**3. The discussion on mechanism of regional tectonic deformation**

Many previous studies divided the deformation between the eastern Sichuan Basin and the Xuefeng Shan into the narrow anticline style fold belt, narrow syncline style fold belt and the basement-involved fold belt, which mainly resulted from the detachment folding and thrusting [10, 26, 27, 41–45]. This study showed that the so-called narrow-anticline style and narrow-syncline style folds in the eastern Sichuan Basin were named just after their geometries at regional scale; the exact original mechanism for these folds is different from that of the Jura type structures.

As mentioned in above sections, the deformation in the study region could be divided into two parts, one is the deformation of the detachment layers and the other is the deformation of rocks in the hanging walls. The bedding parallel detaching and many small-scale A-type, A Discussion on the Detachment Structural Deformation and Its Influence on Pore Structure… http://dx.doi.org/10.5772/intechopen.72245 53

**Figure 6.** Widespread thrust folding in the eastern Sichuan Basin (location: Wanfo Village, Gaoleshan Town, Xianfeng County, Hubei).

northeast-southwest; some AB-type inverted folds whose hinges strike north-east are also found. The axial planes of these folds mainly dip to the southeast with dip angle ranging from 20 to 60°. And some inverted folds with axial plane dipping to the northwest are also developed. In addition, many minor thrust faults top to the northwest are also developed in the layer. These structures imply that the top-to-the-northwest thrusting parallel to the bedding is the main deformation mechanism, and the inverted folds with axial plane dipping to

**Figure 5.** B-type shear folds with SE dipping axial planes in the Silurian detachment layer (location: (a, b) Shaoha Village,

Shaoha Town, Yongshun county, Hunan; (c, d) Heixi Town, Pengshui county, Chongqing).

Many previous studies divided the deformation between the eastern Sichuan Basin and the Xuefeng Shan into the narrow anticline style fold belt, narrow syncline style fold belt and the basement-involved fold belt, which mainly resulted from the detachment folding and thrusting [10, 26, 27, 41–45]. This study showed that the so-called narrow-anticline style and narrow-syncline style folds in the eastern Sichuan Basin were named just after their geometries at regional scale; the exact original mechanism for these folds is different from that of the Jura type structures.

As mentioned in above sections, the deformation in the study region could be divided into two parts, one is the deformation of the detachment layers and the other is the deformation of rocks in the hanging walls. The bedding parallel detaching and many small-scale A-type,

**3. The discussion on mechanism of regional tectonic deformation**

the northwest resulted from the back thrusting.

52 Tectonics - Problems of Regional Settings

**Figure 7.** Multi-layer detaching-thrusting deformation section shown by the seismic reflection. Tt<sup>3</sup> x, upper Triassic Xujiahe Fm; Tt<sup>1</sup> j, lower Triassic Jialingjiang Fm; Tp<sup>2</sup> , upper Permian; Tp1 , lower Permian; Ts2 , middle Silurian; Ts1 , lower Silurian; To1 m, lower Ordovician Meitan Fm; TЄ<sup>2</sup> , middle Cambrian; TЄ<sup>1</sup> , lower Cambrian.

derived from the bedding parallel shearing along the underlying detachment layers led to the deformation of overlying rocks. The above deformation was the results of thrusting and

A Discussion on the Detachment Structural Deformation and Its Influence on Pore Structure…

http://dx.doi.org/10.5772/intechopen.72245

55

The deformation between the Huayin Shan and the Xuefeng Shan is the typical of multilayer detachment. From the Cambrian System to the Jurassic System, the different deformations between four detachment layers and strata between detachment layers are discordant (**Figure 8**). The detachment and thrusting in every detachment layer controlled the deformation of the overlying strata. The main detachment layers controlling the deformation are different in different belts from the east to the west in the study region. Previous studies suggested that the so-called narrow-anticline style fold belt between the Huayin Shan and the Xuefeng Shan was controlled by the Silurian detachment, and the narrow-syncline style fold belt was controlled by the detachment layer between marine covering layers and the metamorphic basement, and these two types of folds are all typical Jura type detachment

**4. Detachment layers in shale of Longmaxi Formation and its** 

To obtain information about the influence of tectonic deformation on shale pore characteristics, the geochemical, mineralogical, structural and textural properties analysis, porosity and pore structure feature investigations are performed using two sets of shale (deformed shale and undeformed shale) collected from the same shale bed of the Longmaxi Formation (Lower Silurian) of southeast of Sichuan Basin, China. Previous studies have shown that the Upper and Middle Yangtze plates have superior marine hydrocarbon geological conditions and immense potential for shale gas. Compared to North America, the geological conditions of gas shale reservoirs in South China are highly diversified and complicated due to the detachment structural deformation, which transformed the structure of shale seams and resulted in structure deformed shale with unique reservoir properties. The detachment layers in the study region show a wide range of deformation styles caused by shearing along the layers: A-type; S-C fabric; sheared puddings; cleavage and thrusts. As the main detachment structure belt [35], the Longmaxi Formations shale layer developed multilayer subdivided slip structural deformation. The primary structure of deformed shale is damaged and the parallel bedding has almost disappeared due to deformation. The plastic deformation of shale is obvious, and the cleavage structure and cleavage surface are smooth with finegrained powder coatings. The detachment fault mirrors (FMs), scratches and micro-fold deformation phenomena were commonly present in Longmaxi shale outcrops. A suite of samples were subset to two sets of deformed shale and undeformed shale for this study due to their variability of texture, fabric and structure properties (**Figures 9** and **10**). To evaluate the influence of detachment deformation on shale reservoir characteristics by comparing the deformed shale and undeformed sample subsets, 14 shale samples (9 undeformed samples from 3 beds) were collected from this Yongshun outcrop (**Figure 9**). However, not all the 14 samples were examined. All samples were analyzed to determine the organic geochemistry and mineralogy, the undeformed samples taken from same bed have similar composition

detachment, called as detachment-thrusting mechanism.

**structural deformation characteristics**

folds [10, 46].

**Figure 8.** Block model of the deformation assemblage of multi-layer detachment and thrust fault between the Huayin Mountain and the Xuefeng Mountain. T-J, Triassic-Jurassic; T<sup>1</sup> -P2 , lower Triassic-upper Permian; P1 -D, lower Permian-Devonian; S, Silurian; Є<sup>2</sup> -O, middle Cambrian-Ordovician; Є<sup>1</sup> , lower Cambrian; Z, Sinian; Pt, Precambrian (Banxi Gp); DS1, detachment layer and its number.

B-type and AB-type inverted folds, recumbent folds and small thrust faults character the deformation of the detachment layers, which generally imply the top-to-the-northwest detachment. And, the symmetrical large open folds and large reclined folds character the deformation of the non-detachment layers, and most geomorphic highs are located at the large reclined folds, such as the Huayin Shan, Tongluo Shan, Mingyue Shan, Fangdou Shan and Qiyao Shan.

Most anticlines between the Fangdou Shan and the Huayin Shan (where the narrow-anticline style folds developed) are narrow with steep northwest limbs dipping at 70–85° and shallow dipping southeast limbs dipping at 40–65°. Between the anticlines are the open synclines. And most northeast trending anticlines between the Fangdou Shan to the west and the Xuefeng Shan to the east (where the narrow-syncline style folds formed) are large open asymmetrical folds with steep northwest limbs and shallow southeast limbs except for some folds with steep southeast limbs such as the asymmetrical folds in Wanzhou along the east bank of the Yangtze River. The field observation showed that thrust faults often developed in the cores and steep limbs of these asymmetrical folds, and the thrust faults in the cores did not cut through the folds (**Figure 6**); similar characteristics could be found in the seismic profiles (**Figure 7**). These large reclined folds resulted from the thrusting; however, the thrust faults derived from the bedding parallel shearing along the underlying detachment layers led to the deformation of overlying rocks. The above deformation was the results of thrusting and detachment, called as detachment-thrusting mechanism.

The deformation between the Huayin Shan and the Xuefeng Shan is the typical of multilayer detachment. From the Cambrian System to the Jurassic System, the different deformations between four detachment layers and strata between detachment layers are discordant (**Figure 8**). The detachment and thrusting in every detachment layer controlled the deformation of the overlying strata. The main detachment layers controlling the deformation are different in different belts from the east to the west in the study region. Previous studies suggested that the so-called narrow-anticline style fold belt between the Huayin Shan and the Xuefeng Shan was controlled by the Silurian detachment, and the narrow-syncline style fold belt was controlled by the detachment layer between marine covering layers and the metamorphic basement, and these two types of folds are all typical Jura type detachment folds [10, 46].
