Abstract

The Early Cretaceous Liaonan metamorphic core complex (MCC), eastern North China craton, provides a field setting to evaluate progressive middle-upper crustal subhorizontal shearing, doming, and detachment faulting. The MCC is bounded by a western Jinzhou detachment fault zone (JDFZ) and a southern Dongjiagou shear zone (DSZ) that were primarily suggested to be two segments of the master detachment fault zone. Integrated structural, microstructural, quartz c-axis fabrics, and fluid inclusion analysis and zircon U-Pb dating on mylonites and syn-kinematic granites along the DSZ and JDFZ reveal that the DSZ possesses deformation characteristics that are obviously different from those along the JDFZ. The DSZ is composed of a Lower Unit of sheared Archean gneisses and an Upper Unit of sheared Neoproterozoic metasedimentary rocks, between which there is an obvious tectonic discontinuity contact (TDC). Rocks from below and above the TDC possess structures and fabrics with consistent geometries and kinematics with those along the JDFZ. A metamorphic break exists between the two units that were sheared at contrasting deformation conditions. Dating of zircons from syn-kinematic granitic dikes from DSZ yields an age of ca. 134 Ma, which is similar to the ages of early shearing along the JDFZ. It is concluded that the Jinzhou and Dongjiagou faults formed parts of a detachment faulting with top-to-the WNW kinematics. Exhumation of the Liaonan MCC shearing initiation along both the JDFZ and DSZ at an early stage (ca. 133~134 Ma), subsequent progressive shearing, and doming during slow cooling and exhumation before ca. 120 Ma, followed by fast cooling and rapid exhumation of the MCC by detachment faulting along the JDFZ until ca. 107 Ma.

1. Introduction

The concept of metamorphic core complex (MCC) was defined by studying the Tertiary Cordilleran extensional tectonics where a group of isolated, denuded, domal uplifts of anomalously deformed, metamorphic, and/or plutonic rocks are overlain by a tectonically detached and unmetamorphosed cover [1, 2]. As one of the most important lithosphere extensional structural styles, MCCs have been identified in many Phanerozoic orogenic belts (e.g., [313]) and Precambrian terrains (e.g., [14, 15]). By definition, a typical MCC contains three basic structural elements: (1) a detachment fault zone with gentle dips and large displacement around a domal high; (2) a lower plate of high-grade gneisses, sometimes intruded by syn-kinematic granitic plutons; and (3) faulted, but non- to low-grade metamorphosed supracrustal rocks and supradetachment basins in the upper plate [16]. The detachment fault zone consists of a discontinuous brittle fault surface (i.e., master detachment fault) underlain by a zone of cataclastic to mylonitic rocks usually <2 km thick [17, 18]. The fault zone, therefore, separates the high-grade lower plate from the weakly faulted non- to low-grade metamorphosed upper plate. The top of the back-dipping mylonitic zone is referred to as the mylonite front [19, 20] or localized-distributed transition [18] where the undeformed upper plate sedimentary rocks are in direct contact with deep level mylonitic rocks of distributed deformation along the detachment fault zone. The mylonites are not directly related to the detachment fault but were “captured” by the fault as it descended below the brittle-ductile transition [21]. The development of the mylonitic front was attributed to detachment fault that cuts through the brittle-ductile transition and soles out at the transition from localized to distributed ductile deformation [19, 21].

There have been controversy explanations of mechanisms of exhumation of deep-seated rocks related to flow of the middle to lower crust during regional tectonic extension [2227]. Studies on the fault rocks along the detachment fault zone and definition of mylonitic front led to the popularly cited model by Lister and Davis [17] who stressed the multigenerations of décollement from an initially subhorizontal ductile shear zone at depth developing a domal culmination, as the result of unloading and isostatic effects of granite intrusion. Moreover, the widely accepted “rolling-hinge” model refers to the progressive flexural-isostatic shallowing of an initially steep fault by unloading of the footwall during extension (e.g., [2831]). However, exhumation of high-grade metamorphic rocks from the footwall of the detachment fault may have close genetic relationships with regional doming in orogenic belts that sometimes MCCs are considered as a type of dome structure [32]. Some studies, in particular, stressed the importance of detachment faulting along low-angle normal fault (LANFs) zones in the exhumation of lower plate rocks [9]. Other studies (e.g., [32]) related the exhumation of MCCs to deep crustal-mantle processes and suggested that the rapid delamination resulted in isothermal decompression and crustal thinning and a transition from gneiss dome formation to MCC exhumation [32]. It is plausible, in such models, that diapiric emplacement of hot lower crustal masses occurred in an early stage of exhumation [32].

The Early Cretaceous Liaonan MCC in the southern Liaodong peninsula, North China craton (Figure 1(a); [33]), is taken as an example to highlight the importance of middle-lower crustal (subhorizontal) shearing in exhumation of MCCs. The Liaonan MCC was proposed to be a typical Cordilleran type MCC that has two branches of detachment zone (Figure 1(b); [12]), i.e., the Jinzhou detachment fault zone (JDFZ) and Dongjiagou shear zone (DSZ). The present study, for the first time, reports the existence of obvious contrasts in structural characteristics and deformation history between the JDFZ and the DSZ. We show from structural, thermometric, fabric analysis, and geochronological dating of the poorly studied DSZ that exhumation of the Liaonan MCC is attributed to the combined effects of progressive subhorizontal shearing, doming, and detachment faulting.

2. Geological Overview

The Liaonan MCC consists of three major elements, i.e., a master detachment fault zone, an upper plate, and a lower plate (Figure 2(a)). The lower plate contains a TTG (tonalite-trondhjemite-granodiorite) igneous suite and supracrustal rocks of amphibolite facies that are dated as Archean protoliths [34] which are intruded by Early Cretaceous syn-extensional granites [35, 36]. The upper plate is mainly composed of an Early Cretaceous supradetachment basin sitting on Neoproterozoic to Paleozoic sedimentary rocks. The basin is normal fault-bounded and filled with Early Cretaceous volcanic-sedimentary rocks [35]. The master detachment fault zone of the Liaonan MCC was proposed to have two branches, i.e., the Jinzhou detachment fault zone (JDFZ) striking NNE and the Dongjiagou shear zone (DSZ) striking ENE (Figures 1(b) and 2(a); [12, 36]). Field observations and geological mapping show that the DSZ was truncated by the JDFZ near Jinzhou (Figure 2(a)). Foliations along the two branches dip to NW (JDFZ) and SE to S (DSZ), respectively. The detachment fault zone, lower plate metamorphic rocks, and syn-kinematic granite plutons possess stretching lineations of the same orientations that are consistent with striations on the Jinzhou master detachment fault surface (WNW-ESE). The JDFZ has corrugations with hinges parallel to the WNW-ESE trending stretching lineation in underlying mylonites [36]. Various shear sense indicators along both zones show consistent top-to-the-WNW shearing [12, 3537]. Geochronological data from syn-kinematic granitic dikes from the JDFZ and intrusions from the lower plate constrain the shearing from 134 to 116 Ma [35, 36, 3840]. The posttectonic magmas were dated as 115 Ma and 113 Ma [36, 41]. Based on the combination of the U-Pb and Ar-Ar ages, the lower plate of the Liaonan MCC was exhumed progressively and sequentially in two different stages (an early, slow exhumation from ca. 135 to 120 Ma and a late, rapid exhumation from ca. 120 to 107 Ma) during crustal extension [35, 42].

3. Deformation Structures of the Liaonan MCC

Deformation of the two shear zones, i.e., the JDFZ and DSZ, characterizes the structural evolution of the Liaonan MCC (Figure 2(a)). Also, the weakly folded upper plate of the DSZ in Dalian City shows similar structural characteristics and kinematics to the above shear zones and may provide important information on the exhumation of the Liaonan MCC.

3.1. JDFZ

The JDFZ is the typical detachment fault zone in the western of Liaonan MCC. The most significant feature of the JDFZ is the occurrence of a thick sequence of fault-related rocks that record the detachment faulting at different crustal levels [12, 35, 43, 44]. From a typical cross-section across the JDFZ (Figure 2(c)), fault rocks include, from the lower plate to the master detachment fault, gneisses (Figure 2(c1)), mylonites (Figure 2(c2)), microbreccias (Figure 2(c3)), brecciated mylonites (Figure 2(c4)), and fault gouges and pseudotchylites (Figure 2(c5)).

3.1.1. The Mylonite Zone in the Lower Plate

The mylonite zone is about 1.5 km thick, characterized by macroscopic banded or augen structures. Feldspar porphyroblasts in generally fine-grained matrix have rotated and stretched tails of quartz grains. Quartz grains in the mylonites across the JDFZ show transitional characteristics from grain boundary migration (GBM), subgrain rotation recrystallization (SGR) to bulging dynamic recrystallization (BLG), correlating with their c-axis fabrics from X-axis to Y-axis and Z-axis maxima, respectively [43]. These characteristics are evidence for plastic deformation of quartz from high to low temperatures or from amphibolite facies to greenschist facies, which imply that the lower plate rocks experienced exhumation from the middle-lower crust [12, 35, 4345].

The mylonites taken along the JDFZ (Figure 2(a); Sample SL18050) are characterized by progressive shearing in the following sequences: (1) the formation of mylonitic foliations accompanying emplacement of early granitic dikes and formation of boudinage structures (Figure 3(a)); (2) progressive mylonitization inducing S-C (Figure 3(b)) and C fabrics; (3) as well as superimposed folding at the necking position of the dike (Figure 3(a)); and (4) the emplacement of a dioritic dike that was relatively weakly sheared (Figure 3(c)). Mylonitic foliations and early folds are truncated by the dikes.

Various structural patterns, e.g., shear bands in banded mylonites (Figure 3(d)), rotated blocks of mylonitic gneisses (Figure 3(e)), and asymmetrical tectonic lenses, indicate the top-to-the-WNW shearing along the JDFZ. Different from the banded mylonites, the blocks and lenses are relatively massive and blocky with penetrative mylonitic foliations, which record higher temperature shearing in an early stage (Figure 3(e)).

3.1.2. Sedimentary Rocks in the Upper Plate

The upper plate consists of Neoproterozoic to lower Paleozoic sedimentary rocks, which are mainly non- or weakly-deformed limestones and quartz sandstones (Figure 3(f)). The bedding of the sedimentary rocks in the upper plate dips predominantly to the ESE (Figure 2(c6)), which is opposite to the dipping directions of foliations in the mylonites of the lower plate. Some sedimentary layers in the hanging wall are folded when approaching the master detachment fault surface, which also indicates top-to-the-WNW shearing along the master fault.

3.2. DSZ

The DSZ is a shear zone in the southern margin of the Liaonan MCC, which consists of two major tectonic units, i.e., a Lower Unit of Archean gneisses and an Upper Unit of Neoproterozoic metasedimentary rocks (Figure 2(d)). The two units are separated by a tectonic discontinuity contact (TDC, [46]). Rocks from Archean gneisses and Neoproterozoic metasedimentary rocks have significant metamorphic contrast. A metamorphic break exists between them, although they possess consistent structural elements (lineations, foliations, and kinematics) formed during the metamorphisms.

3.2.1. The Lower Unit

The Lower Unit consists mainly of Archean gneisses, identical to the lower plate of the JDFZ. Rocks of the Lower Unit are granitic mylonites, transitional to biotite plagioclase gneisses. The granitic mylonites near the contact are banded and have strong foliations and lineations. Feldspar grains form augens of aggregates that have subrounded shapes. Trailing tails of the augens are indicative of top-to-the-WNW shearing. The biotite plagioclase gneisses have subrounded feldspar grains or form feldspar fish due to high-temperature shearing [47].

Rocks from the Lower Unit experienced progressive shearing, preserved by the deformation structures and microstructures of rocks. Biotite plagioclase gneisses located at SL18087 (Figure 2(a)) show the following sequences: (1) feldspar augens with fish shapes formed in the early mylonitization (Figure 3(g)) indicate a relatively high-temperature during early shearing [47]; (2) foliations and lineations are developed due to mylonitization of early granitic dikes and gneisses (Figure 3(h)); (3) granitic dikes that crosscut the mylonitic foliations experienced weak shearing. They are weakly folded (Figure 3(i)); (4) localized mica-rich shears with striations crosscut early granitic dikes (Figure 3(j)).

The granitic mylonites located in SL18057 (Figure 2(a)) possess characteristics of progressive deformation in the following sequences: (1) early-stage medium to high-temperature mylonitization indicated by feldspar augens or subrounded porphyroclasts; (2) late-stage low-temperature simple shearing shown by intensive deformation of quartz veins that are parallel to the foliations. Stretching lineations are developed along the contacts with their surrounding mylonites (Figure 3(k)).

3.2.2. The Upper Unit

The Upper Unit consists of metamorphosed Neoproterozoic metasedimentary rocks, e.g., phyllonite, mica schist, metamorphic quartz sandstone, and calcareous slate. Shearing foliations are parallel to bedding. The planar fabrics (bedding and foliations) dip predominantly to the SSE near the TDC, which is consistent with the Lower Unit (Figure 2(d)). Progressive shearing was also developed in rocks from the Upper Unit, which is shown as: (1) the development of intrafolial folds in calcareous siltstones and (2) stretching lineations on the discrete foliations by late shearing (Figure 3(l)).

3.3. Weakly Folded Upper Unit in Southern Liaonan MCC

Far away from the TDC in the upper plate of the DSZ, Neoproterozoic rocks, including the Qingbaikou and Sinian Systems, have foliations with various dip directions and low dip angles forming the weakly folded Upper Unit in Dalian City (Figure 4(a)). Archean gneisses are not exposed in the area. Quartz sandstone, slate (Figure 3(m)), and quartzite (Figure 3(n)) of the Qingbaikou System have well-developed foliations and stretching lineations and preserve lens-shaped structures. Although the foliations dip in various directions in the dome-shaped structural configuration (Figure 4(c)), the stretching lineations also strike WNW-ESE (Figure 4(b)). Shear sense indicators are compatible at different places with top-to-the-WNW shearing (Figure 3(n)), which is consistent with the JDFZ and DSZ.

4. Methods and Samples

Much work has been conducted along the JDFZ in previous studies, but the role of the DSZ in the formation of Liaonan MCC has been neglected. To clarify the detailed structural characteristics and constrain the tectonic evolution of the DSZ, a series of samples were collected from different structural horizons along the DSZ (Figure 2(a)), using the following experimental approaches.

4.1. Methods

4.1.1. Thermometric Analysis and Laser Raman Spectra of Fluid Inclusions

Doubly polished thin sections (0.30 mm thick) were made of numerous quartz veins. Fluid inclusions were carefully detected to identify their genetic and compositional types and spatial clustering. Microthermometric measurements were performed using the Linkam THMSG600 heating-freezing stage and employing standard procedures at the Institute of Mineral Resources, Chinese Academy of Geological Sciences. The estimated precision of the measurements is ±0.1°C for temperatures lower than 30°C and ±1°C for temperatures higher than 30°C. Salinities, densities, and temperatures of inclusions were calculated using the freezing point [48, 49]. Densities of the inclusions were calculated using the Flincor procedure [50]. The temperatures were calibrated using the P-T diagram proposed by Lu [51]. In the experiment, the primary inclusions were selected to calculate the temperatures, while the secondary inclusions were strictly ignored.

Compositions of inclusions were identified using Laser Raman spectroscopy at the Institute of Mineral Resources, Chinese Academy of Geological Sciences. An argon laser with a wave length of 514.53 nm was used as a laser source at a power of 20 mW. The spectral resolution is 1 ~ 2 cm-1 with a beam size of 1 μm. The scanning range is 100~4500 cm-1. Instrumental setting was kept constant during all analyses. The representative fluid inclusions with different types were selected for Raman analysis.

4.1.2. EBSD Analysis

The crystallographic preferred orientation (CPO) of quartz, especially the study of quartz c-axis fabric, is widely used in structural geology, such as defining deformation temperature, determining shear direction, and analyzing deformation history. XZ sections (paralleling to lineation and normal to foliation) were cut from the samples and polished using Buehler Mastermet colloidal silica and Buehler grinder-polisher. The LPO data acquisition were carried out on a Hitachi S-3400 N-II scanning electron microscope mounted with Nordlys EBSD Model NL-II detector with the thin section surface inclined at 70° to the incidental beam. The new technique can provide fast data acquisition of mineral grains or parts of mineral grains of interest, with 0.1 μm spatial resolution and 0.5° angular resolution. Acceleration voltage of 15 kV and a working distance of 18.4 mm are applied. EBSD analysis was carried out using the HKL Channel 5 software package. The EBSD analysis was performed at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Beijing).

4.1.3. LA-ICP-MS Dating

Zircon grains were separated from whole-rock samples using conventional techniques. After crushing and sieving of the samples, heavy minerals were concentrated by panning and magnetic separation. Zircon grains were mounted in epoxy and carefully polished until their cores were exposed. Cathodoluminescence (CL) images of zircons combined with reflected and transmitted light images were used for analyses. U-Pb dating analyses of zircon were conducted at the Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) microanalysis laboratory, affiliated to the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing. Laser sampling was performed using a Coherent’s GeoLasPro-193 nm system. A Thermo Fisher’s X-Series 2 ICP-MS instrument was used to acquire ion-signal intensities. Helium was used as a carrier gas. Argon was used as the make-up gas and mixed with the carrier gas. All data were acquired on zircon in single spot ablation mode at a spot size of 32 μm with 6 Hz frequency in this study. Zircon 91500 was used as an external standard for U-Pb dating (1062 Ma; [52, 53]). The detailed analytical procedure refers to [54]. Meanwhile, zircon Mud Tank was used as a monitoring standard for each analysis (732±5Ma; [55]). Time-dependent drifts of U-Pb isotopic ratios were corrected using linear interpolation (with time) for every five analyses according to the variations of 91500. Each analysis incorporated a background acquisition of approximately 20 s (gas blank) followed by 50 s data acquisition from the sample. Off-line selection and integration of background and analyte signals and time-drift correction and quantitative calibration for trace element analyses and U-Pb dating were performed by ICPMSDataCal [56]. Data reduction and Concordia plotting diagram were carried out using the Isoplot 4.5.

4.2. Samples

4.2.1. Samples of Fluid Inclusion Studies and EBSD Analysis

The deformation conditions of the DSZ can be determined by the trapping temperature and pressure of primary inclusions in the syn-kinematic quartz veins which developed penetratively paralleled to foliations [5759], i.e., the Sample SL18057 (Figure 3(k)). Syn-kinematic quartz veins of samples SL17048, SL18057, SL18088, and SL18089 are composed of quartz aggregates, taken from different structural horizons (Figure 2(a)) for fluid inclusion temperature measurement and Laser Raman testing. The sample SL17048 from the Lower Unit has quartz grains with extensive grain boundary migration recrystallization (Figure 5(a)). Quartz grains in the sample SL18057 from mylonites at the Lower Unit near the TDC are distributed in a striped grain aggregate. The small new grains are characteristic of subgrain rotation recrystallization (Figure 5(b)). Bulging dynamic recrystallization is the major mechanism of grain size reduction in the samples SL18088 and SL18089 from the mylonized upper plate rocks (Figure 5(c)). Some coarse grains in the sample SL18089 exhibit weak undulose extinction.

Samples SL18087-5/-6, SL18057/SL18057-2, and SL18088-2/-3 were collected from different structural horizons (Figure 2(a)) for EBSD analysis. Samples SL18087-5/-6 are mylonitic gneisses from the Lower Unit, which show the dispersive lens-shaped feldspar (Figure 5(e)) and connected biotite grains along foliations (Figure 5(f)). Samples SL18057/SL18057-2 were collected from a quartz vein paralleled to foliations with quartz grains possess subgrain rotation recrystallization (Figure 5(b)) and wall rock of granitic mylonite with muscovite grains connected to the foliations (Figure 5(d)), respectively. Samples SL18088-2/-3 are calcareous siltstone with C fabrics (Figure 5(g)) and phyllonite with S-C fabrics (Figure 5(h)) collected from the Upper Unit.

4.2.2. Geochronology Samples

Two granitoid samples of syn-kinematic dikes from the DSZ were selected for zircon U-Pb dating. The granitoid was emplaced into the mylonites of the Lower Unit and experienced the shearing event. The geochronological data of these dikes with different structural characteristics are essential to constrain the timing relations of JDFZ and DSZ. Sample locations are shown in Figures 2(a) and 6, respectively.

Sample SLP1702 (39° 9 46.6 N, 122° 3 35.8 E) was collected from Dalijia Village, which is a folded light-gray granitic dike intruded into mylonite (Figure 6(a)). The hinge of the fold is nearly parallel to the lineations, indicating the characteristics of “a” type fold [60, 61]. The dikes cut the foliations. In the hinge area, the dike is penetrated by the foliations, which is important evidence of syn-kinematic dike (Figure 6(b)). This sample is a fine-grained mylonitized granite that consists mainly of K-feldspar (20-25%), plagioclase (25-30%), quartz (30-35%), biotite (5%), and chlorite (3%). Quartz grains have been strongly deformed into ribbons that have largely recrystallized into smaller grains through subgrain rotation recrystallization (Figure 6(c)). The long axes of the grains are parallel to the foliation (Figure 6).

Sample SL17046 (39° 8 45.9 N, 121° 56 7.2 E) was collected from a late syn-kinematic dike from Desheng Town (Figure 6(d)). The dike is characterized by an irregular shape at the outcrop. Part of the granitic dike developed parallel to the foliations, while other parts cut off the foliations (Figure 6(e)). The sample is a medium-grained, light-gray mylonitized granite invaded into the gneiss, which consists mainly of K-feldspar (30-40%), plagioclase (10-15%), quartz (30-40%), and biotite (~10%). Intragranular fractures in remnant feldspar crystals and the subgranular shapes of lobate quartz grains indicate a subsequent low-temperature deformation event (Figure 6(f)).

5. Results

5.1. Thermometric Analysis and Laser Raman Spectrum of Fluid Inclusions from syn-Kinematic Quartz Veins

Primary inclusions in quartz grains from the syn-kinematic veins were selected, which have irregular shapes, such as elliptical or fusiform, ranging in sizes of 2-20 μm. Most inclusions have sizes from 4 to 10 μm. Secondary inclusions are smaller, showing linear distribution along the fracture in general. The secondary inclusions are neglected in the current study. There are two types of the primary inclusions. One is two-phase inclusions of gas and liquid, which account for more than 90% of the inclusions. They are isolated or distributed in small groups (Figures 7(a)–7(d)). The other are three-phase inclusions containing CO2, with a distinct “double eyelid structure.” They are only developed in syn-kinematic quartz veins from the Lower Unit (sample SL17048; Figures 7(e) and 7(f)).

Freezing point temperature (Tm) and homogenization temperature (Th) of more than 30 primary fluid inclusions are measured. The salinity is calculated by the formula as follows [62].
(1)WNaCl=0.00+1.78Tm0.0442Tm2+0.000557Tm3.

According to the T-ρ phase diagram of the NaCl-H2O system proposed by Bischoff [49], the corresponding density range is plotted using the homogenization temperature and calculated salinity presented in Table 1.

The modal values of salinity are included in the Supplementary Materials. The fluid inclusion calculation software Flincor [50] was used to estimate the crystallization temperature and pressure of quartz veins on the basis of homogenization temperatures (i.e., 185°C (SL17048), 215°C (SL18057), and 225°C (SL18088/SL18089)) and the modal values of salinity at these temperature conditions (Supplementary Materials a). The results show that the crystallization temperatures and pressures of samples SL17048, SL18057, and SL18088/SL18089 are ca. 630°C, 470°C, and 350°C and about 700 MPa, 420 MPa, and 200 MPa, respectively. These results can be confirmed using the P-T phase diagrams of different salinity in H2O-NaCl systems proposed by Bodnar and Vityk [63]. Furthermore, they are consistent with microstructural observations. Accordingly, the depths of quartz vein formation are about 25 km (SL17048), 15 km (SL18057), and 7 km (SL18088/SL18089), respectively (applying a density of crustal rocks of ca. 2700 kg/m3).

The liquid compositions in the liquid phase of the inclusions in most samples are mainly H2O (peak 3310~ 3610 cm-1; Supplementary Materials (c)). Only the gas phase component of inclusions of the sample SL17048 has CO2 (double peaks are 1285 and 1388 cm-1; Supplementary Materials (c)). This is consistent with the characteristics of fluid inclusions observed under the microscope. There are abundant CO2-bearing three-phase inclusions in the sample SL17048 that were taken from the lowermost part of the DSZ. The production of CO2 may be related to the large-scale magma activities that led to degasification of magmas [64] or syn-kinematic decarbonization of marble layers [65].

5.2. Quartz c-Axis Fabrics

Point maxima or double-maxima superpositions are the dominant fabric patterns, which show the structural superposition under the same tectonic stress field. The fabrics of all samples indicate a history of progressive deformation from medium-high to low-temperature [6668]. The pole figures and the minimum and maximum multiples of uniform density (MUD) of quartz are represented by the contour lines in Figure 8. The MUD provides us a quantitative indication of fabric strength [69]. The fabric data for the samples show different maxima consistent with field observations for progressive shearing.

Samples SL18087-5 and SL18087-6 are mylonitized gneiss, located at the base of DSZ. The c-axis fabric of SL18087-6 consists of a group of type I symmetrical maxima parallel to the Y-axis and a type II symmetrical maxima located between the Z-axis and Y-axis; the maxima distributed independently as a point (Figure 8(a)). The type I symmetrical maxima parallel to the Y-axis are the result of the prism <a> slip at medium high-temperature condition (550-650°C) which is consistent with the microstructure characteristic of quartz grain boundary migration recrystallization and the temperature estimate of the fluid inclusion of SL17048 (630°C). The maxima located between the X-axis and the Y-axis are attributed to the rhomb <a> slip under the medium low-temperature (400-550°C). The c-axis fabric of sample SL18087-5 maxima is along the Y0- and Z0-axis which indicates that low-temperature basal <a> slip superimposed on high-temperature prism <c> slip (Figure 8(b)). It is consistent with the observation in the field that the relatively low-temperature discrete foliations superimposed on the high-temperature penetrative foliations.

Samples SL18057 and SL18057-2 are granitic mylonites, located at the top of the Lower Unit of the DSZ. The c-axis fabric of SL18057-2 is composed of a combination of type I maxima and type I crossed girdle (Figure 8(c)). Type I maxima are composed of several point maxima groups that are not located along the Y-axis, forming an irregular pattern. This kind of fabric results from rhomb <a> slip at medium low-temperature conditions (400-550°C) or prism <a> slip at medium high-temperature condition (550-650°C). This corresponds to mylonitization of the granite. The symmetry of type I crossed girdle patterns is attributed to relatively large pure shear strain components, and the saturation of fabrics is limited by the magnitude of strain. The c-axis fabric of SL18057 shows maxima parallel to the Y0-axis, which is slightly extended along the Y0Z0 direction (Figure 8(d)). The asymmetry of the distribution and density of the maxima indicate that the simple shear deformation plays a dominant role. But at the same time, a relatively weak Z0 maxima developed which indicates the superposition of simple shear deformation under the low-temperature condition in the later stage. This suggests that the quartz vein precipitated in the later stage under relatively low-temperature conditions (470°C from the estimate of the fluid inclusion) and the microstructure of the quartz vein expressing subgrain rotation recrystallization.

Samples SL18088-2 and SL18088-3 are calcareous siltstones and phyllonites, located at the Upper Unit of the DSZ. The quartz c-axis fabric of SL18088-3 is a symmetrical type I crossed girdle consisting of four type IV point maxima which is caused by rhomb <a> slip under medium low-temperature (400-550°C, Figure 8(e)). The symmetrical pattern also indicates the deformation environment with a relatively large pure shear component which corresponds to the penetrative folding in the field. The quartz c-axis fabric of SL18088-2 and SL18088-3 is similar. Sample SL18088-2 also shows Z0 point maxima. The fabric is monoclinic symmetrical, which indicates that basal <a> slip at low temperature (corresponding to 350°C from the estimate of the fluid inclusion) plays an important role and represents the superposition of simple shear deformation (Figure 8(f)).

5.3. Geochronology of Granitic Dikes in the DSZ

Zircons in sample SLP1702 are commonly colorless, with a glassy-greasy luster, and have good transparency. They are mostly rounded, long-columnar, and semi-cone-shaped. The grain sizes are 90-220 μm with long-short axis ratio of 1 : 1-3 : 1 (Figures 9(a) and 9(c)). In the CL image, zircons have various characteristics: (1) some are typical magmatic zircons with well-developed oscillatory zoning; (2) the cores and edges of some magmatic zircons are obviously different, but all of them developed oscillatory zoning, which is formed by the superposition of multistage magmatic zircon growth; (3) some are inherited zircons. There is oscillatory zoning in the margin, but the core is cloudy; (4) the metamorphic zircons are more common, and hydrothermal dissolution structures are visible inside. Eighteen data points show nearly consistent group distribution, with a 207Pb/206Pb weighted average age of 2387±29Ma (MSWD=0.35, n=18), which are the inherited zircons (Figures 9(a) and 9(b)). Another eleven data points show nearly consistent group distribution, with a 206Pb/238U weighted average age of 138.3±1.6Ma (MSWD=0.81, n=10), representing the age of the zircon crystallization from the magma and therefore the early stage shearing along the DSZ (Figure 9(d)).

Zircons in sample SL17046 are mainly colorless and transparent, and some crystals are fragmented, with a length of 70-170 μm and long-short axis ratio of 1.2 : 1-4.5 : 1 (Figure 9(e)). In the CL image, zircon grains are tabular with no oscillatory zoning. Because of serious lead loss, the weighted age has low concordance. Only 10 data points show a nearly consistent group distribution, with a 206Pb/238U weighted average age of 134.8±2.6Ma (MSWD=0.59, n=10), which represents the age of emplacement of syn-kinematic dikes and therefore the age of late-stage shearing along the DSZ (Figure 9(f)).

6. Discussion

6.1. Contrasting Deformation Characteristics and Structural Evolution between the JDFZ and DSZ

From similarities of orientations of stretching lineations (WNW-ESE) and kinematics (top-to-the-WNW) along both the JDFZ and DSZ, previous studies regarded them as a unified detachment fault zone [12, 36, 41, 70, 71]. The detailed analysis presented in the current study, however, shows that there are significant differences between the two shear zones.

The JDFZ and DSZ both consist of three components. The JDFZ possesses characteristics of most detachment fault zones: (1) a master detachment fault zone of a thick sequence of tectonites from gneissic mylonites near the lower plate to chloritic breccias and fault gauges near/along brittle rupture surface (including mylonite, Figure 10(b)), (2) an upper plate of a supradetachment basin sitting on undeformed Neoproterozoic to lower Paleozoic sedimentary rocks (Figure 10(a)), and (3) a lower plate of high-grade Archean gneisses metamorphosed up to lower amphibolite facies. In contrast, the DSZ is a large-scale shear zone encompassing (1) the Lower Unit of highly sheared Archean gneisses metamorphosed up to amphibolite facies (Figure 10(e)), (2) the Upper Unit of metamorphosed Neoproterozoic sedimentary rocks to lower greenschist facies (Figure 10(d)), and (3) a major tectonic discontinuity contact (TDC) between the Lower and the Upper Units. An obvious metamorphic contrast exists across the TDC [46]. However, rocks from both sides of the TDC possess structures and fabrics for identical shearing deformation that is consistent with shearing along the JDFZ. Comparing with the stratigraphic components of the DSZ, there is a loss of the bottom section of the lower section of Neoproterozoic strata, i.e., the Upper Unit (Pt31; Figure 10).

The TDC along the DSZ bears similarities but obvious distinctions to the mylonitic front [17, 21]. The typical mylonitic front is characterized by (Figure 11(a); [21]) as follows: (1) a detachment fault in the classic model that transects the mylonitic foliation at the mylonitic front; (2) the upper plate is undeformed with brittle fractures. In contrast, the TDC along the DSZ occurs as an invisible tectonic discontinuity surface. Rocks above and beneath the TDC along the DSZ are sheared by identical shearing processes. They possess parallel structural characteristics, i.e., foliations and stretching lineations, of consistent top-to-the-WNW shearing (Figure 11(b)) but of contrasting deformation conditions. The Upper Unit above the TDC comprises rocks of stratigraphically younger sedimentary rocks (Neoproterozoic to Ordovician) that experienced low-grade syn-shearing metamorphism up to lower greenschist facies. The Lower Unit of the TDC is composed of highly sheared Archean gneisses metamorphosed up to lower amphibolite facies during ductile shearing. The above differences of the TDC from the mylonitic front may imply that the TDC along the DSZ may have formed differently to that of the subhorizontal detachment shear zone forming the mylonitic front, involving subhorizontal shearing throughout the whole middle-lower crust. The present mylonitic sequences from the Upper Unit above and the Lower Unit beneath the TDC are the representation of exposed middle to lower crustal subhorizontal shear zones. The contrast between the Upper Unit may be ascribed to climbing-up subhorizontal faulting during progressive extension and exhumation of the middle to lower crustal rocks.

Structural analysis reveals that rock assemblages along the JDFZ exhibit transitions in time and space (upwards in the complex) from fault-related gneiss to mylonitic schist and gneiss, retrograded schist and gneiss, brecciated mylonite, microbreccia, pseudotachylite, and gouge, which documents a progressive overprinting of the lower plate rocks from depths below the crust’s brittle-ductile transition to near-surface conditions during exhumation [12]. Detailed examination of the deformation microstructures of the sheared rocks beneath the master detachment fault zone reveals that rocks at the lower most part of the lower plate are characterized by high-temperature deformation [43]. Intermediate temperature deformation microstructures, however, are preserved in the porphyroclasts in the brittle fault rocks near the master fault. Such a difference may imply that the protoliths sheared by the JDFZ may have originated from preexisting fault rocks at different crustal levels.

The above discussion suggests that the fault rock assemblage along the JDFZ was similar to those along the DSZ at their original stage of shearing, but only the middle to lower crustal fault rocks are preserved in the present exposures. Detachment faulting along the JDFZ that overprinted the early shear fabrics during progressive extension is also revealed by quartz c-axis fabric analysis of fault rocks along the JDFZ [43].

6.2. Timing of Shearing and Exhumation of JDFZ and DSZ

Geochronology of sheared rocks along JDFZ and syn-kinematic plutons of Liaonan MCC has been studied systematically to discuss the tectonic evolution and magmatic events during the exhumation of Liaonan MCC [3537, 3942, 44, 70, 72, 73]. Shearing along the JDFZ initiated before ca. 134 Ma [35], followed by slow cooling and exhumation of the lower plate accompanied a major magmatic event from 130 Ma to 120 Ma subsequently [36, 3840, 44]. The postkinematic emplacement within the JDFZ is exemplified by the Zhaofang granodiorite pluton (113 Ma; [36]) and a small granite-porphyry dike (115 Ma; [41]).

New U-Pb zircon age data of syn-kinematic dikes from DSZ in the current contribution constrain the crystallization ages to 138 Ma (SLP1702) and 134 Ma (SL17046), which are similar to those of the early syn-kinematic dikes along the JDFZ (134 Ma; [35]). Compilation of thermochronological results revealed the importance of two stages of tectonic extension on the exhumation of the Liaonan MCC, i.e., an early stage of slow cooling and exhumation before ca. 120 Ma and a late stage of rapid cooling and exhumation after 120 Ma [13, 35]. From these results, therefore, we would argue that both the JDFZ and DSZ experienced identical early shearing between ca. 138 Ma and 120 Ma. The shear zone deformation was accompanied by intensive magmatic activities, which resulted in the intrusion of most granitic magmas into the JDFZ, DSZ, and their lower plate. Subsequently, shearing along the JDFZ superimposed on the early stage shear structures was localized along the JDFZ from 120 Ma to 105 Ma [13, 41]. Faulting along the JDFZ and a subsidiary detachment fault (i.e., the Wanfu detachment fault) led to rapid cooling and exhumation of the lower plate of the Liaonan MCC [13, 35].

6.3. Three-Stage Evolution of Crustal Extension in Liaonan MCC

6.3.1. Subhorizontal Shearing and Doming

A general conclusion from regional stratigraphic studies is that the DSZ deformed a sequence from Archean gneisses in the Lower Unit, through Neoproterozoic layers at the Upper Unit, to the Cambrian rocks (Figure 10). The TDC between the Lower Unit and the Upper Unit marks a break in metamorphic grades in the stratigraphic sequence. Structural, metamorphic, and geothermometrical analysis reveal that due to intensive Early Cretaceous shearing, the rocks were transformed into tectonites of highly sheared Archean gneisses (the Lower Unit, metamorphosed up to lower amphibolite facies) and sheared Neoproterozoic metasedimentary rocks (the Upper Unit, metamorphosed up to lower greenschist facies). Fluid inclusion studies of quartz veins indicate a decrease in the deformation temperatures from 630°C and 470°C in the Lower Unit to 350°C in the Upper Unit. The results suggest that rocks on both sides of the TDC along the DSZ were sheared at the same time but at different crustal levels, which is consistent with a subhorizontal shearing system prevailing during the formation of the DSZ. In addition, geochronological dating of syn-kinematic granite dikes from the DSZ (134 Ma) is similar to the ages of shearing at the core (134 Ma; [35]) and along the JDFZ (133 Ma; [35]). The similar characteristics of mylonitization, identical kinematic indicators, and the coeval shearing along the JDFZ, the DSZ, and within the lower plate demonstrate that they all experienced the early subhorizontal shearing (Figure 12(a)) during an early stage of deformation in the Liaonan MCC. Doming was the succeeding stage after the subhorizontal shearing along the JDFZ and DSZ, from ca. 134 Ma to 120 Ma (Figure 12(b)). The slow cooling of the lower plate was partly due to subhorizontal shearing [35]. Meanwhile, syn-kinematic emplacement of granitic intrusions also contributed to the thermal budget which may also affect the cooling path of the lower plate rocks. At the same time, the emplacement of syn-kinematic granite plutons promoted the doming process, which is different from the concept by Lin et al. [73, 74] and refers to an early stage of slow exhumation of the middle to lower crustal rocks at a cooling rate of approximately 7°C/m.y. between ca. 134 Ma and 120 Ma prior to major detachment faulting along the JDFZ [35].

6.3.2. Shearing along the JDFZ and the Rapid Exhumation of the Liaonan MCC

We describe a typical sequence of extensional detachment faulting-related tectonites distributed along the JDFZ [12, 35, 75]. Relatively lower temperature superposition structures and fabrics are well-preserved in the tectonites, indicating progressive superposition of a late stage shearing on early deformation fabrics and gradual exhumation of middle and lower crustal rocks [12, 35]. The detachment faulting along the JDFZ resulted in an omission of sheared Neoproterozoic rocks (Pt31; Figure 10), which makes the highly sheared rocks in the lower plate in direct contact with the undeformed sedimentary rocks in the upper plate. Thermochronological dating reveals that progressive shearing along the JDFZ prevailed from ca. 120 to 113 Ma when the undeformed postkinematic dikes intruded into JDFZ (113 Ma; [36]) or to 107 Ma when the lower plate cooled to below the K-feldspar closure temperature [35, 42]. The detachment faulting, together with faulting along the Wanfu fault, led to fast cooling at rates of up to 40-55°C/m.y. and rapid exhumation of the lower plate [35]. Identical kinematics of shearing along the JDFZ and the DSZ, i.e., consistent stretching lineations (striking to 110°-290°) and shearing indicators (top-to-the-WNW), demonstrate that the development of JDFZ is a continuation of early subhorizontal shearing and doming.

Could subhorizontal shearing induce middle-upper crustal doming? Although various mechanisms have been proposed for the dynamic cause and kinematic characteristics of domes [7678], the following observations support the possibility of doming on the premise of subhorizontal shearing: (1) instability induced by vertical variation of viscosity [77], (2) stacking of the anticline [76], and (3) isostatic compensation after the detachment of crustal extension [27, 79]. Partial melting and magmatism may contribute to the low effective viscosity of the crustal rocks, which allows bulk vertical thinning of the crust and favor exhumation processes [18]. None of the above models is applicable to the formation of the domal configuration of the Liaonan MCC. In the case of the Liaonan MCC, progressive subhorizontal middle to lower crustal shearing may induce small-scale perturbations, which gave rise to fluctuations of flow lines. Being consistent with the study on the Ajjaj shear zone, shear zones can be active at a very deep crustal level and for a long time [80]. As a result, deep rocks were exhumed slowly by doming after high progressive strain accumulation (Figure 12(b)), which is compatible with a slow cooling and exhumation of the lower plate of the Liaonan MCC as revealed by thermochronological studies [35].

Exhumation of the Liaonan MCC from subhorizontal shearing and doming to late detachment faulting is a progressive process in the unified extensional setting. In the process of gradual doming, the strain was localized along the western limb of the MCC that further evolved into the JDFZ. A fast rate of extension may have resulted in rapid cooling and exhumation of the lower plate due to detachment faulting along the JDFZ (Figure 12(c); [35]). Brittle degradation and fragmentation are superimposed on the ductile sheared rocks and fabrics during progressive faulting [12, 35, 37, 42]. Detachment faulting could be the primary mechanism of the final unroofing and the juxtaposition of formerly deep rocks and upper crustal rocks. In this case, a lower to middle crustal mylonitic zone, instead of a mylonitic front, occurs along the DSZ.

7. Conclusions

The present study, for the first time, reports the existence of obvious contrasts in structural characteristics and deformation history between the JDFZ and the DSZ in the Liaonan MCC, which provides deep insights into mechanism of exhumation of MCCs. We show, from integrated structural, thermometric, and fabric studies of the DSZ, that the shear zone possesses a TDC separating highly sheared Archean gneisses metamorphosed up to lower amphibolite facies in the Lower Unit and sheared Neoproterozoic metasedimentary rocks up to lower greenschist facies in the Upper Unit, whereas the JDFZ forms a typical detachment fault zone with a sequence of progressively sheared tectonites in direct contact with undeformed upper plate. Rocks from the lower and upper units of the DSZ possess consistent geometries and kinematics, but a metamorphic break exists at the TDC in an upward degrading metamorphic zonation. We refer to these characteristics as the result of early subhorizontal shearing followed by detachment faulting during tectonic extension. U-Pb dating of zircons from syn-kinematic granitic dikes from DSZ yields similar ages to those of early shearing along the JDFZ (ca. 133~134 Ma), implying simultaneous initiation of shearing along both shear zone. The exhumation of the Liaonan MCC therefore includes three stages of progressive deformation, from initiation subhorizontal shearing (ca. 134 Ma) followed by slow cooling and exhumation by doming (ca. 134~ 120 Ma) to shearing along the JDFZ and rapid exhumation of the lower plate of the detachment fault zone (120~ 107 Ma).

Data Availability

Data used to support the results of this study can be found in this manuscript text, the supplemental material file, and previous publications discussed in the text, and primary data can be acquired from a permanent repository at OSF (https://osf.io/csbwt/).

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

We thank Weishi Chen at the Chinese Academy of Geological Sciences for fluid inclusion analysis. This research was financially supported by the National Key Research and Development Plan of China (2016YFC0600108-01) and the National Natural Science Foundation of China (41430211, 90814006).

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