi Group.

Sedimentary characteristics of the strata in other basins

Many other Mesozoic basins have experienced Cretaceous sedimentary processes (Table S1). Some representative pictures are included in this paper to show the characteristics of the extensive growth strata.

The Ningdu Basin in southern Jiangxi Province consists of the Upper Cretaceous Ganzhou Group (K₂GZ) and Guifeng Group (K₂GF) in ascending order (Chen et al., 2019; Figure 10). The Ganzhou Group, containing the Maodian Formation (K₂m) and Zhoutian Formation (K₂z), is mainly composed of feldspathic quartz conglomerate, sandy conglomerates, gravel-bearing anisomeric sandstone, sandstone, siltstone, lithic siltstone and calcareous siltstones and mudstones. The Guifeng Group, including the Hekou (K₂h) and Tangbian (K₂t) formations, is mainly composed of composite conglomerate, quartz conglomerate, pebbled complex sandstone to pebbled complex sandstone, siltstone, calcareous siltstones and mudstones. Several boreholes and prospecting lines along the NW–SE-oriented profiles reveal the deep features of the Ningdu Basin. One of these sections (Figure 10B) shows clear evidence of type III growth strata.

FIGURE 10. Sedimentary characteristics in the Ningdu Basin, southern Jiangxi Province (for location see Figure 1). (A) Geological map of the Ningdu Basin. (B) Explanatory section showing the blind growth strata.

The Xingguo–Ganzhou Basin and Yudu Basin in southern Jiangxi Province have similar compositions as the Ningdu Basin. A riverbank exposure in the eastern Xingguo County, north-east margin of the Xingguo–Ganzhou Basin, exhibits clear sequences of the Hekou Formation (Figure 11A). The thickness of a single tilted bed increases slightly from the higher to the lower end, exhibiting the shape of growth stratum. On the western margin of the Yudu Basin, the thickness increases, and synsedimentary faults within the Hekou Formation (K₂h) appear simultaneously at the highway slope (Figure 11B), indicating that they were shaped during a regional extension period.

FIGURE 11. Sedimentary characteristics of the strata in other basins (for location see Figure 1). (A) Growth strata in the Xingguo–Ganzhou Basin, southern Jiangxi Province. (B) Growth strata in the Yudu Basin, southern Jiangxi Province. (C) Growth strata in the Shaxian Basin, western Fujian Province (after Li, X.H.(Xianghui) et al., 2009).

Sections in other basins exhibit characteristics of syntectonic sedimentation (e.g., the Yong'an Basin in Fujian Province, Figure 11C, Xu, S. et al., 2019a). Because of limited space and similarity, no additional detailed descriptions are given here.

DISCUSSION Geometries and mechanism of the growth strata

Worldwide, in the last two or three decades, there has been renewed interest in growth strata and their link with tectonic structures (Suppe et al., 1992; Poblet et al., 1997; Gawthorpe & Hardy, 2002; Ghiglione et al., 2002; Patton, 2004; Ahmadi et al., 2013; Bodego & Agirrezabala, 2013; Santolaria et al., 2015; Fennell et al., 2017; Lewis et al., 2017). Accurate analysis of growth strata can reveal their relevance to structural kinematics and the timing of deformation (Vergés et al., 2002; Zhang et al., 2020). Different geometries of growth strata in both compressive and extensive settings is recognised as a useful tool for determining the kinematics of structures (Ford et al., 1997; Storti & Poblet, 1997; Casas-Sainz et al., 2002; Rafini & Mercier, 2002; Pochat et al., 2009; Santolaria et al., 2015; Lin et al., 2022). Unfortunately, the depositional morphology and geometries of growth strata in the SECB have previously received limited attention.

On the basis of the integral analysis of the above mentioned various growth strata and summarising the worldwide literature, three kinds of extensional growth strata are identified in the SECB, namely, blind or buried growth strata in graben basins, growth strata governed by the linked fault system (same or opposite dip directions), and growth strata with border surface (limb) rotation in half-graben basins. Each type exhibits different characteristic patterns and distribution.

Type I growth strata with border surface (limb) rotation (Figure 12) in half-graben basins are often developed at the boundary of an angular unconformity in a half-graben basin and can be easily observed in the field. Unlike synorogenic compressional growth strata with limb rotation controlled by reverse faults (Casas-Sainz et al., 2002; Espurt et al., 2012; Ortner et al., 2016), the unconformity surface scarps of half-grabens rotate from gentle to medium angles because of regional extension. When extension proceeds, the border migrates outward, and the dip angle of the border scarp gradually increases. The sediments are thus deposited gradually and approximate the shape of extensional growth strata, similar to, for example, the southern part of the Huangshan Basin, the south-eastern part of the Jinhua–Quzhou Basin, the south-eastern side of the Xinjiang and Yongfeng–Chongren basins and the Xingguo–Ganzhou Basin, etc.

FIGURE 12. Kinematic model of type I growth strata. (A) Initiation of the extensional basin. (B) Process of the deposition of growth strata caused by the migration of the border outward and clockwise rotation of the border line. (C) Further depositional process of growth strata. (D) Explanation of all processes.

Type II growth strata are governed by the linked fault system (involving a listric border fault) in half-graben basins (Figure 13). This kind of growth strata must have formed by directional change in the sediment source. The sediments of type II strata mainly come from the northern and north-western parts of the basins (like the Jinhua–Quzhou Basin), i.e., the source is far from the footwall of the normal faults of the half-graben basins (Seidel et al., 2007).

FIGURE 13. Kinematic model of type II growth strata. (A) Retreat of the footwall of a normal fault and receiving sediments. (B) Rotation of the fault surface and receiving sediments, and thus shaping growth strata in the basin.

Type III growth strata are governed by the linked normal fault system and often blind or buried in deep graben basins (Figure 12A through H). Schlische and Olsen (1990), Schlische (1991), and Hamblin and Rus (1989) provided some half-graben basin-filling models to generalise the sedimentary characteristics. Obviously, the deposition rate is less than the basin subsidence rate, meaning that the area has undergone a rapid extension process (Monaldi et al., 2008; Liang & Wang, 2019). As mentioned by Sharp et al. (2000), the type III growth strata are distributed at or near the bottom of the basin and are ended directly by normal faults (involving listric border faults). Thus this kind of growth strata may be revealed by drill holes or seismic tomography. Crustal extension and the consequent rotation of the block-bounding faults, coupled with footwall flexing, resulted in the passive steepening of mesoscale antithetic faults and associated half-grabens during domino faulting. In contrast, mesoscale synthetic faults were rotated to shallower angles (Sharp et al., 2000). Three sections (Figure 6) from the Jinhua–Quzhou Basin clearly show this type of growth strata within the Zhongdai Formation and Jinhua Formation in the hanging walls of normal faults. The profile across the Ji’an–Taihe Basin (Figure 8C) also shows type III growth strata within deep Cretaceous strata.

The three kinds of extensional growth strata proposed above are based on a consideration of various basin types and their associated filling features worldwide (Bodego & Agirrezabala, 2013; Gunderson et al., 2014; Fennell et al., 2017; Liu et al., 2018; Lin et al., 2022; and references therein). Each type exhibits special geometries. Type I strata are caused by rotation of the unconformity surface scarp from a gentle angle to a medium angle (Figure 12). Type II strata are formed relative to a varied normal fault (Figure 13). Type III is governed by the linked fault system (Figure 14). When regional extension enlarges the basin, the hanging wall of the normal fault moves outward (Figure 14A,B), and the sediments collapse into the void (Figure 14C), thus resulting in a bent basal surface (Figure 14D) that receives sediments (Figure 14E,F). The difference in the thickness of sediments is more obvious when extension is sustained (Figure 14G,H). Compared to types III, type II strata are shaped by both the dip angle changing and the footwall moving outward faster than the hanging wall (Figure 13). Perhaps the sediments are sourced from the rear of the footwall (consult with Seidel et al., 2007). When the dip angle of the normal fault surface gradually changes, the sedimentation on the hanging wall slowly forms growth strata (Figure 13B). These characteristics indicate that the sedimentary area has undergone an extensional process with the expansion and deepening of the depositional basin. Other types of growth strata are all depositional records of regional extension.

FIGURE 14. Kinematic models of type III growth strata. (A–F) Type IV growth strata with: (A and B) the hanging wall of the normal fault moves outward; (C) the sediments collapse into the void area; (D and E) the bended bottom surface for receiving sediments; (F) sediments are received and growth strata are shaped. (G and H) Samples of blind or buried Type III growth strata in deep basins.

These three types of growth strata have been found in the SECB. In the case of the Huangshan Basin, the lower Huizhou Formation has variable dip angles from medium to gentle towards the basin centre, and the beds exhibit slightly increasing thicknesses downwards from the upper to the lower part (Figure 3), thus approximating the shape of growth strata (type I). This pattern agrees with the development of rotational offlap–onlap geometries at the boundaries between tectono-sedimentary units in cross-sections. Accompanying regional extension and basin enlargement, the border of the sedimentary area migrates outward, and the border surface (the bottom surface in the margin of a sedimentary basin) is rotated by increasing dip angles towards the basin centre (Figure 12). In the Jinhua–Quzhou half-graben basin, different kinds of growth strata developed in different places. Type III develops deep in the basin and is revealed by geophysical methods and checked by drilling holes. Type II strata have been found in the northern part of the Jinhua–Quzhou Basin, while type I strata have been found in the southern part. In the Xinjiang Basin and Yongfeng–Chongren basins, type I growth strata have been found in the south-eastern parts of the basins.

Regionally, in addition to the nearly E–W-trending Huangshan Basin, the long axes of almost all red basins in the SECB have a NE-trending direction, which is the same direction as the normal faults. In addition, the red beds exhibit a preferred NE–SW-trending orientation (Table S1) which approximates that of red basins and relative basin marginal faults.

Temporal controls on the initial timing of extension from the growth strata

As mentioned above, the tectonic regime in the late Mesozoic and the precise timing of the initiation of extensional tectonics are debatable. Researchers agree that multiple episodes of shortening and stretching occurred in the SECB during the late Mesozoic (Zhou et al., 2006; Shu et al., 2009; Li, X.H. et al., 2010; Li, X.H. (Xianghui) 2019b; Ji et al. 2018; Chu et al., 2019; Yu et al., 2020). Accordingly, although regional extension during the Cretaceous possibly occurred in the Early Cretaceous and exerted a stronger influence than during the Jurassic, numerous examples have provided evidence that intracontinental extension in eastern South China began during the Early Jurassic (Yu, X.Q. et al., 2010a, 2020; Cen et al., 2016; Gan et al., 2017). Regarding extension during the Cretaceous, the consensus is that extension began in the late Early Cretaceous, ca 120 Ma (Lapierre et al., 1997; Shu et al., 2007, 2009; Sun et al., 2007, 2013; Li, Z. et al., 2014c; Yang, J.B. et al., 2018a Zhang et al., 2019) or 84 Ma (Tang et al., 2014b) or at least not earlier than 130 to 125 Ma (Wong et al., 2009; Jiang et al., 2011, 2018; Liu, L. et al., 2012a, 2014, 2016; Sun et al., 2015; Zhao et al., 2016; Wang et al., 2020; Yue et al., 2020). This contrasts with a number of papers that argue that extension began during 142–140 Ma (Li, Z.L. et al., 2013c; Deng et al., 2014; Huang et al., 2019), began at 151 Ma (Yang et al., 2021), or was separated into several stages (Li, J.H. et al., 2014a, 2014b; Suo et al., 2019). To date, an increasing number of interpretations indicate that Early Cretaceous crustal extension began ca 140 Ma in response to the slab rollback and retreat of the Palaeo-Pacific plate after intense shortening during the Late Jurassic to Early Cretaceous (ca 155–140 Ma; Ji et al. 2018; Chu et al., 2019; Li, B. et al., 2015, 2016; Li et al., 2020; Liu et al., 2017).

Dating the age of growth strata determines the timing of tectonic deformation (Santolaria et al., 2015). Those volcanic rocks underlying or paralleling with the growth strata are one of the best dating targets. In the Huangshan Basin (Figures 4 and 15), several bentonite beds (which are partly altered from volcanic ash) and volcanic tuffs are found in the bottom part of the Huizhou Formation (this bottom part was once known as the Xintan Formation; Anhui BGMR, 1987; Ren et al., 2016), meaning that volcanic activity continued from the end of Shiling Formation deposition to the beginning of Huizhou Formation sedimentation (Figures 4A and 15). Two stage U–Pb ages of volcanic rocks are obtained from the Shiling Formation and indicate that there are at least two intervals of volcanic activity. The first of these occurred at 153 Ma (Figures 3A and 4A, Liu et al., 2019) or 155 Ma (Yu et al., 2016), with a later episode at 139 Ma (Liu et al., 2019) or 136 Ma (Tang et al., 2016). It is obvious that sedimentation of the Huizhou Formation (K₁h) began at ca 140 Ma, because the upper beds of the Shiling Formation and the bottom part of the Huizhou Formation are parallel (Figure 15). In addition, Ren and Zhang (2021) obtained a U–Pb age of 139.7 ± 1.2 Ma from volcanic rocks of the Douling Group (Figure 2F), which underlie the sandstone and conglomerate of the Chishi Group in the Jianning Basin, Fujian Province, also indicating that continental sedimentation began at ca 140 Ma.

FIGURE 15. Cartoon showing the contact relationship between the Shiling Formation and Huizhou Formation in the Huangshan Basin, southern Anhui Province.

The beginning of sedimentation in other basins discussed above seems to have occurred later than that of the Huangshan Basin. For example, the youngest detrital zircons from the Xinjiang Basin have U–Pb ages of 137 to 170 Ma (Wang et al., 2019), meaning that the extension of the Xinjiang Basin must not have begun earlier than 137 Ma and is therefore slightly later than that of the Huangshan Basin. Therefore, the initial timing of extension is decreased but the scope is increased from north-west to south-east (Deng et al., 2014; Li et al., 2015, 2016; Suo et al., 2019). The initial extension coincides with magmatic activities in the study area. A series of Cretaceous A-type granites formed between 142 and 125 Ma (Jiang, et al., 2011; Wu et al., 2012; Gu et al., 2017; and references therein) in a postorogenic lithospheric extensional setting, similar to the formation of the growth strata. All these reasons support the hypothesis that extension began between ca 140 and 137 Ma.

Temporal controls on the duration of extension

Agreement is lacking regarding the duration of this extensional regime (Li, J.H. et al., 2014b; Liu et al., 2014; Chen, C.H. et al., 2016a; Ji et al., 2017a; and references therein). Li, J.H. et al. (2014b) suggested that this back-arc extension ended at 91 Ma due to NW–SE compression during the early Late Cretaceous and then began a N–S extensional regime that was generated in a back-arc tectonic setting originating from Neo-Tethyan subduction during the Late Cretaceous. Ji et al. (2017a) proposed that extension continued to 90 Ma as a response to slab rollback of the Palaeo-Pacific plate, followed by rift basin suites (56–38 Ma). Chen, C.H. et al. (2016a) suggested that the extensional domain would have continued until the Palaeogene (56–38 Ma). A number of studies suggest that the Cretaceous extensional regime continued until approximately the end of the Cretaceous (Meng et al., 2012; Cui et al., 2013; Li et al., 2015, 2016; Gu et al., 2017; Ji et al., 2017a, 2017b; Jiang et al., 2018; Huang et al., 2019; Li, X.H.(Xianghui) et al., 2019b; Suo et al., 2019). Others argue that a compressive tectonic regime existed within the Cretaceous (Faure et al., 1989; Li, J.H. et al., 2012a, 2020; Yang, 2013; Niu et al., 2015; Chu et al., 2019). Li, J.H. et al. (2012a) report that in the late Early Cretaceous, the tectonic regime changed to a compressional regime with NW–SE compression and NE–SW extension, causing the inversion of this extensional basin. Lin et al. (2015) suggest that a back-thrust event developed ca 130–105 Ma resulted in the collision of the West Philippine microcontinent with the South China Block rather than this occurring as a consequence of simple oceanic subduction. Kong et al. (2021) argue that the South China Block was under a compressive tectonic regime during the late Early Cretaceous and that this compression continued until at least 103 ± 1.6 Ma. Similarly, it has been argued that the extensional regime must have migrated eastwards (south-eastwards) in response to the lower-angle, medium-angle and rollback of Palaeo-Pacific plate subduction (Wong et al., 2009; Liu et al., 2014; Li et al., 2015, 2016; Zhao et al., 2015; Ji et al., 2017a; Suo et al., 2019). Simultaneously, several evolutionary models during the Cretaceous have been proposed (Xu, X.B. et al., 2016b; Chu et al., 2019; Zhang et al., 2019; Li et al., 2020).

According to the growth strata shown above, especially the two sets of growth strata (like those that developed in the Huangshan Basin), it appears that the extensional event in the SECB was short lived and did not persist until the end of the Cretaceous and that the two stages of the extension should be divided. The first stage is from 140 to 120 Ma, and a second is after ca 105 Ma. The nondepositional hiatus and reverse fault that occurred at the end of the Cretaceous (or continuing to the early Palaeogene in the coastal area) indicate that the second-stage extensional event ended at approximately the end of the Cretaceous (Li, J.H. et al., 2012a, 2014a; Chu et al., 2019; Li, X.H.(Xianhua) et al., Li, Suo, et al., 2019a; Yu et al., 2020).

Tectonic implications of the growth strata in the SECB

In the SECB, a compression and shortening stage occurred during the gap between the Jurassic and Cretaceous (Ji et al., 2018; Chu et al., 2019; Li et al., 2020), forming thrust faults (Xiao et al., 1998; Li, J.H. et al., 2014a; Li, B. et al., 2015; Yu et al., 2020); then, this stage was immediately followed by an extension event at ca 140 Ma that formed extensional growth strata. These geological representations are formed by late Mesozoic flat-slab subduction and rollback of the Palaeo-Pacific plate (Ji et al., 2018; Chu et al., 2019; Li et al., 2020; Yu et al., 2020). Many studies regarding late Mesozoic lithospheric extension have been presented in recent decades (Yu et al., 2003, 2011, 2020; Li, X.H. et al., 2010; Li, J.H. et al., 2020; Ji et al., 2018; Chu et al., 2019; and references therein). Wang et al. (2017) reported on the back-arc extensional environment of the late Permian within the Indochina plate, Ji et al. (2018) focussed on the Jianghan Basin, Chu et al. (2019) examined the Xuefengshan Belt, and Li, J.H. et al. (2020) emphasised the Lianhuashan fault zone. These studies provided two interpretations of the extensional environment in South China during the late Mesozoic. The first interpretation considers that the tectonic environment was a persistent extensional regime (Li, 2000; Li et al., 2010; Zhou et al., 2006). The second interpretation regards the tectonic environment as a compressional regime alternating with strike-slip and extensional movements (Cui et al., 2013; Li, J.H et al., 2012a, 2014a, 2014; Wang et al., 2017; 2021). This interpretation corresponds to changes in the Palaeo-Pacific plate during the late Mesozoic and possibly includes the following events: (1) variations in the dip angle of the subducting plate (Zhou & Li, 2000; Liu, L. et al., 2012a, 2014); (2) changes in the movement direction of the oceanic plate (Sun et al., 2007, 2013; Liu, Q. et al., Liu, Yu, et al., 2012b); (3) slab breakoff and rollback of the subducting plate (Li & Li, 2007; Li, Z.X. et al., 2012c; Li, Z. et al., 2014c; Li, J.H. et al., 2020; Ji et al. 2018; Chu et al., 2019; Yu et al., 2020): and (4) ridge subduction (Ling et al., 2009; Sun et al., 2010; Li, H. et al., 2012b; Wu et al., 2012). At present, the model with changes in the direction or rollback of the subducting plate plays a dominant role in the interpretation of the tectonic and geodynamic evolution of the SECB, despite ongoing debate (Li, J.H. et al., 2014a; 2020; Ji et al. 2018; Chu et al., 2019; Yu et al., 2020). Overall, these interpretations have not confirmed the specific timing of the extensional event during the late Mesozoic.

As explained above, the growth strata developed in half-graben basins. The formation mechanism of the half-graben basins (Schlische, 1991; Seidel et al., 2007), as well as the graben basins (Stewart, 1971; Illies, 1981; Artyushkov et al., 1991; Schultz-Ela & Walsh, 2002), is believed to have been basement uplift, lithospheric stretching and thinning (Xu, Q.J. et al., 2019b). Thus, it is important to establish how and when the subduction and rollback of the Palaeo-Pacific plate generated these half-graben basins. Because of the shortage of research on growth strata, most previous models indicate that the formation of Early Cretaceous potassic magmas during the subduction of the Pacific–Izanagi plate resulted in the upwelling of the asthenosphere, which provided the required high temperature to melt the lower–middle crust in the SECB (Li, J.H. et al., 2013a, 2020; Li, B. et al., 2015, 2016; Liu et al., 2014; Zhao et al., 2016; Gu et al., 2017; Ji et al., 2017b, 2018; Yang, Y. et al., 2018b; Zhang et al., 2018; Chu et al., 2019; Huang et al., 2019). In fact, the upwelling of the asthenosphere also provided the geostress for lithospheric (or crustal) uplift and pull apart near the surface. Figure 16 illustrates the formation mechanics of the half-graben basins. When asthenospheric upwelling occurs, it produces vertical maximum principal stress (σ1) and derives horizontal minimum stress (σ3), combined with a conjugate shear fault system (Figure 16D). In response, single or coupled normal faults are formed at or near the surface. The footwall uplifts and the hanging wall warps down, thus producing a half-graben or graben basin. The thickness of sediments and duration of sedimentation reflect the process of asthenospheric upwelling. As the process continues, the downfaulted basin enlarges and deepens. The bent bottom surface of the downfaulted basin results in sediments in the shape of growth strata.

FIGURE 16. Schematic illustration of the subduction model between the Eurasian and Palaeo-Pacific plates for the petrogenesis of late Mesozoic magmatic rocks in the East Yangtze Block. (A) Location of the established extensional tectonic model. The boundary between the Yangtze and Cathaysia blocks is from Ji et al. (2017). (B) Rollback of the Palaeo-Pacific plate between 140 and 120 Ma (revised after Liu et al., 2016). (C) Rollback of the Palaeo-Pacific plate after 105 Ma (revised after Liu et al., 2016). (D) Forming mechanism of half-graben basins. (E) Possible mechanics of a near W–E-trending oblique extension in the Huangshan Basin and inheritance of the structural framework from the basement.

This interpretation predominates in the Palaeo-Pacific subduction (140–120 Ma; Figure 16B) and slab migration (after 105 Ma; Figure 16C) with an increasing dip angle of the subducted slab, i.e., slab rollback, resulting in the migration of the magmatic belts and the enhancement of lithospheric extension (Figure 16), which would lead to the oceanward retreat of the trench and account for the Cretaceous geodynamics (Li, J.H. et al., 2013a; Li, B. et al., 2015; Ji et al., 2017b, 2018; Yang, Y. et al., 2018b; Chu et al., 2019; Huang et al., 2019). This process generally caused tensile fractures/faults and basins oriented in a SW–NE direction (Li, Z. et al., 2014c; Liu et al., 2016; Zhang et al., 2018; Huang et al., 2019), especially in the south-eastern part of South China (Zhou & Li, 2000; Zhou et al., 2006). With the oceanward retreat of the trench ongoing after ca 140 Ma, the scope of the extension gradually enlarged and rejuvenated south-eastwards until the end of the Cretaceous.

As a special case, the Huangshan Basin is nearly E–W-trending (N75–85° E) (Figure 3). This trend may be more prevalent in shallow-level structures than in deep-level structures. This prevalence is possibly due to the combined effects of plate rollback and ground response. The ground responses of deep upper–middle crustal processes, such as tight folds, penetrative foliations and crystalloblastic textures, are affected by the former tectonic fabrics of the Neoproterozoic basement rocks (Yu et al., 2011; Jiang et al., 2016; Hu et al., 2020, 2021). The structural framework of the basement left a series of nearly E–W-trending foliations, folds and faults; thus, the basin and magmatic intrusions nearby were distributed in response to the local structural frameworks, especially between 140 and 135 Ma (Figure 16E). With the extension of NE–SW-oriented tectonic lines north-eastwards to the junction of the Zhejiang–Jiangxi–Anhui Provinces, the lines were reoriented in an E–W direction by intersecting with the former E–W-trending tectonic fabrics. These faults hence change from a SE–NW extension to an E–W direction sinistral strike-slip motion and provide suitable conduits for granite intrusion and massive accommodation spaces for red strata growth (Figure 3 and Figure 16E).

To sum up, the growth strata considered in this study initially formed during the Early Cretaceous at ca 140 to 135 Ma under the influence of the rollback of the Palaeo-Pacific plate. The oceanward progression of the trench proceeded next as the extension gradually enlarged and rejuvenated south-eastwards until the end of the Cretaceous, except when interrupted by a short compressional event ca 120–105 Ma.

Multiple extension events occurred during different geological episodes and in different locations (e.g. Neoproterozoic extension, Rodinia breakup, Li, Z.X. Li, Li, et al., 2003b; Wang & Li, 2003; Yu et al., 2023; Late Devonian-Early Carboniferous, Zapata et al., 2019; Cretaceous, references see above). The Cretaceous extension event of the South-eastern China Block must belong to a geological event with global significance.

CONCLUSIONS

1. The dip angles of some sedimentary layers in the red basins in the SECB vary from medium to gently dipping towards the basin centre, and the thicknesses exhibit a slightly increasing downwards trend within the same bed. These phenomena are consistent with the shape of extensional growth strata. Three types of growth strata are revealed from different basins.
2. Dating of the volcanic rocks related to red beds reveals that the sedimentary basins were enlarged and deepened when the Early Cretaceous strata were deposited in the SECB since ca 140–137 Ma. It belongs to one of a number of worldwide extensional events.
3. Under the influence of Palaeo-Pacific plate rollback since ca 140 Ma, the SECB exhibits a geostress field that leads to lithospheric uplift and pull-apart structures near the surface, causing the half-graben basins to receive sedimentation. Although this extensional event was interrupted for a short period between 120 and 105 Ma, with the continued oceanward retreat of the trench, the area of extension gradually enlarged and rejuvenated south-eastwards until the end of the Cretaceous.
4. The nearly E–W-trending bed strikes in the Huangshan Basin were transformed from SW–NE-oriented structural lines by intersecting and inheriting the previous E–W-trending tectonic fabrics.

ACKNOWLEDGEMENTS

The National Nature Sciences Foundation of China (nos. 41872201, 91955205) financially supports this work. The English editing service (AMERICAN JOURNAL EXPERTS, their website: https://secure.aje.com/r/QXMNHN84) has helped us to improve the English. The constructive remarks from two anonymous reviewers helped to improve the manuscript.

DATA AVAILABILITY STATEMENT

Authors confirm that data supporting findings of study are available within article and its supplimentary materials.