Three mylonitic bounding shear zones, which named as the Xiaotian-Mozitan shear zone (XMSZ), Shangcheng-Macheng shear zone (SMSZ), and Shuihou-Xishui-Tuanfeng shear zone (SXTSZ) (Figure 2), separate the NDC from the Beihuaiyang belt to the north, the exhumed rock slice of Xinxian-Hong’an UHP/HP complex to the west, the SDC to the south, and the CDHC to the east, respectively.

They are marked by 0.5–1.5 km thick high-strain zones of granitic, biotite-granitic or granodioritic proto-mylonite to ultra-mylonite sequences (Figures 3A–C). The mineral assemblages of chlorite + epidote + biotite + muscovite and amphibolite + biotite + plagioclase are commonly observed, indicating green schist facies to amphibolite facies conditions for the deformation. Mineral shape fabrics in the zones are dominated by typical S-L tectonites. Fine-grained matrix of highly recrystallized quartz, mica, and some feldspar forms clear cleavage domains; and strongly stretched quartz grains and their aggregates, alignment of porphyroclasts (e.g., feldspar, amphiboles) form pronounced stretching/mineral lineations (Figures 4A,B; Figures 5A,B; Figures 6A,B). Most quartz grains were dynamically recrystallized by sub-grain rotation (SGR) and/or high-temperature grain boundary migration (GBM), accompanied by variable amounts of bulging (BLG) recrystallization of feldspar and various crystalline cracks or fractures on amphiboles (Figures 4A,B; Figures 5A,B; Figures 6A,B). These fabrics indicate a thermal condition of 450°C–530°C (Passchier and Trouw, 2005), corresponding to upper greenschist facies to lower amphibolite facies for the deformation (Bucher, 2023). In addition, our field and microscope observation results demonstrate that a consistent fabric gradient traverses the mylonite zones toward the interior of the NDC, which is characterized by a growing degree of compositional segregation, gradually amplified grain sizes and increasingly distinctive lobateness in dynamically recrystallized quartz grains. In particular, recrystallization type of the quartz shifts from predominantly SGR to a combination of SGR and GBM, and ultimately to independent GBM. This trend demonstrates that the deformation temperature is gradual increasing, whereas associated flow stress simultaneously decreases.

In general, there is a notable consistency in both geometry and kinematic/strain-orientation among these zones. The strikes of the shear zones exhibit curvature in different portions. The XMSZ has a gentle north-ward bulging at its middle part, and the SXTSZ shows an S-shaped strike. Even the seemingly straight-striking SMSZ deflects NE-ward at its northern end. Although periodic occurrences of patchy granitic intrusions interrupt the shear zones, these curves indicate a potential for continuity between these framing shear zones surrounding the NDC. Mylonitic foliations generally show outward dipping (with reference to the metamorphic NDC as the inside core) with a wide range of dipping angles. The lineations in the mylonite exhibit a remarkable concentration of plunging directions in the fields of 300/120 ± 20° (azimuth notation), featuring plunging angles ranging from sub-horizontal to sub-vertical and pitches varying from strike-slip type to down-dip type (Figures 2A–C). Asymmetric intrafolial folds, σ- or δ-type porphyroclasts, S-C fabrics, and shear band boudinages (Figures 4A–C; Figures 5A–C; Figures 6A–D) universally indicate top-to-the-NW/NWW kinematics. These observations yield apparent sinistral shearing for the XMSZ (Figure 4C) and dextral shearing for the NW-SE striking segment of the WXTSZ (Figure 6C) in map view, and normal slipping for the SMSZ (Figure 5C) and reverse slipping for the NNE-SSW striking segment in cross-sections (Figure 6D).

The bounding mylonitic shear zones were overprinted by sub-ductile shearing and brittle faulting sequentially (Figures 2A–C). The sub-ductile shearing was processed heterogeneously in centimeter-to-meter thick small shear zones that run parallel to or cut at small angles with the dominant foliations, resulting in either parallel (Figure 4D; Figure 5D) or anastomosing distribution patterns. Chloritization of amphiboles and sericitization of plagioclase prophyroclasts are common retrograde expressions in this small shear zones, leading to darkening of the small shear zones at outcrops (Figure 4D). Lineations/striations on the sub-ductile slipping surfaces are dominated by oblique type (Figures 2A’–C’), corresponding to a universal down-ward motion (Figure 4D; Figure 5D) and concurrent lateral displacements along the strikes (Figures 2A–C). Sharply dipping brittle normal faults cut both the ductile mylonite (Figure 5C; Figure 5E) and sub-ductile shear zones. The major breccia zones are generally located at the outer edge of the ductile mylonitic zones, along which Xiaotian and Macheng volcano-siliciclastic basins were developed in the north and west respectively (Figure 3).

In addition to voluminous plutons forming sharp intrusive contact with the mylonite (Figure 5E), multiple dykes in the mylonitic shear zones show various occurrences. Some dykes with straight boundaries intersect the country mylonites at high angles with the foliation and their interior portion does not exhibit ductile deformation (Figures 4E,F). Some dykes occur parallel to the mylonitic foliation, with straight boundaries and foliated borders (Figures 5G,H). Some dykes were injected parallel to the foliation of the country mylonite, and were progressively sheared into asymmetric (Figure 6F) or symmetric (Figure 8C) boudinages. In addition, some fish-shaped felsic veins are sandwiched between mylonitic layers (Figure 6E). These dykes are usually used as ideal chronometers for determining the deformation timing of the shear zones (Hanmer et al., 1997; Brown and Solar, 1998; Ring and Collins, 2005; Boffadossi et al., 2020).

The inner area of NDC, known as the high-grade basement culmination, serves as the footwall of the inclined mylonitic bounding shear zones. Based on the trajectories of foliation, two dome structures are roughly identified. They are the Luotian dome with a migmatitic core at Huantuling and the Yuexi dome with a migmatitic core near Yanzihe (Figure 2D). Field mapping results demonstrate that both domes generally comprise three types of tectonic lithologies. Their limb areas are dominated by ribbon-striped gneiss (S-L type) at the outer portions and banded gneiss (S>L type) with veined/banded structures at the interior portions, respectively. The core areas mainly dominated by stromatic/layered migmatite (S>>L).

According to their kinematic styles, three flow domains can be identified (Figure 2). The first is the top-to-the-NW non-coaxial flow domain, which corresponds roughly to the ribbon-striped gneissic areas. The second is the NW-SE oriented co-axial dominated flow domain, primarily developed in the banded gneissic areas. The third is the top-to-outward diapiric flow domain that is mainly localized in the stromatic migmatitic areas.

A total of fifteen samples were collected for U-Pb dating. These samples can be classified into three main categories based on lithology and structural occurrence. The first category consists of mylonite or mylonitic gneiss, banded gneiss, and stromatic migmatite, including samples XMa1, XMa3, SM2, SW1, XT2, YX1, and LT20. The second category comprises granitic dykes, dyke-derived boudins, and intrusive pegmatitic veins, including samples XMa2, SM3, SW2, YX2, XT1, and LT21. The third category is composed of granitic plutons, consisting of samples SM1 and YZ13. Their lithological and structural information are presented in Table 2. Methodology for experiment and data processing is available in supplementary documents.

Mineral morphology, internal texture, Th/U ratios, and weighted mean ²⁰⁶Pb/²³⁸U ages of analyzed zircon grains are presented in Table 3. The dating results and detailed description of each sample are presented in the Supplementary documents. Concordia diagrams and representative CL images of the dated zircons are displayed in Figures 10, 11.

A total of 286 concordant ages within the error bars were obtained from the 15 analyzed samples. The ages can be divided into two distinct populations: the Neoproterozoic ranging from 750 Ma to 789 Ma and the Early Cretaceous spanning from 125 Ma to 144 Ma. Neoproterozoic age data were obtained from zircon domains of oscillatory zoning of intact zircon grains (sample XMa1, SM2, SW1, and XT2) and inherited cores (sample XMa3, YX1, and LT20) (Figures 10, 11). Th/U ratios for the analyzed domains range from 0.35 to 1.85. Cretaceous ages dated from felsic mylonite (sample XMa3), granitic plutons (sample SM1 and YZ13), granitic dykes (sample XMa2 and SM3), a felsic vein (sample SW2), granitic boudinages (sample XT2 and XY2), and a granitic pegmatite (sample LT21) are all magmatic in origin for their oscillatory zoning of the internal texture in the analyzed zircons, and relatively high Th/U ratios (>0.2). These ages represent timing of magmatic events (Corfu et al., 2003; Hoskin and Schaltegger, 2003).

All analyzed zircon grains of banded gneiss sample YX1 have a very low and variable Th content of 25–251 ppm and Th/U ratios of 0.01–0.09. Some of them have a core-rim texture, but others do not have core-rim texture with patched or sector zoning or non-zoning. They are likely crystallized from precipitation of metamorphic fluid without breakdown of high Th minerals such as allanite and monazite (Wu et al., 2007b; Rubatto, 2017). They yield a weighted mean ²⁰⁶Pb/²³⁸U age of 144 ± 2.1 Ma (MSWD = 0.26), representing the timing of fluid-favored metamorphism.

Sample LT20 from a leucosome of stromatic migmatite at Huantuling area yields concordia ²⁰⁶Pb/²³⁸U ages of 137–127 Ma. All zircons have a weak or distinct oscillatory zoning in CL images (Figure 11). They have a relatively higher and variable Th content of 90–989 and Th/U ratios of 0.31–0.69. These zircon grains are metamorphic origin as well, but they are more likely crystallized from metamorphic melt (Wu et al., 2004 and Wu et al., 2007c; Rubatto, 2017) at higher temperature conditions in which breakdown of Th rich minerals like allanite and monazite occurs. Their ages record the time of extraction and segregation of the felsic melts from the host rocks to form the leucosome.

Previous researchers initially interpreted the NDC as a crustal terrane that was entirely created by Cretaceous extension-related magmatism (Xue et al., 1997; Hacker et al., 1998). This interpretation is mainly supported by Cretaceous zircon ages of the foliated granitoid rocks known as the grey gneisses regionally. However, the specific nature of the ages was not precisely confirmed at that time. Subsequent research provided geochronological evidence suggesting that the majority of the gneisses in this region may originate from Neoproterozoic protoliths, as indicated by the presence of magmatic-type Neoproterozoic ages in most of the dated gneissic rocks (Xue et al., 2002; Bryant, 2004; Liu and Xue, 2007; Zhao et al., 2008; Xie et al., 2010). In addition, a number of Cretaceous ages in the gneisses were proved to be metamorphic in origin (Wu et al., 2007a; Wu et al., 2007b; Yang et al., 2020).

All the deformed/metamorphic rocks (sample XMa1, XMa3, SM2, SW1, XT2, YX1, LT20) performed by LA-ICP-MS zircon U-Pb dating in our study yield Neoproterozoic ages of magmatic in origin. Four samples (XMa1, SM2, SW1, XT2) preserve intact magmatic zircon grains without inherited cores or overgrown rims. They were all collected from mylonitic rocks on the bounding shear zones. The other three samples yield Neoproterozoic ages dated from the inherited magmatic cores, with overgrown rims resulting from metamorphism (YX1 and LT20) and magmatism (XMa3) in the Early Cretaceous, respectively.

Considering the lithological features and comparable age range of orthogneiss in the CDHC and the Luzhengguan Group in the Beihuaiyang belt (Huang et al., 2016; Jiang et al., 2017), we propose that regionally the protolith of the majority of the orthogneiss in North Dabie is similar to those in the Central Dabie and Luzhengguan Group. These Neoproterozoic magmatic rocks are probably formed during rifting of the Rodinia supercontinent (Li et al., 2003; Zheng, 2004; Bader et al., 2013). Locally, some Early Cretaceous magmatites were involved in dynamic metamorphism and associated deformation, such as the sample XMa3.

Syn-kinematic plutons typically exhibit a geometric concordance between the country rock and the intrusion margins. Essentially, a fabric gradient is usually observable, transitioning from solid-state deformation at the margins to hypersolidus flow within the intrusion (Ratschbacher et al., 2000; Nzenti et al., 2006; Jolivet et al., 2021). Ductile syn-kinematic dykes are commonly parallel to the pervasive foliation of the country rock, exhibiting foliation along their borders or throughout their entire volume. Pinch-and-swell structures, rotation of winged pegmatitic/aplitic inclusions, and intrafolial folding of dykes are additional indicators of syn-kinematic characteristics (Ring and Collins, 2005; Grasemann et al., 2018). Additionally, pervasive precipitation of metamorphic fluid and crystallization of metamorphic melt within the high-grade deforming metamorphic rocks signify syn-kinematic metamorphism (Corona-Chave et al., 2006; Ganzhorn et al., 2014).

Based on the criteria illustrated above, the following features should belong to syn-kinematic intrusions: the granitic dyke with mylonization at the edges in the SMSZ (Figures 7F,G), the granitic vein of a fish-like shape on the Shuihou-Wuhe segment of the SXTSZ (Figure 6E), the delta-shaped felsic boudinage with oriented augen feldspar porphyroblasts on the Xishui-Tuanfeng segment of the SXTSZ (Figures 6F,G), the geometrically symmetric granitic boudinage with weakly foliated edges (Figure 8C), the granitic pluton carrying a foliated carapace (Figure 8D), and the foliated large Tiantangzhan granite complex (Figure 7D). In addition, the banded gneiss (Figure 8C) and nebulitic leucosome (Figure 9A) are interpreted as syn-kinematic metamorphic rocks. The protoliths of the granitic mylonites (Figure 4F; Figure 5F; Figure 6E) are considered as the pre-kinematic intrusions, while the granitic dykes and plutons (Figure 4F; Figure 5E) and pegmatite (Figure 9A) that did not experience ductile deformation are classified as post-kinematic intrusions.

According to the analysis elaborated above, the ductile deformation timing of the XMSZ is suggested to occur after the crystallization of its protoliths, which have been dated as 750.4 ± 7.4 and 142.1 ± 1.4 Ma, respectively. Furthermore, it is proposed to have occurred before the intrusion of the undeformed granitic dyke, dated at 129.4 ± 1.7 Ma, which is consistent with the oldest undeformed plutons reported in the shear zone in a previous study (Wang et al., 2011). Considering the previously reported hornblende ⁴⁰Ar/³⁹Ar ages (Wang et al., 2011), the age of 132 Ma is likely to be synkinematic, while the ages of 127 and 128 Ma likely represent subsequent heating reset ages. The age of 142 Ma is consistent with zircon ages produced in this study and probably represents the crystallization timing of the Early Cretaceous granitic protolith, which survived the later magmatic reheating.

The ductile deformation timing of the SMSZ can be constrained to occur after the protolith age of 743 ± 9.0 Ma and prior to 129.0 ± 1.4 Ma, which is the oldest lower bound reported for undeformed intrusions in the shear zone (Wang et al., 2007 and references therein). Considering the syn-kinematic crystallization age of 136.5 ± 1.7 Ma and hornblende ⁴⁰Ar/³⁹Ar age of ca. 131 Ma (Ji et al., 2017), the ductile deformation timing of the SMSZ is likely to be in the Early Cretaceous and prior to 129 Ma.

Similarly, the ductile deformation of the WXTSZ took place as early as 136.1 ± 1.4 Ma and lasted to 132.0 ± 1.6 Ma in the Early Cretaceous, both of which are syn-kinematic crystallization ages. The oldest undeformed granitic pluton that intruded the mylonitic shear zone was dated as 129 ± 0.8 Ma (Wang et al., 2018), indicating the cessation of the ductile shearing.

In the ribbon-striped gneissic areas, where top-to-the-NW non-coaxial plastic flow dominates the deformation, the syn-kinematic Tiantangzhai granite complex yields LA-ICP-MS and SHRIMP zircon U-Pb ages of 139 ± 3 to 131 ± 4 Ma for the intrusion (Deng et al., 2014). Wu et al. (2007a) obtained SHRIMP zircon U-Pb age of 137 ± 4 Ma for precipitation of the metamorphic fluid in the striped gneisses. In contrast, the ductilely undeformed Zhubuyuan and Baimajian granites intruded into the North Dabie metamorphic complex during 117 ± 11 to 130 ± 3 Ma (Zhao et al., 2004; Wang et al., 2007; Xu et al., 2007; Zhao et al., 2007). These results demonstrate that the non-coaxial flow in this domain commenced as early as 139 Ma and terminated before 130 Ma.

In the banded gneissic areas, where NW-SE oriented coaxial plastic flow controlled the deformation, the syn-kinematically boudinaged dyke in the southern Luotian area yields crystallization age of 138.6 ± 1.8 Ma, while the syn-kinematically foliated pluton in the northern Yanzihe area is dated as 139.9 ± 1.5 Ma. In addition, 143.6 ± 2.1 Ma syn-kinematic precipitation of metamorphic fluid in the southern Luotian area corresponds to the same metamorphic process taking place at 140 ± 2 to 138 ± 2 in the northern Yanzihe area (Wu et al., 2007b). These results collectively suggest that the NW-SE oriented coaxial dominated middle-lower crustal flow occurred during ca.144 ∼ 138 Ma.

The leucosome of the migmatite in the lower/core domain of the southern Hutuling migmatitic dome defines the metamorphic melt activities at 137 Ma to 127 Ma, which is coeval with the 137–129 Ma for migmatites (Wang et al., 2018) in the northern Yanzihe area. In addition, undeformed pegmatite yields a crystallization age of 124.7 ± 2.0 Ma in our study. These results demonstrate that diapiric-style submagmatic flow in the North Dabie probably took place during ca. 137–125 Ma.

The closure temperature of the ⁴⁰Ar/³⁹Ar system in biotite is constrained to 310°C ± 40°C (Harrison et al., 1985; Grove and Harrison, 1996; Oriolo et al., 2018). At this temperature range, felsic rocks deform sub-ductilely in common geological situations, with quartz exhibiting plastic behavior and feldspars deforming in a brittle manner (Passchier and Trouw, 2005; Fossen, 2016). Biotite ⁴⁰Ar/³⁹Ar ages therefore serve as reliable measure for timing of the sub-ductile deformation. Mylonites in the XMSZ yielded biotite ⁴⁰Ar/³⁹Ar ages of 123 Ma and 121 Ma (Y Wang et al., 2011), consistent with ages of 126–124 Ma of the SXTSZ (Ratschbacher et al., 2000; Hou et al., 2007; Lin et al., 2015; Ji et al., 2017). A syn-kinematically boudinaged granitic pegmatite in the secondary sub-ductile shear zones of the SMSZ yielded a crystallization timing of 126 ± 4.2 Ma (Wang et al., 2007). These age data (Figure 12) demonstrate that the three bounding shear zones successively evolved into a ductile-brittle regime and experienced sub-ductile deformation during ca.130∼120 Ma. Analyzed K-feldspar age spectra in the North Dabie complex indicate an overall cooling of the metamorphic complex at ca. 200°C during ca. 120 Ma. This implies that the North Dabie metamorphic complex uplifted into a brittle regime from that point onwards (Chen et al., 1995; Eide et al., 1994; Hacker et al., 1995; Hacker et al., 2000; Ji et al., 2017; Ratschbacher et al., 2000; Wang et al., 2011).

Consistency in deformation is evident across multiple aspects between the bounding mylonitic shear zones, leading to a compelling conclusion. Firstly, a consistent NW-SE directed lineation and a uniform presence of top-to-the-NW/WNW ductile motions are observed within all shear zones, although their strikes and dips are variable. Secondly, through meticulous dating work, we have successfully established the temporal synchronicity of the ductile deformation between these bounding shear zones. Lastly, the thermal conditions of the ductile shearing show remarkable similarities, suggesting a uniform depth of the deformation if assuming a consistent thermal gradient existed at that time. These results demonstrate that these shear zones probably originated from a single, nearly flat-lying shear zone (Lin et al., 2015; Ji et al., 2017).

The NW-SE directed top-to-the-NW/WNW flow not only permeates the mylonites in the bounding shear zones but also extends into the ribbon-striped gneisses of the interior limb areas of the domed metamorphic core complex in the region, during the same geological period. This observation gains further credence when considering the discernible fabric gradient and the scarcity of crosscutting relationships. These results demonstrate that the large-scale shear zone experienced a downward thickening, forming the mylonitic lithologies in the upper part of the middle crust and the gneissic lithologies in the middle part of the middle crust. Consequently, a large-scale detachment processing zone was delineated.

Combining the NW-SE oriented coaxial flow domain occurring in between and the diapiric-styled submagmatic flow domain located at the core, a refined kinematic model can be better established for the North Dabie composite extensional complex. This extensional complex consists of the exhumed HP/UHP rock slices and juxtaposed supra-detachment basins as the rigid upper plate, while the viscously flowing mylonitic-gneissic-migmatitic-magmatic crust acts as the soft lower plate. The upper plate behaves in a brittle manner, whereas the lower plate shows ductile flow in various styles (Figure 13A).

The flowing lower plate exhibits distinct structural characteristics: a large-scale detachment processing zone at the top, a coaxial plastic flow zone in the middle, and a sub-magmatic diapiric uprising flow from below (Figure 13A). The detachment processing zone accommodates intense strain at upper-middle levels of the middle crust (upper greenschist facies to middle amphibolite facies) through top-to-the-NW shearing (Figure 13B). The coaxial plastic flow zone, stretched in the same direction, develops in the middle-lower levels of the middle crust (upper amphibolite facies). The migmatitic subsolidus flow ascends from deeper levels in the lower crust, exhibiting a top-to-outward diapiric kinematic style under the granulite facies conditions. Syn-kinematic melts inject into the flowing rocks, further weakening the crust and enhancing their efficiency of flow. Isostatically driven doming and diapiric flow (inflation and buoyancy) unevenly uplift the detachment processing zones and the coaxial flow zone, resulting in the formation of the North Dabie gneiss-migmatite dome system. When the upper plate is eroded, this uplift exposes them as a metamorphic window. Consequently, the detachment mylonitic zone separates into the Xiaotian-Mozitan shear zone to the north, the Shangcheng-Macheng shear zone to the west, and the curved Shuihou-Xishui-Tuanfeng shear zone to the south and east (Figure 13A).

Post-collisional collapse of the DOB has been widely employed by numerous researchers to explain the Early Cretaceous tectono-thermal event especially the magmatic activities in this period within the DOB (Zhao et al., 2004; Zhao et al., 2005; Zhao et al., 2007; Zhao et al., 2008; He et al., 2011; Li et al., 2013; He et al., 2022). However, the undeformed Jurassic sedimentary rocks unconformably overlie the deformed pre-Triassic rocks in the northern Behuaiyang belt and the southern Yangtze foreland fold-and-thrust belt, indicating the termination of the orogeny prior to the earliest Jurassic. Geochronological data also demonstrate a >100 Ma temporal gap between the initial collision of NCB–SCB during ca. 270–252 Ma and the onset of the Early Cretaceous magmatic activities (ca. 143 Ma). Therefore, it is difficult to link the Early Cretaceous tectono-thermal event with the Late Permian-Triassic orogenic processes.

Considering the tectonic settings of the DOB in the Early Cretaceous, several dynamic factors potentially contributed to this extensional tectonics. A thickened lithosphere with a crust over 50 km thick in the Dabie orogenic belt (He et al., 2011) was formed through a robust continental collision between the North China Block (NCB) and the South China Block (SCB), resulting in significant gravitational instability. To the north, the closure of the Mongol-Okhotsk Ocean caused a large field of contractional regime until the Early Cretaceous (Cogné et al., 2005; Fritzell et al., 2016; Sorokin et al., 2020). To the southwest, the consumption of the Meso-Tethys Ocean along the northern margins of Lhasa-Indosinia (Metcalfe, 2013; Fan et al., 2021; Liu et al., 2022) potentially causes far field compression to the Dabie region. To the east, the paleo-western Pacific plate subducted beneath the East Asian continent accompanied with a widely proposed slab roll-back during this period (Zhu et al., 2017a; Zhu and Xu, 2019).

Regionally, extensional structures, such as MCC type extensional gneissic domes, detachment faults, and extension-related basins, are widespread from Trans-Baikal to SCB, over more than 3,000 km along the eastern Asian continent (Figure 14) (Huang et al., 2021; Lin et al., 2015; Wang et al., 2011; Zhou et al., 2012). These extensional structures exhibit a consistent NW-SE oriented maximum stretching direction. They were predominantly developed during the Early Cretaceous of ca. 140–120 Ma (Lin et al., 2015; and references therein). Obviously, these extensional structures likely share a common geodynamic origin. It is considered that the far-field back-arc extension caused by the northwest-directed subducting paleo-western Pacific plate may contribute the main force to drive the large-scale extension (Lin et al., 2015; Zhu et al., 2017b; Jiang et al., 2017; Li et al., 2019).

Together with the spatiotemporal configuration of the structures and associated metamorphism and magmatism in the NDC, a schematic multi-stage tectono-structural evolution for the extensional deformation is outlined as follows:

(1) Development of NW-SE coaxial flow in the middle-lower crust at 144–138 Ma (Figure 15A)

The exhumed HP-UHP rock slices overlay as a cover of the DOB before the extension started to take place (Hacker et al., 2000; Faure et al., 2003), contributing to substantially thickening of the Dabie lithosphere. As a result, gravitational instability was fostered and eclogitization probably occurred at the bottom of the lower crust (Wang et al., 2007). Strong rollback of the subducting Paleo-Pacific plate and fast trench retreat in the eastern Asia during the Late Jurassic-Early Cretaceous (Meng et al., 2014; Zhu et al., 2017a; Li et al., 2019) likely induced a significant change in tectonic regime for the DOB from N-S direction dominated contraction into NW-SE direction dominated extension/transtension.

In light of these circumstances, it is evident that the lithosphere of the DOB experienced a significant phase of NW-SE-directed coaxial-dominated flow within the coherent middle-lower crust. This flow began at ca. 144 Ma and lasted to ca. 138 Ma, marking the initiation of the regional extension. This crustal flow introduced fluctuations into the highly active, non-equilibrium orogenic system of the DOB. This dynamic process also facilitated mobility of metamorphic fluids and locally triggered decompression-related adakitic melts, which originated from the eclogitic bottom of the lower crust.

(2) Formation of the detachment processing zone and initiation of migmatitic diapric flow at 140–130 Ma (Figure 15B)

The emplacement of adakitic granites heterogeneously softened the crust in the middle levels. As a result, regional extension was primarily localized into the weakened upper-middle part of the middle crust, giving rise to the development of a large-scale detachment processing zone. Deformation of the zone likely initiated around 140 Ma, slightly later than the onset of the magmatic activities (Ca. 143 Ma according to Wang et al., 2007). This zone accommodated the extensional strain by means of top-to-the-NW/NWW non-coaxial ductile flow, which lasted to ca. 130 Ma.

Structural weakening during the detachment processing enhanced permeability of the deformation domain (Fossen and Cavalcante, 2017), and made it an effective channel path for crustal melts to transfer (Brown and Solar, 1998; Jolivet et al., 2021). The syn-kinematic Tiantangzhai granite complex was arrested during this course.

Pervasive micro-mesoscopic partial melting and perlocative flow of the melts within the migmatites in the lower crust induced decrease of its density and viscosity (Gilbert and Merle, 1987; Rey et al., 2009). These changes prompted rheological instability, which further initiated a diapiric flow of the migmatitic lower crust (Yin, 2004) at ca. 137 Ma. Radial stretching lineations and top-to-outward kinematics characterized this flow.

The detachment-related arching and the diapric flow induced differential uprising incrementally domed the region; isostatic-rebound caused by the extensional thinning increasingly uplift the metamorphic lower plate up to higher levels.

(3) Heterogeneous doming inside and uniform uplift along the bounding shear zones at 130–120 Ma (Figure 15C)

The extraction of melts from the eclogitic bottom of the lower crust persistently increases its density, which ultimately triggered mantle delamination (Zhao et al., 2007; He et al., 2011; Xu et al., 2012; Li et al., 2013; Zhang et al., 2015; Li et al., 2021). This allowed upwelling mantle to pump mafic-ultramafic magma up into the crust, and to heat the lower crust to produce normal granites. Magmatic activities reached the peak at this stage. The powerful magma inflation and forceful buoyancy heterogeneously uplifted the detachment processing zone and the metamorphic complex beneath it. Consequently, the dome-shaped zone wrapped the gneiss-migmatite core, with its roof part covering the core and the wall part surrounding the core.

Meanwhile, incremental localization of the extensional strain resulted in networks of outcrop-scale strike-slip-normal sub-ductile shear zones that primarily overprinted on the tilted wall part of the detachment processing zone. This structural style accounted for a uniform uplift of the entire North Dabie metamorphic complex to a higher level of approximately 6–7 km (200 °C) in the upper crust around 120 Ma. During the same process, this sub-ductile shearing separated the roof part of the dome-shaped detachment from its wall part, and progressively split the initially continuous wall part into different segments–the rudiments of the individual bounding shear zones.

(4) Thinning by superposition of normal brittle faulting after 120 Ma (Figure 15D)

Dwindling of magmatism after 120 Ma in the Dabie orogen (Xu et al., 2002; Wang et al., 2007) coincides with the shallowing dip of the westward-subducting Paleo-Pacific plate (Zhu et al., 2017a). Intensity of the extensional deformation probably decreased, which is indicated by a slow-down of the uplift speed of the North Dabie metamorphic complex after this time (Wang et al., 2014). Normal brittle faulting superposed on the metamorphic complex. This brittle deformation in particular enhanced activity of the already superposed bounding shear zones, and transferred them into major normal faults that controlled the growth of the detachment-related basins such as the Xiaotian basin to the north and the Macheng basin to the west (also Figure 12). The exhumed HP/UHP slices served as considerable sedimentary sources to feed the basins. This is evidenced by Triassic aged zircon grains found in the Early Cretaceous sediments and volcanic rocks on the north foot of the Dabie Mountains (Zhu et al., 2017b).

As denudation continuously took place, the mylonites in the roof part of the arched detachment were removed; the ribbon-stripped gneisses, banded gneisses and migmatites were consequently exposed in the metamorphic window. The separated segments which were originated from the wall part of the detachment are finally located individually around the metamorphic window, as the Xiangtian-Mozitan shear zone to the north, the Shangcheng-Macheng shear zone to the west, and the Shuihou-Xishui-Tuanfeng shear zone to the east and south.