Dating Strike‐Slip Ductile Shear Through Combined Zircon‐, Titanite‐ and Apatite U–Pb Geochronology Along the Southern Tan‐Lu Fault Zone, East China

Dating the timing of ductile shear deformation is critical to the understanding of complex, multi‐episode kinematics that commonly occur within the study of continental tectonics. The Tan‐Lu fault zone (TLFZ) in East China is a lithospheric discontinuity that records multistage sinistral strike‐slip shearing during Mesozoic time. Herein, based on field mapping, pressure‐temperature estimates via microstructures, quartz lattice preferred orientations, and geothermobarometer analysis, and U–Pb geochronology, we present a detailed examination of a portion of the southern TLFZ known as the Fucha Shan ductile shear zone. Two phases of sinistral ductile shear deformation in the shear zone are identified. The main phase occurred under amphibolite‐facies metamorphic conditions (500°C–650°C, 2.8–5.2 Kbar). U–Pb dating of synkinematic titanite indicate deformation occurred ca. 142–140 Ma. This age information is consistent with U–Pb dating of zircon from crosscutting granitic dikes that provide an upper limit of the shearing of ca. 134 Ma. A later, but less well‐developed phase of sinistral ductile shearing variably overprints older structures within the Fucha Shan shear zone at greenschist‐facies metamorphic conditions. U–Pb dating of apatite grains in rocks that record this later event are interpreted to indicate that shearing occurred ca. 118–108 Ma. These results provide new insight into Early Cretaceous shear deformation history within the southern TLFZ in the context of the late Mesozoic intracontinental deformation in East Asia.

2 of 34 employed in the assessment of deformation timing: (a) applying the 40 Ar-39 Ar method to synkinematic K-bearing minerals such as amphibole, mica, and K-felspar, which record the time at which the minerals closed to diffusion of Ar from the crystal structure, which is often at or below the temperature of deformation (Dunlap et al., 1991;Kellett et al., 2013;McDougall & Harrison, 1999), or (b) using the U-Pb system on uranium-bearing minerals, such as zircon, monazite, titanite, apatite, and rutile, that (re)crystallized synkinematically within a shear zone (Cao et al., 2011;Resor et al., 1996;Searle et al., 2010;Tapponnier et al., 1990).
The application of these geochronological techniques to ductile shear zones, however, requires paired detailed textural and structural interpretations and, as such, should be applied cautiously. This is especially true given the potential of the K-Ar ( 40 Ar/ 39 Ar) system in mica to be perturbed by post-kinematic, ephemeral tectono-thermal events (Hacker & Wang, 1995;Ratschbacher et al., 2000). The U-Pb system, in contrast, is relatively stable under medium-to low-grade metamorphic conditions for most minerals (R. Parrish, 2014;Rubatto, 2013;Yakymchuk et al., 2017). In this regard, the U-Pb geochronology of monazite, titanite, apatite and rutile may be more effective than the 40 Ar/ 39 Ar systems for dating shear deformation under amphibolite-to greenschist-facies metamorphic conditions (Chew & Spikings, 2015;Kohn, 2017; R. R. Parrish, 1990;Zack & Kooijman, 2017), especially when fluid mediated (re)crystallization occurs (e.g., Ribeiro et al., 2020). For instance, Giuntoli et al. (2020) dated protracted shearing under mid-crustal conditions during large-scale thrusting in the Scandinavian Caledonides by in situ U-Pb analysis of synkinematic titanite, while Odlum and Stockli (2020) used apatite U-Pb geochronology to investigate the timing of deformation and metasomatism along the Main Mylonitic Band in the Northern Pyrenees.
The Tan-Lu fault zone (TLFZ) in East China is a lithospheric boundary that juxtaposes the North and South China Blocks and offsets two high-pressure (HP)-to-ultra-high pressure (UHP) metamorphic zones, the Su-Lu to the east and the Tongbai-Dabie Shan to the west, with ∼600 km left-lateral displacement (Gilder et al., 1999;Okay et al., 1993;Q. Zhang et al., 2007;Zhao et al., 2016;Zhu et al., 2009). The TLFZ extends over a length of ∼2,400 km across East Asia, branching toward the NNE into several smaller scale structures and terminating southward on the eastern side of the Dabie Shan UHP metamorphic zone (Figure 1; Gu et al., 2016;J. Xu & Ma, 1992;J. Xu et al., 1987;Y. Q. Zhang and Dong, 2019;Q. Zhang et al., 2005;Zhu et al., 2018). It has been characterized as a long-lived continental-scale fault with a multiphase deformation history from Triassic to Cenozoic (Jolivet et al., 1994;Mercier et al., 2013;Y. Q. Zhang, Ma, et al., 2003;Zhao et al., 2016;Zhu et al., 2018). Deformation along this fault zone has been variably attributed to: (a) Triassic indentation of the Yangtze Block with the North China Craton (A. Yin & Nie, 1993;Zhao et al., 2016); (b) southward subduction of the NCC below the Qinling-Dabie terrain (Jahn & Chen, 2007;S. Z. Li et al., 2017); or (c) late Mesozoic oblique subduction of the Paleo-Pacific Plate beneath the eastern margin of the Asia continent (J. Xu & Zhu, 1994;D. Zhang et al., 2018;Zhu et al., 2005).
The steeply dipping shear zones with well-developed ductile fabrics that define the TLFZ have been investigated by many researchers (e.g., Y. Wang, 2006;Q. Zhang et al., 2007;Zhao et al., 2016;Zhu et al., 2005Zhu et al., , 2009Zhu et al., , 2010. Some of those studies interpret 40 Ar-39 Ar geochronology on amphibolite, muscovite, and/or biotite to quantify the timing of shear deformation. The age results reported, however, span a wide range of time, in 240-100 Ma, reflecting the complex thermal history of this shear zone associated with Triassic collision-related strike-slip faulting and/or with late Mesozoic intracontinental deformation (X. Chen et al., 2000;Wang, 2006;Q. Zhang et al., 2007;Zhao et al., 2016;Zhu et al., 2005Zhu et al., , 2009Zhu et al., , 2010. Zircon U-Pb geochronology has also been used to investigate the timing of shear deformation (Han & Niu, 2016;Han et al., 2015;Z. Li et al., 2020;Lu et al., 2023;W. Wang et al., 2015W. Wang et al., , 2016D. Zhang et al., 2018;Zhu et al., 2018). Due to the high closure temperature of U-Pb isotopic system in zircon and relatively low tendency to recrystallization under amphibolite-to greenschist-facies metamorphic conditions of zircon (Corfu et al., 2003;Hoskin & Schaltegger, 2003;Lee et al., 1997), these studies only provide information on the upper bound of the deformation timing obtained by zircon U-Pb ages of the post-kinematic granitic veins but fail to directly date the timing of mylonitic rocks formed.
To help quantify the history of ductile shear deformation along the southern TLFZ, within the Fucha Shan shear zone, we present the results of detailed field mapping, synkinematic P-T determinations, and U-Pb geochronology of zircon, titanite, and apatite. The minerals examined via U-Pb geochronology were separated from both mylonite and crosscutting, undeformed leucogranite dikes to attempt to directly and indirectly date the timing of ductile shearing. These new geochronological data help inform the history of movement along the southern TLFZ and its potential role during late Mesozoic intracontinental deformation in East Asia.   Zhao et al., 2016. Geochronology data from Song et al., 2003;Wang, 2006;Zhu et al., 1995Zhu et al., , 2001Zhu et al., , 2005Zhu et al., , 2010 Abbreviations in (a) YYF = Yilan-Yitong Fault, MDF = Mishan-Dunhua Fault, YRF = Yalu River Fault, EDO = East Dabie Orogen).
The NNE-striking Fucha Shan strike-slip shear zone developed within the Feidong complex and associated Neoproterozoic granitic intrusions is defined by a ∼5 km-wide high-strain zone of ductile strike-slip shear fabrics (Song et al., 2003;Zhu et al., 2005) (Figure 2). The shear deformation across the Fucha Shan structure is localized on several mylonitic zones, 10-300 m wide, characterized by a near-vertical foliation and near horizontal lineation. Sheath folds and L-tectonite observed along its eastern margin have been interpreted as Triassic in age . Further geochronological information can be extracted from Early Cretaceous granites and granitic veins that intrude or crosscut the shear zone and have been dated between ca. 134 and 103 Ma (Niu et al., 2008;Zhu et al., 2010Zhu et al., , 2018, which indicates the shear zone formed before 134 Ma.

Field Observation
Detailed field structural mapping of the ductile shear zone was conducted at three quarries across the Fucha Shan area ( Figure 2).
The "railway" quarry, located on the western side of the Fucha Shan shear zone (Figure 2), exposes a ∼500 m wide NNE-striking zone of high strain within foliated Neoproterozoic granites and plagioclase amphibolite ( Figure 3a). The sheared amphibolite, which ranges between 0.5 and 60 m wide, is characterized by banded leucosome and melanosome (Figures 3, 4a, and 4d). The foliations in the high-strain zones are subvertical (>70°), dipping to SE or NW (Figure 3a). The lineations, defined by aligned hornblende, biotite, elongate quartz, and/ or feldspar, plunge gently (0°-25°) to the NE or SW. Asymmetric folded veins, rotated porphyroclasts, and S-C fabrics indicate sinistral sense strike-slip shear (Figures 4e and 4f). Unfoliated Early Cretaceous granitic dikes, 0.5 m to several meters wide, crosscut the steeply dipping, foliated Neoproterozoic granitic and amphibolite mylonites at a high angle with relatively straight boundary (Figures 4b and 4c).
The Xiwei quarry, located in the center of the Fucha Shan shear zone (Figure 2), exposes a ∼130 m wide, NE-striking mylonitic to ultramylonitic high-strain zone ( Figure 5) characterized by a steeply dipping (>60°) foliation and a gently plunging (<20°) lineation. The mylonite comprises 2.4-2.0 Ga biotite amphibole plagiogneiss and gneiss of the Feidong Group that locally record post-mylonitization K-feldspar-chlorite alteration. Two differently oriented, generally unfoliated, Early Cretaceous granitic dikes were observed within the shear zone: one that crosscuts the mylonitic foliation ( Figures 5, 6a, and 6e), and a second that parallel with the mylonitic foliation ( Figures 5 and 6a). These textural relationships indicate that the main shear zone had formed prior to the emplacement of the granitic dikes. Narrow, ∼0.2 to <10 m wide late mylonitic bands (blue dashed lines in Figure 5) cross-cut granitic dikes and the main mylonitic fabric locally ( Figures 5, 6e, and 6f). The foliation within these late mylonitic zones also strikes NE, dips steeply (55°-80°) and plunges gently.
The Mawa Village exposure of the Feidong gneissic group (Figure 7), along the eastern margin of the Fucha Shan shear zone, includes mylonite with quartz defining a shallowly plunging (0°-25° to SW) stretching lineation (Figures 7b and 7c) within the steeply (>70° SE or NW) dipping foliation. Asymmetric plagioclase porphyroclasts and S-C fabrics in the foliated Early Cretaceous pegmatite indicate sinistral sense strike-slip shear ( Figure 7d). As in the previous exposures, the Feidong gneissic group rocks are intruded by Early Cretaceous granitic dikes.

Microstructures
Microstructures observed in rock specimens, as discussed below, were characterized in rock thin sections cut parallel to the mesoscopic mineral stretching lineation and perpendicular to the foliation to approximate the XZ plane of the finite strain ellipse, assuming non-complex flow symmetry. The high strain zone at the "railway" quarry ( Figure 3) dominantly occurs within plagioclase amphibolite and granite. The strongly foliated plagioclase amphibolite mainly consists of hornblende, plagioclase, biotite, and quartz with minor diopside (Figures 8a and 8b). The hornblende and biotite in the plagioclase amphibolite marked the mylonitic foliation and underline the stretching lineation. The mineral assemblage in foliated granite is quartz, K-feldspar, and plagioclase with minor biotite. Accessory minerals such as zircon and titanite are common in these rock types.
The quartz in both the plagioclase amphibolite and the foliated granite shows lobate boundaries, pinning and "window" structures ( Figure 8c Mylonite at the Xiwei quarry, developed in Feidong group gneiss, is composed of quartz, plagioclase and biotite with minor hornblende. Quartz in the Xiwei protomylonite shows lobate boundaries with "window" structures The rocks from late, narrow, crosscutting mylonitic zones observed at the Xiwei quarry and at the Mawa Village exposure have mineral assemblages of quartz-plagioclase-biotite-K-feldspar, with accessory zircon and apatite. Quartz in these mylonitic rocks form ribbons, small neoblasts formed along the irregular grain boundaries of old grains and old grains show undulate extinction (Figures 8h-8l). The feldspar in these rocks shows internal fractur- ing, bend twins, "book-shelf" and core-mantle structures (Figures 8h-8l). Microscopic shear-sense indicators in these mylonitic rocks, including asymmetrical porphyroclasts, quartz oblique foliation, S-C structures, and feldspar "book-shelf" textures, are all consistent with sinistral-sense shear (Figure 8h-8l), matching macro-scale observations.

EBSD Analysis of Quartz and Titanite
Electron backscattered diffraction (EBSD) analysis of quartz crystal lattice orientation was performed in nine specimens of different mylonitic rocks from both the main and late phase shear zone, summarized in Table 1. Titanite EBSD analysis was also applied to the titanite grains in plagioclase amphibolite with the purpose of quantifying potential recrystallization prior to in situ titanite U-Pb age dating. 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) patterns 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 HKL Channel 5 software (Oxford Instruments). The LPO distributions are presented as one-point-per-grain representations.

Quartz c and a-Axis Fabrics
Samples FCS-8-3b and FCS-10-1 are plagioclase amphibolite, and sample FCS-8-6 is a leucosome collected within a foliated amphibolite. All three samples were collected from the shear zone at "railway" quarry. The quartz c-axis orientations in FCS-8-3b are mainly dominated by Y-maximum with a minor concentration in the X direction ( Figure 9a). Such orientations are consistent with dominant prism 〈a〉 slip (Hobbs, 1985; Schmid & Casey, 1986) with minor prism 〈c〉 slip (Mainprice et al., 1986). The quartz LPO measured in FCS-8-6 and FCS-10-1 are also dominated by Y-maxima (Figures 9c and 9d). The quartz a-axis of these have similar distributions, with three maxima asymmetrically disposed about the macroscopic fabric axes (Schmid & Casey, 1986). The LPO of FCS-8-13, a sample of foliated granite from the main shear zone at "railway" quarry, has a similar, though less well-defined, Y-maxima dominant distribution, with additional orientations indicating rhomb and basal 〈a〉 slip ( Figure 9b). Taken as a whole, the quartz c-axis distributions in the samples examined from the "railway" quarry are consistent with a significant component of simple shear (see Lister & Hobbs, 1980). The asymmetry of both the quartz c-and a-axis distributions indicate sinistral shearing.
Two samples (FCS-19-7b and JS-6-15) from the Xiwei quarry were examined for quartz LPO analysis. Sample JS-6-15 shows a dominantly single girdle c-axis fabric pattern and strong a-axis maximum at the margin of the east-west direction with an angle less than 30° (Figure 9e). The quartz slip-systems in the sample include both rhomb a (Passchier & Trouw, 2005;Toy et al., 2008) and basal 〈a〉 ( Figure 9f). The quartz c-axis distributions in sample FCS-19-7b, in contrast, form a poorly defined (with J-index of 〈c〉 = 1.17, M-index = 0.03) type I crossed girdle ( Figure 9e). The asymmetry of the c-axis distributions in both FCS-19-7b and JC-6-15 is consistent with sinistral-sense shear.
Samples JS-10-2, CH-2-5, and CH-2-3 were collected from locally developed late phase shear zones at Xiwei quarry. The quartz c-axis orientations of JS-10-2 and CH-2-5 define type I crossed girdles with a small opening angle (Figures 9i and 9l). The dominant quartz slip systems active in these two samples appear to be rhomb 〈a〉 and basal 〈a〉. Sample CH-2-3 shows a partial single girdle c-axis pattern with a significant prism, rhomb and basal 〈a〉 slip (Figure 9h). The quartz a-axis of these three samples have an intermediate maximum near the line- ation direction at a relatively small angle with respect to the horizontal. The asymmetry of both the c-and a-axis in these specimens indicates sinistral sense shearing.
Finally, a foliated pegmatite, sample FCS-29-2, was collected from the Mawa Village exposure ( Figure 2). Quartz in this sample yields a single girdle pattern of c-axes with strong asymmetry, and an a-axis maximum is situated at the margin of the lineation direction with an angle less than 20° (Figure 9g). The activate slip system includes prism 〈a〉, basal 〈a〉 and minor rhomb 〈a〉. Asymmetric a-and c-axis fabrics exhibit sinistral sense of shearing.

Deformation Temperatures Estimated by Quartz LPO and Microstructure
Previous studies have demonstrated that the LPO pattern or activity of certain slip systems of quartz can be temperature dependent (Schmid & Casey, 1986;Stipp et al., 2002;Toy et al., 2008) though it should be noted that quartz LPO patterns may be complicated by complex bulk flow patterns or strain partitioning process (e.g., Jeřábek et al., 2007;Larson et al., 2014). When combined with detailed microstructural observations, the quartz LPO distributions can be used as an additional line of evidence to make rough estimates about the relative deformation temperatures experienced by different rocks specimens (Kurz et al., 2002;Law, 2014;Passchier & Trouw, 2005). Such interpretations, however, may overlook the potential effects of strain rate, critically resolved shear stress and/or hydrolytic weakening on quartz deformation. Assuming those effects are minor relative to temperature, in the following sections we combine field and microstructural observations to distinguish two distinct rock deformation events.

Main Phase Strike Slip Deformation Under Amphibolite-Facies Condition
Amphibolite-facies deformation condition (500°C-650°C, dominant deformation phase): Quartz slip systems in the amphibolite, foliated granite and mylonite from the main ductile shear zone (tens of meters to ca. 300 m wide) are dominated by prism 〈a〉 and rhomb 〈a〉 slip (Table 1), and contain evidence of quartz grain boundary migration dynamic recrystallization (i.e., lobate boundaries, pinning and "window" structures), feldspar bulging (BLG)-subgrain rotation (SGR) recrystallization, as indicated by core-mantle structures, and myrmekite development, all of which are consistent with high deformation temperature (500°C-650°C) (Passchier & Trouw, 2005;Stipp et al., 2002;Toy et al., 2008).  Figure 9. Quartz c-and a-axis distribution diagrams of the mylonitic rocks from both the main phase shear zone (a-f) and late phase shear zone (g-i), X parallel to the stretching lineation and Z parallel to the pole of the mylonitic foliation, lower hemisphere, equal area, half width 15° and cluster size 5°; mud = multiples of uniform density. The strength of the LPOs were evaluated by the J-index 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) (

Late Phase Strike Slip Deformation Under Greenschist-Facies Condition
Greenschist-facies (350°C-500°C, late, localized phase) deformation condition: quartz c-axis fabrics of mylonitic rocks from the late, cross-cutting phase ductile shear zones (<10 m) at Xiwei quarry are single girdles and type-I crossed girdles with basal 〈a〉 slip, which is generally consistent with relatively low deformation temperature (<500°C; Schmid & Casey, 1986;Toy et al., 2008). Microstructures show dominant dynamic recrystallization in quartz via SGR, as evidenced by low-strain neoblastic grains formed along irregular boundaries and ribbon peripheries, and feldspar "book-shelf" structures and internal microfractures without obvious dynamic recrystallization. Such a combination of textures in the two mineral phases are consistent with greenschist-facies conditions (∼350°C-500°C) (Passchier & Trouw, 2005;Stipp et al., 2002).

Titanite EBSD
Two titanite aggregates with preferred orientations were observed in sample FCS-8-3b, collected from amphibolite mylonitic rocks at "railway" quarry. The titanite grains coexist with plagioclase, hornblende and biotite. Every grain is elongate parallel or subparallel to the foliation (Figures 10a and 10b). Most titanite grains have low grain orientation spread (GOS) values (Cross et al., 2017), <5° (Figures 10c and 10d) and kernel-average maps show small (<1°) but pervasive misorientations (Figures 10e and 10f). The misorientation angle distribution of the correlated pairs shows a peak at very low angle (<5°) and the same metric for uncorrelated pairs show a distribution that deviates signifi- cantly from a random curve (Figure 10h). The M-index (Skemer et al., 2005) of the titanite aggregate 1 is 0.339 and of the titanite aggregate 2 is 0.504. The pole figures of these grains display strong LPOs (Figure 10g). Poles of the {100}, {010} crystallographic planes show girdle-shaped patterns in lower hemisphere-equal area projections with a small angle (<30°) to stretching lineation. In the {001} pole figure most points are distributed in the margin with a ∼45° angle to the stretching lineation. Combining the strong SPO, LPO, low internal grain strain and high M-index value, we speculate that these titanite grain aggregate could be formed in two scenarios: (a) these titanite grains may have formed by oriented growth during deformation (Kamb, 1959;Passchier & Trouw, 2005); (b) or they have been completely, dynamically recrystallized through sub-grain rotation recrystallization (Papapavlou et al., 2017;Passchier & Trouw, 2005). Both scenarios are consistent with titanite grains (re)crystallizing synkinematically.

Amphibolite Thermobarometry
To gain further information about the potential metamorphic conditions during deformation, we applied amphibole-plagioclase and Ti-in-biotite geothermobarometers to the amphibolite samples (FCS-8-5 and FCS-8-8) from "railway" quarry ( Figure 3). The strongly elongate hornblende, plagioclase and biotite in the amphibolite samples define the main foliation of amphibolite indicating they were recrystallized during deformation (Figures 8a and 8b). Thus, the thermobarometers of these minerals may provide information about pressure-temperature conditions during mylonitic foliation development. Electron Probe Microanalyzer analyses (EPMA) of the different mineral phases were conducted at the State Key Laboratory for Marine Geology, Tongji University (China), using a JEOL JXA-8230 electron microprobe. Natural and synthetic mineral standards (SPI) were used to calibrate all quantitative analyses and a ZAF correction were used for data reduction. Operation conditions and data processing followed the methods described by Cheng et al. (2018). 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. Data were corrected using standard ZAF correction procedures.
Temperature estimates were also calculated using the Ti-in-biotite geothermometer (Henry et al., 2005). Biotite from both samples has similar Ti contents (0.09-0.12 a.p.f.u.) and Mg# (0.58-0.64). The estimated temperatures vary in the range of 595°C and 655°C (±24°C; estimated precision), which overlaps the results obtained by the amphibole-plagioclase thermobarometer.

Zircon U-Pb Geochronology
To determine the protholith ages of the mylonitic rocks and granitic dikes along the Fucha Shan shear zone, we conducted LA-ICP-MS U-Pb geochronology analysis of zircon grains separated from 15 samples (detailed analytical and computational methods for zircon U-Pb geochronologic data collection and processing are presented in Supporting Information S1). Among them, 10 samples were taken from foliated mylonitic rocks, and 5 samples were taken from unfoliated granitic dikes. Sample localities, lithologies, and weighted mean ages are listed in Table 3 (original data can be found in Table S2). Representative zircon cathodoluminescence (CL) images, 206 Pb/ 238 U ages, Th/U ratios and U-Pb concordia diagrams for each sample are presented in Figures 11-13. All the U-Pb geochronological data were plotted with IsoplotR software (Vermeesch, 2018).
JS-6-2 is a mylonitic gneiss sample collected from the Feidong Group at Xiwei quarry. Zircon grains from this sample are euhedral to subhedral with oscillatory zoned texture. Of 16 total analyses, 14 analyses are concordant with high Th/U (>0.4). The concordant analyses define a weighted mean 207 Pb/ 206 Pb age of 2,024 ± 14.0 Ma (MSWD = 0.87; Figure 11e). FCS-19-4 is a gneiss sample with augen-like proto-mylonitic texture taken form the Feidong group at the same Xiwei quarry. Most zircon grains separated from FCS-19-4 show a core-rim structure characterized by concentric oscillatory zoned, CL-dark cores with narrow high CL-luminescent, structureless rims. The Th/U of the cores (>0.3) is typically higher than the rims (0.12-0.54). U-Pb analyses of all rims and most of the cores are disconcordant; only eight analyses of total 24 grains from cores are concordant. The weighted mean 207 Pb/ 206 Pb value for the eight concordant analyses is 2,384 ± 26.4 (MSWD = 1.3). This age is consistent with the upper intercept age of 2,394.8 ± 31.0 Ma for the discordia defined by all analyses (Figure 11f). We interpret the zircon two ages determined from the samples collected at the Xiwei quarry to represent the protolith age of each sample. Two samples (CH-2-3 and CH-2-5) were taken from narrow, mylonitic zones that sheared granitic intrusions at the Xiwei quarry. Zircon grains from these two granitic mylonite samples are euhedral to subhedral, typically with oscillatory zoning (Figureures 12a and 12b). Twenty-one concordant analyses of sample CH-2-3 yielded a 206 Pb/ 238 U weighted mean age of 128.7 ± 1.1 Ma (MSWD = 1.2) and 14 concordant analyses of sample CH-2-5 gave a 206 Pb/ 238 U weighted mean age of 129.6 ± 1.2 Ma (MSWD = 2.1). The dates are interpreted as the emplacement ages of the mylonitic granites. Analyses in both samples hinted at minor Paleoproterozoic inherited components.
One sample (FCS-29-2) was taken from a mylonitic pegmatite at the Mawa Village exposure (Figure 7). Zircon grains from this sample are euhedral or fragmented euhedral grains with long column shape. Zircons show clearly oscillatory zoning with a low Th/U ratio (0.04-0.11). Twenty-four analyses yield a concordant 206 Pb/ 238 U weighted mean age of 131.2 ± 0.9 Ma (MSWD = 1.8), which is interpreted as the emplacement age (Figure 12c). Sample FCS-37-12 was also collected from a mylonitic granite at the Mawa Village exposure. Zircon grains from this samples show oscillatory zoning, most of them show high Th/U ratios (0.48-1.22) (Figure 12d), only five of total 28 grains show relative low Th/U ratios (0.08-0.22). Twenty-eight concordant analyses define a 206 Pb/ 238 U weighted mean age of 125.7 ± 1.1 Ma (MSWD = 0.5) which represents the age of emplacement.
The above zircon U-Pb date from the 10 rock samples fall into three groups: 2.4-2.0 Ga, 775-770 Ma, and 131-126 Ma. These results show that the Paleoproterozoic Feidong Group, Neoproterozoic intrusions as well as the Early Cretaceous granitic rocks were all involved in shear deformation.

Zircon U-Pb Age of Unfoliated Granitic Rocks
Samples JS-21-6 and FCS-5-1 were collected from unfoliated granitic dykes at the "railway" quarry site, where they crosscut the mylonitic shear zone (Figures 3c and 3d). Zircon grains separated from these two samples are euhedral with aspect ratios ranging from 1:1.5 to 1:6, well-developed oscillatory zoning, and high Th/U (>0.3 for JS-21-6 and >1.1 for FCS-5-1) (Figureures 13a and 13b). Zircon U-Pb analysis of these specimens defines yielded weighted mean 206 Pb/ 238 U ages of 131.6 ± 1.4 Ma (JS-21-6; MSWD = 1.4, n = 16) and 129.2 ± 1.6 Ma (FCS-5-1; n = 20; MSWD = 0.26), which we interpret as the crystallization age of the granitic dykes.  Figures 3 and 5).  Based on these zircon ages, the emplacement ages of unfoliated granitic rocks that cross-cut the main ductile shear zone were constraint to 126-134 Ma. However, some Early Cretaceous granitic rocks (131-126 Ma) at the Xiwei quarry and Mawa Village exposure were partially overprint by the late narrow localized green-schist facies shear zone. To further investigate the timing of these two ductile shear zones with different metamorphic grade and spatial distribution feature, we conducted U-Pb geochronology of titanite and apatite separated from specimens of mylontic amphibolite (main deformation phase) mylonite at the "railway" quarry and foliated Cretaceous leucogranite (late deformation phase) at the Xiwei quarry and the Mawa Village exposure.

Titanite U-Pb Geochronology
Given the apparent (re)crystallization of titanite during deformation, titanite U-Pb geochronology was conducted to investigate the timing of deformation recorded in amphibolite samples from the "railway" quarry. U-Pb geochronology of titanite was conducted by LA-ICP-MS at Nanjing FocuMS Technology Co. Detailed analytical and computational methods for data collection and processing are presented in Supporting Information S1.
Titanite ages were determined from analyses of grains separated from sample FCS-10-1 and those in thin section of sample FCS-8-3b (for sample location see Figure 3). In thin section, the titanite grains coexist with biotite, amphibole and plagioclase and are aligned with long axes parallel or subparallel to the main foliation. Titanite is unzoned under BSE, which is consistent with a lack of late-stage fluid (Holder & Hacker, 2019).
Because the titanite in the amphibolite mylonite at the "railway" quarry is characterized by synkinematic (re) crystallization textures with strong SPOs and LPOs (Figure 10), we interpret the lower intercept U-Pb ages of 142.1 ± 4.8 and 140.1 ± 4.5 Ma to reflect the timing of ductile deformation.

Apatite U-Pb Geochronology
Apatite U-Pb geochronology was applied to mylonitic rocks with textures interpreted to reflect greenschist-facies metamorphic conditions (350°C-500°C; Odlum & Stockli, 2020;Ribeiro et al., 2020). These analyses were conducted at the Fipke Laboratory for Trace Element Research (FiLTER) at University of British Columbia, Okanagan and State Key Laboratory of Geological Processes and Mineral Resources at China University of Geosciences. A detailed description of the methodologies is presented in Supporting Information S1.
Given the typical range of closure temperature of the apatite U-Pb system to Pb diffusion (i.e., 400°C-550°C; Chamberlain & Bowring, 2001;Cochrane et al., 2014;Chew & Spikings, 2015), we interpret the three apatite U-Pb ages obtained from the green-schist facies granitic mylonite (deformation temperatures ∼400°C-500°C) to represent cooling and thus reflect the timing of the late shear deformation to or before, at ca. 118-108 Ma.

Two Phases of Early Cretaceous Ductile Strike-Slip Shear Deformation Along the Southern Tan-Lu Fault Zone
Detailed field mapping of well-exposed quarries/new exposures across the Fucha Shan shear zone in the southern portion of the Tan-Lu structure help document two phases of sinistral ductile shearing.
The main phase of ductile shear deformation occurred prior to 134 Ma, as it is crosscut by unfoliated granitic dikes that yield 134-126 Ma zircon U-Pb ages. This early shearing phase strongly transposed the Paleoproterozoic amphibolite-plagioclase gneiss of the Feidong Group and associated Neoproterozoic granitic plutons into a ∼1-2 km wide mylonitic zone. Based on microstructures, quartz LPOs, and geothermobarometry, we confirmed the shearing appears to have taken place under amphibolite-facies metamorphic conditions. Our new titanite U-Pb geochronology results are within uncertainty of a hornblende 40 Ar/ 39 Ar date, also from the mylonitic amphibolite rocks at the quarry outcrop, which is defined by plateau age of 143 ± 1.3 Ma (Zhu et al., 2005). This hornblende date indicates that the rocks in the shear zone cooled through closure to Ar diffusion in hornblende (∼490°C-580°C; Harrison, 1981), by that time. Seven biotite 40 Ar-39 Ar plateau ages (137-125 Ma) obtained from the same area (Zhu et al., 2005) yield younger results that generally follow the record of Early Cretaceous (134-126 Ma) magmatism in the area (Figure 16a).
Late-phase ductile shearing overprinted the main shear zone at the Xiwei quarry and Mawa Village, forming narrow (<10 m) mylonitic to clastomylonite sub-zones at greenschist-facies metamorphic conditions (350°C-500°C). This shear deformation occurred after the ∼124 Ma crystallization of Early Cretaceous dikes and pegmatites (134-124 Ma) deformed within the high-strain horizons (S. Chen et al., 2022;Han and Niu, 2016;Zhu et al., 2018;This study). The new apatite U-Pb ages from the mylonitic Early Cretaceous-age dikes and pegmatite reflect the timing of the late shear deformation to, or before, at ca. 118-108 Ma (Figure 16b).
Whether the documented deformation timing reflects a single protracted event, or two discrete events is important to assess to understand the history of the Fucha Shan portion of the Tan-Lu shear zone. While the metamorphic grade and width of deformation in rocks that record the two different dates vary significantly, such observations are not distinctive and could reflect multiple events at different depths or overprinting in an evolving shear zone (e.g., Dyck et al., 2021). Perhaps more revealing is that the foliation developed in the main phase amphibolite facies shear zone is crosscut by Early Cretaceous dikes. While some of the dikes are partially overprinted by late phase ductile shearing, the microtextures observed in the dikes (e.g., quartz SGR recrystallization, strong SPO and CPO of recrystallized quartz grains, etc.; Figures 8h-8l and 12) indicate solid state deformation under green-schist facies conditions, consistent with a distinct, younger deformation event. There is no evidence that deformation structures formed at amphibolite facies, melt-present conditions (e.g., low dihedral angles, pseudomorphed melt films, pseudomorphed melt inclusions or pockets; Daczko & Piazolo, 2022;Piazolo et al., 2020).

Comparison With Other Fault Branches of the Tan-Lu Structural System
Structural and geochronological data demonstrate that the TLFZ experienced sinistral strike-slip shearing during the Late Jurassic to Early Cretaceous. Mylonitic rocks with steeply dipping foliations and gently plunging lineations have been documented not only along the southern segment of the TLFZ (Z. Zhu et al., 2005), but also along its northern fault branches such as the Yilan-Yitong Fault zone (Gu et al., 2016), the Dunhua-Mishan Fault zone (C. Liu et al., 2018), and the Yalu River Fault Zone (S. . Figure 16 presents a compilation of ages interpreted from various geochronometers from ductile shear zones along these different branches of the Tan-Lu structural system. Ductile shear deformation observed along the eastern edge of the Dabie Orogen is thought to have occurred under greenschist-facies metamorphic conditions (Wang, 2006;Zhu et al., 2005). 40 Ar/ 39 Ar dating of mica separated from this shear zone yields a wide range of plateau ages (162-139 Ma) for muscovite and biotite (130-100 Ma)  (Wang, 2006;Zhu et al., 2005Zhu et al., , 2010. Unfoliated dikes that crosscut the mylonitic foliation of the shear zone were dated as 127-126 Ma by zircon U-Pb geochronology (Y. Wang et al., 2019), which provides an upper bound of the timing of shear deformation. It is likely that the muscovite 40 Ar/ 39 Ar ages of 162-139 Ma with a closure temperature in the range of 350°C-400°C (Harrison et al., 2009) best approximates the actually timing of shear deformation.
The timing of ductile strike-slip shearing along the northern branches of the TLFZ is informed by zircon U-Pb geochronology of foliated and unfoliated intrusions within the shear zone (Figure 16b). Ductile shear zone along the Dunhua-Mishan and Yilan-Yitong Fault zones has been limited to between 161-128 Ma and 160-126 Ma, respectively (Gu et al., 2016;. In the Yalu-River Fault zone, Middle-to-Late Jurassic granitoids dated to 169-157 Ma, and an Early Cretaceous pegmatite dyke, dated as 146 Ma by U-Pb geochronology on zircon, are deformed. Whereas Mid-Cretaceous andesite from Dandong basin, which partially stitches the Yalu-River Fault, is dated to ca. 131 Ma. Thus, the timing of sinistral strike-slip shearing along this fault was interpreted to occur between 146 and 131 Ma (D. . By integrating available geochronological data of shear zones along the Tan-Lu structural system and correlation with magmatism along the Zhangbaling Massif, we can infer that the main (early) phase of sinistral strike-slip shearing along the TLFZ may have started in Mid-late Jurassic, possibly by 162 Ma, and peaked in the early Cretaceous at ∼140 Ma and continued until 135 Ma ( Figure 16).

Tectonic Setting for Post-Collisional Strike-Slip Faulting Along the Tan-Lu System
The formation and development of the TLFZ was originally interpreted to be associated with the Triassic collision between the North China Block and the Yangtze Block (A. Yin & Nie, 1993;Zhao et al., 2016). During this process, the TLFZ is interpreted to have behaved as a transform fault that accommodated left-lateral slip along the segment between the two offset HP-UHP metamorphic zones of Sulu in the east and Tongbai-Dabie in the west (Figure 1b; Q. Zhang et al., 2007;Zhu et al., 2009).
The tectonic setting for Late Jurassic-Early Cretaceous post-collisional strike-slip shearing along the TLFZ has been linked to NW-ward subduction of the Paleo-Pacific Plate beneath the eastern margin of the East Asia continent (J. Xu et al., 1987;D. Zhang et al., 2018;Zhu et al., 2005Zhu et al., , 2018. In this oblique subduction tectonic model, deformation in the overriding plate is partitioned into thrusting and sinistral strike-slip faulting along arrays of NNE-to NE striking faults that were distributed over the marginal zone of the East Asia continent (J. Xu et al., 1987;. Such a model does not, however, account for the role of displacement elsewhere in eastern Asia at the time (Dong et al., , 2018 and the role it might have played in intracontinental deformation. The magnitude of Late Jurassic-Early Cretaceous age displacement accommodated along the TLFZ is key to distinguishing between potential drivers of post-collisional strike-slip motion. The northern margin of the North China Craton was apparently offset by two northern fault branches of the TLFZ (C. Liu et al., 2019)the Mishan-Dunhua Fault with ∼170 km of left-lateral offset, and the Yilan-Yitong Fault with ∼35 km of left-lateral offset for a total displacement is ∼205 km. To the south, the TLFZ terminates just to the southeast of the Qinling-Dabie orogen (Luo et al., 2022; Figure 17a). That termination is consistent with sinistral strike-slip displacement along the southern TLFZ transferring into crustal shortening across the Qinling-Dabie orogen (Y. Q. Zhang & Dong, 2019). As previously recognized, the Qinling-Dabie orogen was formed during the Triassic period through collision between North China Block and the Yangtze Block (e.g., Hacker et al., 2004). Further deformation within the orogen overlaps with movement along the TLFZ during Late Jurassic to Early Cretaceous time (Dong et al., 2013; resulting in the formation of the Daba Shan foreland thrust-fold zone (ca. 143 Ma; Dong et al., 2013;P. Li et al., 2012;W. Shi et al., 2012). The minimum crustal shortening across this foreland thrust-fold zone has been estimated to ∼135 km (J. H. Li et al., 2015;W. Shi et al., 2012), which is consistent with the accumulated coeval displacement along the TLFZ and is interpreted to indicate that the sinistral shearing along fault zone accommodated S-ward motion of the North China Block.
In addition to the Qinling-Dabie intracontinental orogen, many other intracontinental fold-and-thrust belts with approximately E-W trends, were developed during Late Jurassic-Early Cretaceous time including the Yinshan-Yanshan belt (Davis et al., 1998(Davis et al., , 2001 and Yagan belt (Zheng & Wang, 2005) just to the west of the Tan-Lu system (Figure 17a). It is difficult to reconcile WNW-directed movement of paleo-Pacific subduction (Figure 17a; S.  with the formation of both E-W and NE-SW trending thrust-and-folds belts ( Figure 17a) during late Mesozoic intracontinental contractional deformation in East Asia.
Other possible driving mechanisms for deformation at this time include the collision between the Siberia-Trans-Baikal block and the composite Mongolia-North China block along the Mongol-Okhotsk orogenic belt (Cogné et al., 2005;Metelkin et al., 2010;Zorin, 1999) in mid-Late Jurassic to Early Cretaceous time (Davis et al., 1998;Dong et al., 2004Dong et al., , 2018Y. Q. Zhang & Dong, 2019;Y. Q. Zhang, Qiu, et al., 2022) (Figure 17a). This collision was diachronous with an eastward younging trend (Cogné et al., 2005;Pei et al., 2011). Final closure of the east Mongol-Okhotsk Ocean was completed in the early portion of the Early Cretaceous epoch (145-136 Ma;Cogné et al., 2005;Metelkin et al., 2010;Guo et al., 2017). This timing is coincident with our estimate of the main deformation phase along the southern segment of TLFZ at ca. 142-140 Ma.
We thus tie the early Early Cretaceous sinistral shearing documented along the southern TLFZ to the closure of the Mongol-Okhotsk Ocean and collision of the Siberia-Trans-Baikal block with the Mongolia-North China block. The far-field stresses associated with this collision appear to have reactivated orogenic belts in northern China, including the Yin Shan-Yan Shan and Qinling-Dabie orogens with the TLFZ providing kinematic linkage to the block collision to the north. It is likely that the deformation at the time does not reflect the far-field effects of that collision alone, but the interaction of those stresses with those associated with the westward subduction of the Paleo-Pacific Plate subduction under the East Asia continent (Figure 17).
The period of 135-120 Ma was dominated by extensional tectonics in East Asia. This event was manifested along the Tan-Lu system by intrusion of numerous granitic dikes into former ductile shear zones (Y. Q. Zhu et al., 2018, 2021, anddocumented herein) and by the formation of rifted basins and associated volcanism (Z. Q. Xu et al., 1982;Zhu et al., 2012). This extensional tectonic phase was coeval with the Early Cretaceous giant igneous event (magmatic flare-up) in East China (Wu et al., 2005(Wu et al., , 2019; Y. Q.  and the development of extensional structures or metamorphic core complexes (J. L. Liu et al., 2011;T. Wang et al., 2011). The North China Craton may have been thinned considerably through removal of about 100 km lithospheric keel (J. Wu et al., 2019).
After this intensive extensional event, a short-lived sinistral strike-slip shearing event occurred along the southern-middle segment of the TLFZ (Han & Niu, 2016;Zhu et al., 2018;this study  . Though the specific tectonic dynamic mechanism of this short compressional event remains unclear, we infer that it most likely resulted from the global plate reorganization during the late Early Cretaceous epoch and followed acceleration of the oblique subduction of the Paleo-Pacific Plate (111-100 Ma; Matthews et al., 2012;Olierook et al., 2020). Future works including regional structural deformation, magmatism, metamorphism and plate reconstruction are needed to further interpret this event and the role sinistral strike-slip faulting dated herein played.

Conclusions
We draw the following conclusions from this study.
1. Detailed field structural mapping, microstructural, quartz LPO and zircon U-Pb analysis documented two phases of early Cretaceous sinistral ductile shear deformation along the Fucha Shan shear zone of southern TLFZ. 2. The main phase shearing was formed under the amphibolite-facies metamorphic conditions (500°C-650°C; 2.8-5.2 Kbar). Synkinematic titanite U-Pb dating confined the time of this shear deformation at 142-140 Ma. 3. The late phase ductile shearing occurred under the greenschist-facies metamorphic conditions (350°C-500°C).
The deformation age was dated to be 118-108 Ma by apatite U-Pb geochronology. 4. By integrating our new results with previous studies, we linked the first phase strike-slip shearing to late Mesozoic multi plate convergent tectonics in East Asia and the late phase shearing to oblique subduction of the Paleo-Pacific Plate.

Data Availability Statement
Data sets for this research are included in this paper, the supporting information files, and references. Data sets generated by this research can also be found at the online data repository Zenodo (https://doi.org/10.5281/ zenodo.7795841).