Dating Subhorizontal Ductile Fabric in the Feidong Complex via Zircon and Titanite U–Pb Geochronology: Insights into Middle Triassic Transpressional Deformation along the Southern Tan-Lu Fault Zone Open Access

Continental collision is one of the most conspicuous phenomena among plate tectonic processes, often resulting in linear metamorphic, magmatic, and deformation belts that parallel the collided plate boundaries [1, 2]. The collision of two continental blocks with contrasting rheology and irregular boundaries can lead to complex deformation patterns not only at the plate boundaries but also within plate interiors [3-7].

The Mesozoic collision of the North China and the Yangtze blocks [8-10] led to the ultrahigh-pressure (UHP) metamorphism of Yangtze block rocks along the Dabie-Sulu orogenic belt [11-15] and large-scale fold-and-thrust deformation in both blocks [16-18]. Subsequent movement along the southern Tan-Lu fault zone, a narrow (<30 km wide), steep lithospheric-scale structure, resulted in the ~600 km offset of the Dabie and Sulu UHP metamorphic belts, further modifying the positions of the North China and Yangtze blocks (Figure 1(a)) [19, 20]. The Tan-Lu fault zone is widely regarded as forming during the collision of the North China and Lower Yangtze blocks. Tectonic models proposed to explain its development include postcollisional transcurrent fault [21, 22], syncollisional transcurrent fault [16, 23, 24], an indenter boundary [19, 25], a syn-exhumation transform fault [26], plate tear fault [27], and postcollisional rotation [28]. Previous studies have conducted detailed reviews on these different tectonic models [16, 19, 22], the key point of the debates between different tectonic models is how to coordinate the complex deformation patterns of the tectonic elements (e.g. Tan-Lu fault zone, Sulu-Dabie orogens, and foreland fold-and-thrust belts) involved.

While the large-scale structures of the North China and Yangtze blocks, the Dabie and Sulu UHP belts, and the Tan-Lu fault zone have been identified (Figure 1(a)), the detailed structural relationships, kinematics, and timing that resulted in the first-order spatial distribution of tectonic elements remain underexamined. The various ductile structures exposed along the southern Tan-Lu fault zone, specifically, are key to understanding the kinematic relationships between the Sulu-Dabie orogen, the fault zone itself, and the Lower Yangtze fold-and-thrust belts.

The Zhangbaling–Feidong uplift zone exposes former midcrustal rocks along the western margin of the Lower Yangtze block (Figure 1(b)). It includes the Zhangbaling Complex in the north and the Feidong Complex in the south (Figure 1) [19, 24]. The Neoproterozoic Zhangbaling Complex [29, 30] comprises basement rocks with Triassic greenschist-facies metamorphism, which exhibits a subhorizontal foliation (<30°) and stretching lineation with a generally NE to N trend [24, 31, 32]. The Paleoproterozoic Feidong Complex [33-35], in contrast, contains both steeply dipping and subhorizontal ductile fabrics and preserves amphibolite- to greenschist-facies metamorphism [36-38]. The steep fabrics in the Feidong Complex have sinistral strike-slip kinematics that reflect two phases of Early Cretaceous shear [36, 38-40], while the timing and kinematics of the subhorizontal fabrics in the Feidong Complex remain little understood [19, 24, 41, 42]. The lack of detail about the kinematics, spatial distribution, and timing of formation of the subhorizontal ductile structures in the Zhangbaling–Feidong uplift zone hinders a comprehensive interpretation of their significance within the regional tectonic framework.

In this study, we present new structural, quartz electron backscattered diffraction (EBSD), thermobarometry, and zircon-titanite U–Pb geochronology data from a subhorizontal, amphibolite-facies shear zone with top-to-south kinematics in the eastern part of the Feidong Complex. These new data help document Triassic transpressional deformation along the southern Tan-Lu fault zone, which improves the understanding of the tectonic framework during the collision of the North and South China blocks.

The Zhangbaling Complex extends ~80 km along in NNE trend, tapering in width from ~30 km in the north to ~10 km in the south (Figure 1(b)) [43]. It encompasses the lower amphibolite- to greenschist-facies Zhangbaling Group in the west and slightly metamorphosed (i.e. subgreenschist facies) Ediacaran sandstone/siltstone units (i.e. Zhougang–Doushantuo Formations) in the east [24, 30, 44]. The Zhangbaling Group has protolith ages that range between ca. 770 and ca. 635 Ma [29, 45, 46] and experienced metamorphism (~3–7 Kbar, 350–450℃) [44] during Triassic Dabie-Sulu orogenesis (ca. 245 to 235 Ma) [24, 31]. Ductile deformation recorded in the Zhangbaling Complex is characterized by a subhorizontal foliation and a strong NNE-SSW trending stretching lineation with top-to-SSW shear sense [24, 30, 32]. Synkinematic muscovite from ductile rocks in the Zhangbaling Complex yielded ⁴⁰Ar/³⁹Ar plateau dates ranging from 240 to 227 Ma, which has been interpreted to reflect the timing of deformation [24]. To the east of the Zhangbaling Complex, the unmetamorphosed Ediacaran Dengying Formation (dolomite) conformably overlies the subgreenschist facies Ediacaran Doushantuo Formation (slates) [30, 47]; the contact was reworked locally by subsequent thrust faulting/folding [48].

The ~50 km long (5 and 10 km wide) Feidong Complex along the southern part of the Zhangbaling–Feidong uplift (Figures 1(b) and 2) comprises mainly the Paleoproterozoic Feidong Group (plagioclase amphibolite, biotite-plagioclase gneiss, and hornblende-plagioclase gneiss) and Neoproterozoic bimodal intrusions. The rocks of the Feidong Group have protolith ages that range between ca. 2500 and 2380 Ma or ca. 2050 and 1950 Ma [37, 38, 40, 49-51] and experienced amphibolite-facies metamorphism (~4–8 Kbar, 550–700℃) [37, 52]. But the timing of the metamorphism has been estimated differently, ca. 688 Ma [53], or 242 ± 26 Ma [47], or 167–133 Ma [37]. The Proterozoic Feidong Group is unconformably overlain by Meso-Neo Proterozoic phosphorous marble [21, 24, 54, 55]. Neoproterozoic bimodal intrusive rocks are ubiquitous across the Feidong and Zhangbaling complexes (820 640 Ma) and are affected by the Mesozoic metamorphic/deformation events [45-47, 50, 56-58].

As shown in Figure 1(a), the two zones that host UHP metamorphic rocks, the Dabie to the west and the Sulu to the east, are offset by the NNE-striking Tan-Lu fault zone. A compilation of available geochronological data ( online supplementary Table S1 ) from the Dabie-Sulu orogens and along the southern Tan-Lu fault zone (Figure 3) facilitates a comparison of the metamorphism–deformation history associated with collision of the two blocks.

In the Dabie orogen, the record of prograde metamorphism started in the Middle to Late Permian (ca. 267–252 Ma) as zircon, titanite U–Pb, and garnet Lu–Hf geochronological dates [15, 59]. The ultrahigh pressure peak metamorphism (~700–800℃, 3.5–5.0 Gpa) [60, 61] occurred between 240 and 230 Ma (Zircon U–Pb and garnet Sm–Nd) [62-64]. The UHP rocks were subsequently exhumed to the lower-middle crust (~500–600℃, 0.6–1.5 Gpa) between 228 and 210 Ma (zircon-monazite-titanite-rutile U–Pb and garnet Sm–Nd) [65-70]. The same rocks further transitioned to green-schist facies conditions after 210 Ma as recorded by phengite and biotite Rb-Sr data (Figure 3) [62, 65, 71].

Rocks within the Sulu orogen (Figure 1(a)) can be divided into ultrahigh pressure in the north and high pressure in the south. The ultrahigh-pressure rock unit experienced prograde metamorphism from 256 to 240 Ma (zircon U–Pb) [72, 73] with peak metamorphic conditions of 750–850℃ and 3.4–4.0 Gpa reached at 235–228 Ma (zircon U–Pb) [14, 68, 74]. Similar to the Dabie orogen, the ultrahigh pressure rocks in the Sulu orogen were exhumed from >90 km to lower-middle crust level (~550–650℃, 0.7–1.0 Gpa) between 228 and 211 Ma (zircon-titanite U–Pb and garnet Lu–Hf) [74-78], followed by a transition to green-schist facies conditions after ca. 210 Ma (amphibolite-muscovite-biotite ⁴⁰Ar-³⁹Ar and apatite U–Pb dates, Figure 3) [74, 78-81]. The high-pressure rock unit in the Sulu orogen reached peak metamorphic conditions (~500–600℃, 1.0–2.5 Gpa) at ca. 245 Ma or earlier [78, 80] and underwent exhumation to the middle crust between ca. 240–214 Ma, recorded by muscovite-biotite ⁴⁰Ar-³⁹Ar [78, 80, 81].

Most of the Tan-Lu fault zone is buried beneath the Cretaceous sedimentary rocks or the Hefei Basin (19, 48), with only sporadic outcrops of ductile fabrics overprinted on the pre-Cambrian basement rocks of the Lower Yangtze block along the east margin of the Dabie orogen in Lujiang area and in the Zhangbaling–Feidong uplift zone [16, 19, 24, 82]. Muscovite ⁴⁰Ar-³⁹Ar analysis of mylonite from a green-schist facies portion of the shear zone [19, 82] and zircon U–Pb date from granitic gneiss with NE trend foliation from an amphibolite-facies portion of the shear zone [83] indicate that ductile shear deformation occurred approximately between 255 and 203 Ma.

The NNE-striking Fuchashan shear zone is defined by sinistral strike-slip kinematics across a 5- to 10-km-wide ductile high-strain zone within the Feidong Complex (Figures 1(b) and 2) [36, 84]. Our recent investigation of these steeply dipping fabrics identified two phases of strike-slip shearing. The main deformation phase was dated by synkinematic titanite (U–Pb) at ca. 142–140 Ma, whereas the later deformation phase was dated by apatite (U–Pb) at ca. 118–108 Ma [38]. Subhorizontal foliations with NNE-SSW stretching lineations have also been identified in the Feidong Complex’s eastern flank near the Fuchashan shear zone (Figure 2) [19, 24, 42, 47]. Field observations show that the subhorizontal fabrics are crosscut by the steeply dipping strike-slip shear fabrics [38, 39, 47]. Zircon U–Pb dating of a sample from the granitic gneiss that contains the subhorizontal fabric yielded upper and lower intercept dates of 2337 ± 21 and 242 ± 26 Ma, respectively [47], indicating Paleoproterozoic emplacement overprinted by a Triassic metamorphic event.

The subhorizontal ductile fabrics developed in the eastern Feidong Complex form a 1–4 km thick NNE-striking zone of high-strain within the Feidong Group, namely the Lower Yangtze décollement zone (Figure 2). Rocks of this high-strain zone include plagioclase amphibolite, biotite-plagioclase gneiss, hornblende-plagioclase gneiss, and granitic dykes. The foliation within the high-strain zone is dominantly subhorizontal (0° and 35° dip) with variable dip directions (Figures 2(c), 4(a), 4(b), and 4(c)). The foliation is commonly folded (Figures 4(b) and 4(h)), with variable dipping fold limbs (25° and 80°) toward either NW or SE. The fold axe trends in NNE-SSW (~207°) and plunge shallowly (~6°) in SSW (Figure 2(c)).

The lineations on the subhorizontal foliation are defined by aligned hornblende, biotite, elongate quartz, and/or feldspar, trend in NNE-SSW (0° and 40°) and plunge gently (0° and 25°) to NNE or to SSW (Figures 2(c), 4(a), and 4(c)). Locally L-tectonite with sheath folds marked by granitic gneiss surrounding an amphibolite core (Figure 4(h)) are observed. Amphibolite in the high-strain zone shows evidence of migmatization commonly, including the development of banded leucosome and melanosome (Figures 4(f) and 4(g)). The migmatitic rocks experienced strongly ductile shear deformation, the leucosome bands are commonly folded (Figures 4(d), 4(f), and 4(g)) and the mafic minerals (e.g. hornblende and biotite) in melanosome are strongly elongated. The kinematic indicators including the rotated porphyroclasts, asymmetric folds, and S-C fabrics all indicate top-to-SSW sense of shear (Figures 4(d), 4(e), and 4(f)).

Microstructures in the rock specimens described below are observed in thin sections cut parallel to the mesoscopic mineral stretching lineation and perpendicular to the foliation, approximating the XZ plane of the finite strain ellipse, assuming noncomplex flow symmetries.

Specimen FCS-37-11, taken from a migmatitic amphibolite at the “Mawa” exposure (location see Figure 2), comprises primarily hornblende, plagioclase, and quartz with minor epidote, and clinopyroxene (Figures 5(a) and 5(c)). Accessory phases include zircon and titanite. The strongly elongated hornblende defines the main foliation (Figure 5(a)). Quartz is also elongated with lobate boundaries (Figure 5(c)); plagioclase typically has a high aspect ratio with small neoblasts formed along irregular grain boundaries (Figure 5(c)). Minor clinopyroxene occurs as porphyroclast fragments, partially replaced by hornblende (Figure 5(b)). Asymmetrical porphyroclasts, S-C structures, and mineral “fish” all indicate top-to-SSW shear (Figures 5(a)–5(c)).

Specimen FCS-37-3, sampled from a foliated granitic dyke at the “Mawa” exposure (Figure 2), has a mineral assemblage of quartz, plagioclase, biotite, and K-feldspar with accessory zircon, titanite, and apatite; elongated biotite and ribbon quartz mark the foliation (Figure 5(d)). Quartz has lobate boundaries and undulose extinction and exhibits both “dragging,” and “pinning” structures (Figure 5(d)). Feldspar in the specimen forms asymmetric porphyroclasts with serrate boundaries and associated neoblasts (Figure 5(d)) that indicate top-to-SSW shear sense.

Specimen CH-3-1, taken from a leucosome in a migmatitic amphibolite at the “Mawa” exposure (Figure 2), comprises quartz, plagioclase, biotite, and minor hornblende with accessory titanite, zircon, and apatite. Quartz is elongate and exhibits lobate boundaries, pinning structures (Figure 5(f)), and locally “chessboard” extinction (Figure 5(e)). Plagioclase has irregular boundaries, which contain myrmekite locally (Figure 5(e)), and associated neoblasts. Biotite occupies interstitial positions between quartz and feldspar grains (Figure 5(f)); hornblende porphyroclasts are subhedral with irregular boundaries (Figure 5(f)).

Specimens FCS-9-1 and CH-9-4 were collected from migmatitic amphibolite at the Yantou Shan site (location indicated in Figure 2). They both exhibit mineral assemblages of hornblende, plagioclase, quartz ± biotite, with accessory titanite and zircon. The main foliation is defined by strongly elongated hornblende. Elongated quartz exhibits lobate boundaries, undulose extinction, “dragging,” and “pinning” structures (Figures 6(b) and 6(e)). Plagioclase also displays undulose extinction and is moated by small equant grains along seriate grain boundaries (Figures 6(c) and 6(e)). Biotite is only present in specimen FCS-9-4 as interstitial material (Figure 6(e)). The presence of S-C structures, asymmetric porphyroclasts, and the shape-preferred orientation (SPO) of hornblende in both specimens indicates top-to-SSW shearing (Figures 6(a) and 6(d)).

Specimen CH-3-4 is a felsic orthogneiss collected at the Yantou Shan site (Figure 2). It comprises a mineral assemblage of quartz, plagioclase, K-feldspar, and biotite, with accessory zircon, apatite, and titanite. Quartz forms elongated lenses with lobate boundaries characterized by undulose extinction (Figure 6(f)). Plagioclase grains also exhibit undulose extinction, have serrate boundaries with bulging locally, and contain myrmekite along boundaries with K-feldspar porphyroclasts (Figure 6(f)).

In order to determine the dominant slip system in quartz and shear sense during deformation, EBSD analysis of quartz lattice orientations was performed in six specimens. Titanite was also targeted for EBSD analysis to quantify detailed microstructural characteristics of titanite grains in plagioclase amphibolite (specimen FCS-37-11) prior to in situ titanite U–Pb age dating. All EBSD analyses were conducted with a Jeol‐JMS-6490 SEM equipped with an Oxford Instruments Nordlys EBSD detector at the State Key Laboratory for Mineral Deposits Research, Nanjing University (China). Lattice-preferred orientation (LPO) distributions were acquired with a 70° tilted sample geometry, 20 kV accelerating voltage, and 16–24 mm working distance. Diffraction patterns were automatically indexed using Aztec software (Oxford Instruments). Data were processed with Aztec Crystal software (Oxford Instruments). The crystallographic axis distributions in pole figures are presented as one-point-per-grain representations within lower hemisphere, equal area plots contoured with a 15˚ half-width. The strength of the LPOs is evaluated by the J-index of quartz c-axis pole figures and M-index; the J-index ranges from a value of one (a completely random distribution) to infinity (a single crystal), whereas the M-index ranges from zero (a completely random distribution) to one (a single crystal) [85, 86].

The quartz c-axis orientations in the six specimens examined are dominated by Y-maximum distributions [87] with varying asymmetry (Figures 7(a)–7(f)). Such distributions are consistent with dominant prism <a> slip [87, 88], with additional contributions from the rhomb <a> slip system (Figures 7(a)–7(d) and 7(f), legend) [89]. The quartz a-axis distributions within these specimens show a consistent pattern of three maxima asymmetrically disposed about the macroscopic fabric axes with the strongest a-axis maximum at the margin of the X direction with an angle less than 30° (Figures 7(a)–7(f)) [87]. The asymmetry of both the quartz c-axis and a-axis distributions indicate top-to-SSW shearing.

Low-angle (<15°) misorientation axis distribution patterns (see Figure 7(g)) can be used to distinguish the slip system(s) that contributed to the recrystallization of quartz in the specimens [90-92]. In crystal coordinates, misorientation axis distributions (for misorientation angle of 2°–15°) of six specimens generally show a clustering on the c-axis, only FCS-37-11 and CH-3-1 have maxima spreading toward the rhomb position (Figure 7(g)), which indicates the slip systems are dominated by prism <a> slip with minor rhomb <a> slip (Figure 7 legend).

A string of titanite grains found in specimen FCS-37-11 were examined via EBSD. These titanite grains have a strong SPO with long axes (sub-) parallel to the foliation (Figures 8(a) and 8(e)). The poles of the {010} crystallographic plane show a point-maximum distributed about the X direction, the poles of the {001} crystallographic plane display poorly developed maxima distributed near the Z and Y directions, while the poles of the {100} planes define multiple point maxima (Figure 8(d)). The pole figures and high J-index (1.86) and M-index (0.239) values demonstrate that the LPO for these titanite grains is highly ordered. Eighty-five percent of titanite grains have grain orientation spread (GOS) values of <4˚ [93] with the remaining 15% having GOS values between 4° and 6.7° (Figure 8(c)). Moreover, 70% grains show only small, local misorientations (<1°) in kernel-average maps; about 30% have abundant subgrain boundaries with higher local misorientation (1°, and 4°) along the grain boundaries (Figure 8(b)). In summation, these titanite grains are characterized by strong SPO, LPO, general low GOS value, and locally distributed low-angle (1° and 4°) misorientation along grain boundary.

To gain further information about the pressure and temperature conditions during deformation, we evaluated the major element compositions of hornblende and plagioclase from two amphibolite specimens (FCS-9-1 and FCS-37-11) by Electron Probe Microanalyzer analyses (EPMA) analysis. EPMA were conducted at the State Key Laboratory for Mineral Deposits Research of Nanjing University using a JEOL JXA-8100 electron microprobe. Natural and synthetic mineral standards were used to calibrate all quantitative analyses (detailed methods can be found insupplementary materials) (online supplementary Table S2) and a ZAF (atomic number [Z] effect, absorption [A] effect, and fluorescence excitation [F] effect) correction was applied. Operating conditions and data processing followed the methods described by Wu et al. [94]. The analyses were performed with a nominal beam diameter of 1 or 3 µm, at an accelerating voltage of 15 kV, and at a probe current of 20 nA.

Hornblende compositions in the two amphibolite specimens (FCS-9-1 and FCS-37-11) examined a plot in the central area on Mg/(Mg+Fe²⁺)–Si (a.p.f.u) classification diagram (Figure 9(a)); Mg/(Mg+Fe²⁺) ranges from 0.45 to 0.65 while the Si (a.p.f.u) varies between 6.2 and 6.9 (in 23 O atom; Figure 9(a), Table 1). The hornblende of FCS-9-1 fall into the pargarsite–edenite field, whereas for those of FCS-37-11 plot in ferro-pargarsite–ferro-edenite–edenite fields. EMPA analysis of plagioclase in the same two specimens plot in the oligoclase–andesine field in K-Na-Ca ternary space (Figure 9(b)). Plagioclase in FCS-37-11 has slightly higher An (26–36) than those in FCS-9-1 (An = 22–30) (Figure 9(b)).

The strongly elongated hornblende and plagioclase grains in both specimens define/parallel the main foliation (Figures 5(a), 6(a), and 6(d)) indicating that they likely (re)crystallized in the deformation process. Thus, the thermobarometers of these minerals may provide information about pressure–temperature conditions during shearing. Applying the empirically calibrated hornblende–plagioclase thermobarometers [95, 96] for mineral pair data collected from FCS-9-1 returns temperatures between 645 and 720℃ (±40℃; estimated precision) and pressures between 4.3 and 6.6 Kbar (±2 Kbar; estimated precision). FCS-37-11 returns similar results, with calculated temperatures between 640 and 710℃ (±40℃; estimated precision) and pressures between 4.0 and 6.6 Kbar (±2 Kbar; estimated precision).

Zircon U–Pb dating was carried out at the State Key Laboratory for Mineral Deposits Research, Nanjing University, using a ThermoFisher Quad-ICP-MS (Inductively Coupled Plasma-Mass Spectrometry) attached to a GeolasPro 193 nm laser ablation system with an in-house developed sample cell. Detailed analytical procedures are similar to those described by Griffin et al. [97] and Wang et al. [98]. All Zircon U–Pb geochronology data of this study are listed in online supplementary Table S2. U–Pb fractionation was corrected using zircon reference material GEMOC GJ-1 (intercept age of 608.5 ± 1.5 Ma) [99] and accuracy was assessed using the Mud Tank zircon reference material (concordia age of 731 ± 0.2 Ma) [100]. All analyses were carried out using a laser spot size of 32 µm (diameter), fluence of 6.5 J/cm², and a repetition rate of 5 Hz. Raw data were processed using the Glitter 4.4 software program [101]. Correction for common Pb was processed following the method of Andersen [102]. Six repeated analyses of Mud Tank zircon yielded a weighted mean ²⁰⁶Pb/²³⁸U date of 733 ± 17 Ma (mean-squared weighted deviation (MSWD) = 0.04; n = 6/6), which is consistent with the expected age.

U–Pb geochronology of titanite was conducted by Laser Ablation‐Inductively Coupled Plasma‐Mass Spectrometry (LA-ICP-MS) at Nanjing FocuMS Technology Co. Ltd. An Australian Scientific Instruments RESOlution LR laser-ablation system (Canberra, Australia) and Agilent Technologies 7700 x quadrupole ICP-MS (Hachioji, Tokyo, Japan) were used for the analyses. The 193 nm ArF excimer laser, homogenized by a set of beam delivery systems, was focused on titanite surface with fluence of 8.0 J/cm². The ablation protocol employed used spot diameter of 33 µm with a repetition rate of 6 Hz for 40 seconds. Helium was applied as carrier gas to transport the ablated aerosol to ICP-MS. Titanite BRL-1 (²⁰⁶Pb/²³⁸U age of 1047 ± 0.4 Ma) [103] was used as the primary external reference material to correct for instrumental mass discrimination with “Ontario” (²⁰⁶Pb/²³⁸U ID-TIMS age of 1053 ± 3 Ma) [104] used as a secondary reference material to verify the method. Raw data were processed using the ICPMSDataCal software [105]. Six repeated analyses of Ontario titanite yielded a ²⁰⁷Pb corrected [106] ²⁰⁶Pb/²³⁸U date of 1047 ± 7 Ma (MSWD = 2.1; n = 6/6), which overlaps the expected age. The concentrations Si, Ca, Ti, and Zr were measured with the U-Th-Pb isotopes. The glass reference material “SRM NIST 610” [107, 108] was used for primary elemental concentration calibration. Titanite BLR-1 [109] and reference glass “SRM NIST 612” [110] were analyzed as unknowns to verify element concentration results; the precision was better than 9% for the elements analyzed. All the U–Pb geochronological data were plotted with IsoplotR software [111].

Zircon in sample FCS-9-3 (migmatitic amphibolite) taken at Yantoushan site (for location see Figure 2) shows a core-rim structure characterized by concentric oscillatory zoned, high CL-luminescent cores with narrow CL-dark, and structureless rims (Figure 10(a)). The Th/U of the cores (>0.46) is typically higher than that of the rims (0.02 and 0.17; online supplementary Table S3). U–Pb analyses of all rims and most of the cores are disconcordant (Figure 10(a)). The weighted mean ²⁰⁷Pb/²⁰⁶Pb date for the seventeen oldest analyses is 2485 ± 15 (uncertainty is reported throughout as 2 standard error of the mean – 2SE; MSWD = 0.87). These data are consistent with the upper intercept date of 2500 ± 16 Ma for the discordia defined by all analyses (Figure 10(a)). We interpret this Proterozoic date to represent the protolith age of the specimen. The discordia defines a lower intercept date of 245 ± 10 Ma (MSWD = 1.9).

Sample FCS-37-18 is an amphibolite collected at the Mawa exposure (for location, see Figure 2). Zircon grains in this specimen also show a core-rim structure characterized by concentric oscillatory zoned, CL-dark cores with narrow high CL-luminescent, and unzoning rims (Figure 10(b)). The Th/U of the cores (0.23, 1.2) is generally higher than that of the rims (0.05 and 0.53; online supplementary Table S3). U–Pb analyses of all rims and most of the cores are disconcordant. The discordia regression defined by all twenty-seven analyses defines upper and lower intercept dates of 2449 ± 22 and 246 ± 13 Ma (MSWD = 1.3), respectively. The weighted mean ²⁰⁷Pb/²⁰⁶Pb date for the sixteen oldest analyses, 2431 ± 23 (MSWD = 0.45; Figure 10(b)), overlaps that of the upper intercept. This Proterozic date is interpreted to reflect the protolith age of the rock, whereas the 246 ± 13 Ma lower intercept date is interpreted to represent timing of a later thermal event.

Sample FCS-37-3 is taken from a foliated granitic dyke at the Mawa exposure (for location, see Figure 2). Zircon from this specimen shows a typical core-rim structure; the cores are of CL-dark, concentric oscillatory zones; and the rims are narrow high CL-luminescent and structureless (Figure 10(c)). The Th/U of the cores (> 1.3) is higher than that the rims (0.21 and 0.64). Nine concordant core analyses spread between 597 and 830 Ma (²⁰⁶Pb/²³⁸U date). Eleven concordant rim analyses define a ²⁰⁶Pb/²³⁸U weighted mean date of 244 ± 5 Ma (MSWD = 0.12; Figure 10(c)). The zircon cores with Neoproterozoic dates are interpreted as inherited, and the 244 ± 5 Ma date of the rims is interpreted to represent the crystallization age of the dyke.

Titanite dates were determined from in situ analyses of grains in sample FCS-37-11. Backscattered electron imaging of the grains conducted prior to analysis did not reveal obvious zonation (Figure 11(b)), implying a lack of late-stage fluid alteration [112]. The titanite grains analyzed contained U concentrations of ~3.0 to 40.5 ppm (online supplementary Table S4) with low Th/U values (0 and 0.23), consistent with a metamorphic/recrystallized origin [113-115]. When plotted in Tera‐Wasserburg space, the twenty-four analyses define an isochron with a lower intercept date of 242 ± 12 Ma (MSWD = 2.5; Figure 11(a)).

Zr-in-titanite geothermometry was used to determine the temperature of the titanite (re)crystallization. The calibration of Hayden et al. [116] was used with an assumed pressure of 0.5 GPa, based on the average value calculated via hornblende–plagioclase thermobarometry. Values of α_SiO2 = 1.0 and α_TiO2 = 0.75 [117, 118] were estimated for quartz and titanite-bearing, rutile-absent specimen. The twenty-four analyses returned Zr-in-titanite temperatures of ~690–730°C (±20℃; estimated precision; Figure 11(c)), which is consistent with both the hornblende–plagioclase thermobarometer results and the amphibolite facies mineral assemblage.

Previous studies have demonstrated that LPO distributions or activity of specific slip systems in quartz is highly temperature dependent [87, 89, 119]. It should be cautious as quartz LPO distributions may also be influenced by complex bulk flow patterns or strain partitioning processes (e.g. [120, 121]). When combined with detailed microstructural observations, and assuming that the potential effects of differences in strain rate critically resolved shear stress and/or hydrolytic weakening on mineral deformation process between specimens are minimal, the quartz LPO distributions can be interpreted as additional evidence for different relative deformation temperatures in highly strained rock specimens [122-124].

Typical microstructures like lobate boundaries, pinning, and dragging structures of quartz (Figures 5 and 6) we observed in the specimens show evidence of quartz grain boundary migration recrystallization [123]. Both the point-maxima quartz c-axis and misorientation axes distributions (near c-axis in crystal coordinate) of six specimens indicate the dominance of prism <a> slip. Furthermore, some quartz grains with “chessboard” extinction structure indicate the existence of combined basal <a> and prism <c> slip [119, 123, 125]. Feldspar microstructures, specifically equal size neoblast formed along the serrated boundaries of relict grains, are typical of subgrain rotation and/or bulging recrystallization.

All of the above-mentioned characteristics are consistent with amphibolite-facies deformation temperature (600 and 700℃; Table 2) [89, 119, 122, 123]. This estimation overlaps the results calculated using the hornblende-plagioclase thermobarometer (640 and 720℃; 4.0 to 6.6 Kbar) and the Zr-in-titanite thermometer.

Based on strong SPO, LPO, low internal strain characteristics, and locally distributed low-angle (1° and 4°) misorientation along grain boundaries of the analyzed titanite grains, we interpret that these grains formed either through oriented growth with subgrain formation during deformation [123, 126] or by mechanical rotation of the preexisted grains with subgrain formation during deformation [123]. Considering that the Zr-in-titanite thermometer results (690 and 730℃) overlap the estimated metamorphic/deformation temperature (640 and 720℃ by hornblende–plagioclase thermobarometer and quartz microstructures), we consider that these titanite grains mostly like formed through the former scenario outlined above.

Previous studies have shown that the effective closure temperature (Tc) of the titanite U–Pb system is relatively high (>750℃) in natural rocks [104, 114, 127], well above the estimated metamorphic temperature (~640–72˚C) outlined herein. The U–Pb system in titanite is sensitive to deformation process and may re-equilibrate during deformation-induced recrystallization [128-131]. The Zr-in-titanite thermometer estimation and titanite deformation features of the analyzed titanite grains are consistent with the synkinematic development or recrystallization of grains at T < Tc and, therefore, they date the timing of deformation [132]. Consequently, the U–Pb titanite date obtained from sample FCS-37-11 (242 ± 12 Ma) represents the timing of deformation. This age is fairly consistent with the lower intercept/young population zircon U–Pb dates measured in the specimens FCS-37-18 (246 ± 13 Ma), FCS-9-3 (245 ± 10 Ma), and FCS-37-3 (245 ± 5 Ma).

To further elucidate the possible spatial relationship between the Triassic subhorizontal ductile fabrics documented along the Zhangbaling–Feidong Complex uplift zone (outlined in Figure 2) and deep structures beneath the Lower Yangtze thrust-fold zone, we reinterpret a portion of the deep seismic reflection profile (NW11-1) acquired by the SINOPROBE project, which spans the research area of this study (for location see Figure 1). This seismic profile extends approximately 360 km from Heifei Basin to Lower Yangtze area, it was initially designed to probe deep crustal structure of the Yangtze River metallogenic belt and adjacent areas [133]. The segment we focus on (common depth point 1-8801) encompasses the Hefei Basin, Tan-Lu fault zone, and the northwestern Lower Yangtze fold-and-thrust belt. Details regarding raw data acquisition, processing specifications, and the uninterpreted seismic reflection profiles can be found in Lü et al. [133]. All times mentioned below are two-way travel times (TWT).

Two packages of seismic reflections are prominent below the Hefei basin. The first one between 0 and 4 seconds consists of well-defined continuous reflectors with high frequency (marked A in Figure 12(a)), and the second one below 4 seconds corresponds to complex low-continuity reflectors with relatively low frequencies. Furthermore, a number of subhorizontal middle-amplitude reflectors can be observed (marked B in Figure 12(a)). Borehole data show that the Jurassic-Cretaceous (J-K) sedimentary sequences consisting mostly of sandstone-mudstone packages in the Hefei Basin is approximately 4 km thick, transitioning to slightly metamorphosed rocks (slate) at 4 to 5 km depth [134]. As such, we interpret that the first seismic reflector package with well-defined continuous reflectors represents the J-K sedimentary series, and the second set of reflectors with complex low-continuity reflectors and relatively low frequencies represents pre-Mesozoic metamorphosed basement. Their discontinuity indicates thrust shear zones/faults in the basement of the Hefei basin.

In contrast, seismic structure below the Zhangbaling Complex zone seems more complex. We notice numerous subhorizontal to shallow SE-dipping reflectors with middle-frequency traceable in the depth above 5 seconds, but absent below 5 seconds (Figures 12(a) and 12(c)). This seismic feature across the Zhangbaling Complex region was recently recognized by Li et al. [20]. By taking into account the thick, top-to-SW subhorizontal ductile fabrics documented in the greenschist- to amphibolite-facies Zhangbaling–Feidong Complex [24, 30, this study], we attempt to interpret the top continuous reflectors above 5 seconds TWT in the seismic profile to be the décollement in the basement rocks (Figure 12(b)). The seismic transparency below 5 seconds may reflect high-degree metamorphic and intrusive rocks in the lower crust [20, 34, 35, 135-137].

A narrow (<10 km wide) zone with chaotic seismic reflections situated between the traceable, shallow-dipping reflectors of Zhangbaling Complex and Hefei Basin (Figures 12(a) and 12(b)) corresponds to the location of the subvertical Tan-Lu fault [20, 138, 139].

The seismic reflection pattern below the Lower Yangtze fold-and-thrust belt seems to be dominated by NW-dipping reflectors with high frequency at depth above 3.5 seconds. Subhorizontal middle-continuity reflectors with low frequency are observed at depths ranging from 3.5 to 5.0 seconds (Figures 12(a) and 12(b)). In the lower crust below 5.0 seconds is a seismically transparent region with sporadic, moderate subhorizontal reflectors (Figure 12(a)). We interpret that the NW-dipping reflectors in the top part are the thrust-fold structures of the Lower Yangtze fold-and-thrust belt, which are merged downward into a décollement zone (displayed by the subhorizontal reflectors in the middle crust) developed in the metamorphic basement rocks (Figure 12(b)). The subhorizontal décollement marks the transition to the midlower crustal metamorphic and intrusive rocks of the Lower Yangtze block.

The above interpretation of a décollement zone in the middle crust of the Lower Yangtze fold-and-thrust belt aligns with previous seismic and aeromagnetic observations in the same area [18, 20, 27, 139-141]. We consider that this subhorizontal décollement zone beneath the Lower Yangtze fold-and-thrust belt may have been exposed across the Zhangbaling–Feidong uplift through SE vergent thrust faulting along its eastern flank, which can also be inferred from the seismic profile. This spatial correlation implies that large-scale top-to-SW shearing occurred in the basement of the Lower Yangtze block may play a major role in accommodating shortening deformation in the foreland zone during the Triassic collision of North China and South China blocks.

When integrated previously published data, our new findings contribute to delineating a coherent tectonic framework spanning the research area and its surroundings during the Early to Middle Triassic epochs (Figure 13).

As mentioned earlier, the Zhangbaling–Feidong uplift zone exposes Paleo-to Neoproterozoic metamorphic rocks of the lower Yangtze block. Ductile fabrics documented in this basement uplift, form a subhorizontal décollement zone with top-to-SSW kinematics [24, 30, 142, 143, this study]. Precise geochronological information confirms that the development of the subhorizontal décollement was syncollisional [24, this study], and regional deep seismic profiles reveals that such a subhorizontal décollement may be been linked to a deeper structure one that accommodated the horizontal shortening (thrusting and folding) of the upper crust in the lower Yangtze region (Figure 12).

The Tan-Lu fault zone is typically thought to have initiated during Triassic Sulu-Dabie orogenesis as a sinistral strike-slip boundary that juxtaposed the North China block and South China block [16, 19, 23-28]. Questions, however, regarding the initiation and development the Tan-Lu fault zone remain, in part, because Triassic strike-slip shear fabrics have not been reported along the Zhangbaling–Feidong uplift zone. It is inferred that the Triassic Tan-Lu fault trace in the western part of the uplift is buried beneath the Cretaceous sedimentary deposits, as largely informed by a linear feature of magnetic anomalies [138, 139, 144].

As shown in Figure 1(a), the Lower Yangtze thrust-fold zone has a pronounced “S” shaped extending geometry (i.e. the thrust faults and fold axes are parallel the Tan-Lu zone along its western part and gradually turned to E-W in the east). This changing geometry implies a transpressional pattern of deformation, assuming that the Tan-Lu fault zone does be localized in the west of the Zhangbaling–Feidong uplift zone. The interpreted geometric and kinematic relationships between the basement décollement zone in Zhangbaling–Feidong uplift zone, fold-and-thrust zone in the Yangtze block, the Tan-Lu fault zone, and the Sulu-Dabie collisional orogen are illustrated in Figure 13.

In this block-diagram model, strike-slip shearing along the Tan-Lu fault (ca. 250–230 Ma) [19, 82, 83], the development of midcrustal décollement in the Zhangbaling–Feidong uplift (ca. 246–235 Ma) [24, this study] and the Lower Yangtze fold-and-thrust zone (247 and 244 Ma) [145, 146] are contemporaneous with the subduction of the Yangtze block beneath the North China block and subsequent exhumation of high-pressure rocks in the Sulu orogen (ca. 258–240 Ma) [14, 18, 78, 147]. These coeval events may reflect that the collisional process had been governed by the unique shape of the plate boundary. The southeastern margin of the North China Craton exhibits an irregular shape with a convex corner, causing the rheologically weaker Yangtze block to tear at the piercing point [19, 148-150] and subduct along the Dabie-Sulu orogen during the Early to Middle Triassic (see Figure 13(a)) [14, 147, 151, 152].

The collisional geometry during the Early-Middle Triassic would had favored transpressional tectonics at the western margin of the Lower Yangtze block (Figure 13) [24, 32]. In such a framework, the Tan-Lu fault would have acted as a sinistral wrench boundary that accommodated strike-slip displacement between the two blocks. In contrast, horizontal displacement associated with the block collision would have been partitioned into the Lower Yangtze fold-and-thrust. The continued northward movement of the Yangtze block would require the development of a large-scale midcrustal décollement zone that linked the strike-slip and thrust faulting [32, 142].

Finally, two fundamental aspects of the Tan-Lu fault zone emerge from the new results presented herein: 1) the Tan-Lu fault zone developed as a syncollisional fault and 2) a Triassic transpressional deformation system existed at the western margin of the Lower Yangtze block. Consequently, the syncollisional transpressional fault [24], syncollisional transform fault [16, 23], and indentation-induced continent tearing fault [19] appear to be the most plausible scenarios for the origin of the Tan-Lu fault zone. Future work including regional structural deformation, metamorphism, and geophysical observations is needed to elucidate the origin and behavior of the Tan-Lu fault zone during the syncollision process.

We documented a subhorizontal ductile shear zone exposed along the eastern flank of the steeply dipping Fuchashan strike-slip ductile shear zone in the southern Tan-Lu fault zone. Following points can be drawn from this study.

1. Field observation, microstructure, quartz c-axis fabric, and geothermobarometer analysis document that subhorizontal ductile fabrics with top-to-SSW shearing occurred under amphibolite facies metamorphic conditions (640 and 720℃, 4.0–6.6 Kbar).

2. Zircon and titanite U––Pb dates obtained from the high-strain rocks with subhorizontal ductile fabric constrain the timing of shear deformation to be Middle Triassic at ca. 246–242 Ma.

3. By compiling our new results with previous geological and geophysical studies, we propose that the Lower Yangtze block underwent a transpressional deformation during its Triassic collision with North China block, the Tan-Lu fault zone might have developed as a transpressional boundary.

Datasets for this research are included in this article, the supplementary material files, and references. Datasets generated by this research can also be found at Open Science Foundation: https://osf.io/fejd6/?view_only=636929383cc04875812e2a15b868a809

The authors declare that there is no conflict of interest regarding the publication of this article.

This work was financially supported by National Key R&D Program of China (Grant No. 2022YFF0800404) and National Natural Science Foundation of China (Grant No. 42172238).

Graduate students J. Ren (now in Università di Padova) and B. Li of the Nanjing University participated the field work. Discussion with N. Piette-Lauzière, S. Dong, Q. Wang, J. Li, W. Shi and J. Zhu helped clarify some of the details presented. This contribution benefited from the constructive comments of section editor Dr. Tamer S. Abu-Alam, associate editor Dr. Chuan-Lin Zhang and three anonymous reviewers.