1. Ductile shear zone
and other places in South China such as Wugong Mountain and Lushan Mountain with strong ductile deformation (Shu Liangshu et al., 1998; Faure et al.，1996； Shu et al.，1996； Sun and Shu，2111； Shu et al.，1998； Wang et al.， 2111； Lin et al., 2111), the ductile shear strength in the study area is relatively weak and the scale is small. The field kinematic investigation shows that only two ductile shear zones are developed in Zhuguang rock mass. One zone is developed in Tangdong-Fuxi area inside the rock mass, which strikes NE41°, with a length of more than 41km and a width of 211-411 m; The other one is developed in Nanxiong-Zhoutian area along the SE margin of rock mass, with a strike of NE61°, a length of more than 51km and a width of about 2111m m.. The rocks in the two ductile shear zones have undergone greenschist facies metamorphism and Shi Ying-feldspar ductile deformation.
The investigation shows that the ductile deformation of granitic rocks gradually weakens from the center to both sides of the ductile fault in the Tangdong-Fuxi ductile shear zone, which is steeply inclined towards the SE direction, that is, porphyritic granite → eyeball gneiss granite (71 ~ 121m) → gneiss granite (51 ~ 81m) → muscovite sericitized fine-grained feldspar Shi Ying phyllonite or ultramylonite (61 ~ 91m). The deformation width (181 ~ 311 m) of the SE plate (upper plate) is larger than that of the NW plate (lower plate) (61 ~ 111 m), which reflects that the Tangdong-Fuxi ductile fault zone is the center to provide the heat source for rock metamorphism and deformation. For Nanxiong-Zhoutian ductile shear zone, ductile deformation mainly occurs in the outer edge zone of SE side of rock mass; The inner zones are all magmatic crystalline domains under the physical conditions of stable temperature and pressure, characterized by porphyritic structure, with coarse matrix (Shi Ying particles > 5 mm), and microcline porphyritic crystals up to 21mm×51mm, with a content of 21% ~ 31%. From the rock mass outward, the metamorphic degree gradually changed from amphibolite facies to greenschist facies; The characteristic metamorphic minerals have changed from garnet to mica and chlorite, and the ductile shear deformation has been weakened from the rock mass to the outside: porphyritic granite → banded mixed granite → eyeball mylonitized granite → gneissed mylonitized granite → mylonitized granitic gneiss → fine grained feldspar Shi Ying phyllonite or ultramylonite. In view of the fact that high-temperature minerals and strong ductile deformation often occur near plutonic rocks; Far away from plutonic rocks, metamorphism and deformation gradually weaken, so this paper believes that the heat source that leads to metamorphic deformation of rocks in Nanxiong-Zhoutian ductile shear zone is provided by granitic magma. This feature is similar to the profile phenomenon of Wugongshan granite dome extension structure with strong ductile deformation (Shu Liangshu et al., 1998; Lou Fasheng et al., 2112; Shu et al.，1996； Sun et al.，2111）。 The ductile shear zone in this area is the product of metamorphism on the basis of basement metamorphism, according to the analysis of the low-grade metamorphism degree of shallow metamorphic rocks (Paleozoic is the main) of granite surrounding rocks and slate-weak phyllite of basin basement rocks.
2. Kinematics analysis
Tangdong-Fuxi ductile shear zone is developed in the axial part of rock mass and distributed along Tangdong-Fuxi area, and the granites in the zone have been subjected to strong fine-grained and mylonitized deformation. The shear foliation in granitic mylonite is steep, and the occurrence is 311 ∠ 75 (dip ∠ dip angle); The tensile lineation on the foliation plane is nearly horizontal, and the occurrence is 211 ∠ 5 (dip angle to ∠), which reflects the strike-slip shear property. The development of ductile shear deformation structures on XZ plane (motion plane), such as asymmetric feldspar remnant spot system, Shi Ying remnant spot system, feldspar eyeball structure and S-C composite fabric, indicates that this zone is a left-lateral strike-slip ductile shear zone with very clear marks. At the Sanhe River quarry in Renhua County on the SW side of Fuxi River and at the SW side of Tangdong River, the weakly marbled sandy limestone and phyllite have undergone strong asymmetric folds, with developed foliation and visible tensile lineation. Its kinematic signs (asymmetric rocks and folds, etc.) also indicate that the fault is left-handed strike-slip deformation.
Nanxiong-Zhoutian ductile shear zone is mainly developed in Changyuan (Nanxiong North)-Cang Shi-Dayuan Power Station-Baishiling-Zhoutian area at the junction of basin and mountain, with a strike of NE61°, a length of more than 51km and a width of about 2111m. In-band shear foliation and extension lineation are well developed, and the asymmetric kinematic fabric on XZ plane (motion plane) is obvious. Macro-and micro-kinematic fabrics mainly include: (1)σ-type feldspar or Shi Ying residual spot system; (2) the eyeball structure of feldspar or Shi Ying; (3) mica "fishtail" structure; (4)S-C composite foliation; (5) Oblique fibrous Shi Ying sub-particles, etc. There are two groups of tensile lineations on the foliation surface, one of which is dip-slip lineation, and the occurrence of foliation is: 155 ∠ 45 (dip ∠ dip angle); The occurrence of extensional lineation is: 165 ∠ 38 (dip direction ∠ dip angle), which is stable and widely distributed. The kinematic fabric on XZ plane indicates that the hanging wall rocks slide towards SE. The other is strike-slip lineation. The occurrence of foliation is 1.45 ∠ 45 (dip angle ∠), and the occurrence of extensional lineation is 225 ∠ 4 (dip angle ∠), which has cut or even replaced the dip-slip extensional lineation and developed in the junction zone between the basin and the mountain with a width of about 21m.
in addition, on the NW side of Shixing county in the southern margin of Nanxiong basin (GPS location: N24 57 ′ 32 ″, E114 12 ′ 52 ″), a gray Carboniferous carbonate outcrop with a length of about 111m and a width of about 5 ~ 111 m is seen, which has been metamorphic and deformed into mylonitized marble. It is covered by the unconformity of metamorphic late Cretaceous red beds, and the occurrence of glutenite and siltstone is 321 ∠ 15. Carboniferous strata have been strongly crumpled, and extrusion foliation and tensile lineation have developed. However, the outcrop suffered from the intense superposition of brittle deformation in the later period, which led to the wrinkling and bending of tensile lineation, the widespread development of joints, cleavage and extrusion lens, which destroyed the early ductile shear structure traces and trend signs. No credible kinematic trend of ductile deformation was found in the field and indoor oriented slices.
most parts of zhuguang rock mass belong to brittle deformation domain, with developed joints and fractures. Taking Tangdong-Fuxi strike-slip ductile shear zone as the axial part, the sections of brittle faults on the SE side of the zone are all inclined to the SE direction, and the scratches, steps and associated structures developed on each main section indicate that the rocks slide in the SE direction, that is, toward Nanxiong Basin. In the northern Cang Shi town of Nanxiong in the basin-mountain junction zone, the late Cretaceous brick-red siltstone (S1: 151 ∠ 11) and gneiss granite (S1: 155 ∠ 41) are in normal fault contact (F: 151 ∠ 45), and the contact bandwidth is 21m, and the rocks in the zone are strong. 6km north of Mashi Town, Paleogene purplish red glutenite (S1: 145 ∠ 31) and gneiss granite (S1 ：151°∠52°) are in normal fault contact (F: 151 ∠ 52), with a contact bandwidth of 12m, and the granite rocks in the belt are strongly silicified, fractured and angular. In Nanxiong basin, normal faults are also developed, but the scale is small, and the fault distance is mostly in the meter scale (for example, at N25 13 ′ 19 ″ and E114 13 ′ 23 ″ of Nanxiong SW side, the normal fault distance of K2 siltstone is 3.2m). Brittle kinematic signs (scratches, steps, cleavage, asymmetric folds, etc.) indicate that the hanging wall rocks slide in the direction of S-SW.
to sum up, the kinematic directions of ductile rheological layer and brittle deformation layer related to Zhuguang thermal uplift extension structure are completely the same. The evolution law of ductile and brittle deformation reveals that the granite thermal uplift extension structure is the result of continuous deformation from the middle deep to the shallow, and the granite mountain and the Nanxiong basin in front of the mountain are isomorphic into a semi-graben-like structural system.