Vol.17 No.2 (190~199) ACTA SEISMOLOGICA SINICA Mar., 2004 Article ID: 1000-9116(2004)02-0190-10 Lateral migration of fault activity in Weihe basin∗ FENG Xi-jie (冯希杰) DAI Wang-qiang (戴王强) Earthquake Administration of Shaanxi Province, Xi′an 710068, China Abstract Lateral migration of fault activity in Weihe basin is a popular phenomenon and its characteristics are also typical. Taking the activity migrations of Wangshun Mountain piedmont fault toward Lishan piedmont fault and Weinan platform front fault, Dabaopi-Niujiaojian fault toward Shenyusi-Xiaojiazhai fault, among a serial of NE-trending faults from Baoji city to Jingyang County as examples, their migration time and process are analyzed and discussed in the present paper. It is useful for further understanding the structure development and physiognomy evolution history of Weihe basin. Key words: lateral migration; fault activity; Weihe basin CLC number: P315.2 Document code: A Introduction Like the longitudinal migration (DING, et al, 1993) of fault activity, the lateral migration is a common phenomenon in the fault development and evolvement, especially in the piedmont and the margin of lake basin. As a large inland basin beside the northeast margin of Qinghai-Xizang block (DENG, YOU, 1985; Research Group of Active Fault System around Ordos Massif, 1988; Bureau of Geology and Mineral Resources of Shaanxi Province, 1989; PENG, et al, 1992), the lateral migration of fault activity in Weihe basin is also popular and typical. Since the Oligocene when the Weihe basin began to sink and accept sediment, the activities of some faults along the basin margin had been very strong (XU, et al, 1988; HAN, et al, 1987; LI, RAN, 1986; SUN, et al, 1998; HOU, et al, 1993; Chinese Working Group of the Project 206, 1989), and they were the controlling faults for the basin border throughout the long geological period. But to the end of Late Tertiary period and Early Quaternary period, the border-controlling role of some original faults began to decrease and the activities of some faults inside the basin began to intensify to replace gradually the controlling role of the previous faults. In the individual area, the lateral migration of the faults occurred for many times to result in the unique scene of many topographic layers. Through the study on the lateral migration of fault activity in a region, the tectonic development course and geomorphic evolvement history can be known fully and the latest controlling active faults in the region can be recognized deeply. Meantime, the potential seismic risk of each fault can be estimated properly. Thus we can provide the scientific and reasonable bases for the land planning, land development and land utilization in this region. ∗ Received date: 2002-12-24; revised date: 2003-04-09; accepted date: 2003-05-03. Foundation item: Chinese Joint Seismological Science Foundation (100120) and State Key Basic Research Development and Programming Project of China (G199804070102). No.2 FENG Xi-jie, et al: LATERAL MIGRATION OF FAULT ACTIVITY IN WEIHE BASIN 191 1 Migration of strong activity from Wangshun Mountain piedmont fault to Weinan loess platform front fault 1.1 Activity of piedmont fault of Wangshun Mountain As a part of the normal fault on the north side of Taihua Mountain, the Wangshun Mountain piedmont fault is located to the south of Lantian County, extending northeastwards along the piedmont of Wangshun Mountain (Figure 1). The fault formed in the primary Sinian period and revived in the Tertiary and Quaternary periods, dipping N-NWwards with an angle of about 70°~80°. Wangshun Mountain located on the south side of the fault is a mid-height mountain with a height of 2 239 m, which is composed of the monzogranite formed in the Hercynian and Late Caledonian movement. On the north side of the fault, Lishan uplifted block is covered by the Quaternary loess and beneath the loess, there is the river and lake-facies stratum formed in the Early and Late Tertiary period (JIA, et al, 1966; XIE, et al, 1966). The maximum height of Lishan uplifted block is 1 302.2 m. Wangshun Mountain piedmont fault had been the controlling fault on the south border of Weihe basin during the Tertiary period and had limited the Tertiary sediment in the area to the north of the fault. Therefore, Figure 1 Topography and faults distribution of Lishan upthe Tertiary stratum has not been found lifted block and its surrounding areas on the south side so far. By the end of F1. Wangshun Mountain piedmont fault; F2. Lishan piedthe Tertiary period, the activity mont fault; F3. Weinan loess platform front fault; F4: Linstrength of Wangshun Mountain piedtong-Chang′an fault (including F4-1, F4-2, F4-3) mont fault began to decrease and its previous border-controlling role was replaced by Lishan piedmont fault (Figure 2). Because the activity of Wangshun Mountain piedmont fault was weak, the super-thick sediment of the Quaternary period did not form on its north footwall. For instance, the drilling results obtained between Lantian County and Yushan Town have confirmed that the Holocene deposit at the present valley of Bahe is only 20 m thick and its bedrock is purple-red mud of Late Tertiary system (Figure 3). In addition, the purple-red mud rock of Late Tertiary period can be seen in the riverbed and the first-order terrace of the lower reaches of Qingshui River that is a tributary on the left bank of Bahe. By analyzing the phenomenon that the top level of hydatogenetic deposits (height: 700 m; age: 1.3 Ma) on the fifth-order terrace on the left bank of Bahe at the location of Lantian Man Ruins at Gongwangling (SUN, ZHAO, 1991; LEI, ZHANG, 2001) is only 50 m higher than the modern riverbed of Bahe (height: 650 m) (Figure 4), it can be seen that the Quaternary uplift is not very large. The reason for the weak sinking and small uplifting is perhaps related to the conflict of reverse movements of sinking of Wangshun Mountain piedmont fault and uplifting of Lishan block. 1.2 Early Quaternary activity of Lishan piedmont fault Lishan piedmont fault was a section of controlling fault along the south border of Sanmen 192 ACTA SEISMOLOGICA SINICA Vol.17 Lake in the Early Quaternary in the region. As what is mentioned above, Lishan uplifted block is composed of Tertiary system and is covered by the aeolian loess of Quaternary system, so the Sanmen Lake sediment is not found in this area (CAO, et al, 1966). While on the north side of Lishan piedmont fault, Weinan loess platform is covered by the aeolian loess with the thickness of about 110 m and its main bottom is made of the lake-facies sediment in the Early Pleistocene, which indicates that the ancient Sanmen Lake was limited on the north side by Lishan piedmont fault in the Early Quaternary period. According to the paleomagnetic data, both Well Yan 7 and Wujiabao natural loess section in Weinan loess platform, the Sanmen Lake deposit on the main bottom of loess platform ended in period of Early Pleistocene with the age of about 1.2~1.8 Ma (Figure 4) (SUN, ZHAO, 1991; NIE, et al, 1996; YUE, XUE, 1996; WANG, et al, 1999). Thus it proved that the activity of Lishan piedmont fault was very strong from the beginning of Quaternary Figure 2 Geological profile between Wangshun Mountain and Weinan 1. Monzogranite formed in the Hercynian and Late Caledonian movements; 2. Early Tertiary system; 3. Miocene; 4. Pliocene; 5. Sanmenian series; 6. Gravel layer in the Early Pleistocene; 7. Quaternary loess; 8. Sand layer in Late Quaternary; 9. Drillings and their number; 10. Dip and dipping angle of faults and strata Figure 3 Drilling profile and bore-hole location between Yushan Town and Lantian County ZK stand for bore-hole; N 2 stand for Equus sanmeniensis red clay; Q 4 2 2al + pl stand for Holocene deposits No.2 FENG Xi-jie, et al: LATERAL MIGRATION OF FAULT ACTIVITY IN WEIHE BASIN 193 Figure 4 Some typical Quaternary profiles in the studied area (after SUN, ZHAO, 1991) ME denotes magnetic period; MP denotes magnetic polarity; ESRC denotes Equus sanmeniensis red clay; 1.Holocene loess; 2. Late Pleistocene loess; 3. Middle Pleistocene loess; 4. Early Pleistocene loess; 5. Paleosoil; 6. Clayey soil; 7. Clayey sand; 8. Sand layer; 9. Pebble and boulder; 10. Equus sanmeniensis red clay 194 ACTA SEISMOLOGICA SINICA Vol.17 period to the year before 1.2 Ma BP. At that time, it was the wide ancient Sanmen Lake on the north side of Lishan piedmont fault and the loess hills on its south side. After 1.2 Ma BP, the activity of Lishan piedmont fault began to decrease, the activity of Weinan platform front fault began to increase and the basin border began to migrate from the south to Weinan platform front fault. Then, the lakebed between Lishan piedmont fault and Weinan platform front fault rose up to the lake surface and the aeolian loess (S0~L18 or S22) deposited. Through the period of 1.2~1.8 Ma, the present Weinan loess platform had formed. It should be noted that although the activity of Lishan piedmont fault became weak, the fault have not stopped moving since the Middle and Late Quaternary period. The phenomenon of Late Pleistocene loess dislocated by Lishan piedmont fault can still be discovered in the field (Figure 5). Figure 5 Lishan piedmont fault in Miaogou of Yangguo Town (viewing to the west) Activity of Weinan platform front fault in the Middle and Late Quaternary period Weinan platform front fault is a main border-controlling fault in the southern part of Weihe basin in the Middle and Late Quaternary period. There are Weinan loess platform on its south side and Weihe valley and terraces on its north side. According to the deep-hole data of Well Wei 5 and Well Wei 61, the Quaternary sediment on the fault footwall on the north side of Weinan platform front fault deposited completely with the thickness of about 1 205~1 240 m. And the altitude difference of the top surface of Tertiary system between the south side and the north side of the fault is about 900~1 000 m (Figure 4). In addition, according to the drilling data2 on the second-order terrace of Weihe right bank near Zhangyifu Primary School in Weinan city, the paleosoil layer at the bottom of Late Pleistocene loess was dislocated to 5.02 m by Weinan platform front fault. 1.3 2 Activity lateral migration among the second-order faults of Lintong-Chang′an fault As the northwest border of Lishan uplifted block, Lintong-Chang′an fault consist of several second-order ones with certain distances between one another (LI, et al, 1992; Seismological Bureau of Shaanxi Province, 1996). From the southeast to the northwest, there are second-order faults of Dabaopi-Niujiaojian (F4-1), Shenyusi-Xiaojiazhai (F4-2) and Xiekou-Dongda (F4-3), respectively. It was found by the field investigation that the phenomenon of activity lateral migration of the second-order faults is also very obvious with the migration direction from the southeast to 1 2 Third Survey and Exploration Brigade of State Geological Bureau. 1977. Report of Geological Results in the First Stage of Petroleum Survey and Exploration in Fenhe-Weihe Basin, 6~139. Institute of Engineering Seismic Prospecting of Shaanxi Province. 2001. Report of Seismic Risk Evaluation for the Site of General Building of Guangming Power Company of Weinan City, 8~35. No.2 FENG Xi-jie, et al: LATERAL MIGRATION OF FAULT ACTIVITY IN WEIHE BASIN 195 Figure 6 Quaternary geological section from Liujiapo to Fangzhicheng 1. Pliocene; 2. Quaternary loess; 3. Sanmenian series; 4. Paleosoil; 5. Sand layer; 6. Powdery clay; 7. Faults and number; 8. Dip and dipping angle of faults the northwest (Figure 6). 2.1 Activity of Dabaopi-Niujiaojian fault in the Early Quaternary period Dabaopi-Niujiaojian is a second-order fault in the farthest south of Lintong-Chang′an fault zone, which formed the earliest among the second-order faults. It has the strike of NE-NNE, dip of NW-NWW and dip angle of 60°~80°. In the area between Bahe and Chanhe and under the effect of the activity of Dabaopi-Niujiaojian fault, the southeastern region of the fault is a wide and flat loess platform named Bailuyuan with the height of 740~766 m. The northwestern region of the fault is the NE-trending strip-shaped Pancun-Jiangcun loess platform with the height of 621~629 m, being 80~100 m lower than the southeastern area. According to measured results of Liujiapo and Duanjiapo natural loess section, the upper aeolian loess of Bailuyuan loess platform has a height of about 110 m and its main bottom is made of the purple and red mud rock. However, the lake-facies deposit of the Quaternary period has not been found in this area (Figure 4) (SUN, ZHAO, 1991; YUE, 1989, YUE, XUE, 1996). The outcrop of Qingcaigou natural section shows that in the strip-shaped Pancun-Jiangcun plateform on the north side of the fault, the overlying loess has a height of about 78 m with 9 layers of paleosoil (S0~S9) inside. Its main bottom is the gray-green small sand layer of Sanmen Lake facies (there is a spring). According to the age scales for the loess and paleosoil in Weihe basin, the period with the strongest activity of Dabaopi-Niujiaojian fault was from the Early Quaternary to 0.9 Ma BP, in which Dabaopi-Niujiaojian fault controlled the south border of Sanmen Lake. Although its movement became weak and lost the previous controlling role after 0.9 Ma BP, it is still active nowadays, and the paleosoil beneath the Late Pleistocene loess was dislocated to 1~4 m in the sections of Niujiaojian and Dabaopi. 2.2 Activity of Shenyusi-Xiaojiazhai fault in Middle Quaternary period Shenyusi-Xiaojiazhai fault is another second-order fault in Lintong-Chang′an fault zone. In front of Bailuyuan loess platform between Bahe and Chanhe, the strip-shaped platform of PancunJiangcun is on the southeast side of the fault and the loess platform block including only 5 paleosoil layers (S0~S5) is on the north side of the fault. It proves that the activity of Shenyusi-Xiaojiazhai fault began to increase from the time about 0.9 Ma to the present and the strip-shaped loess zone between Pancun and Jiangcun rose quickly. As a result, the water in the 196 ACTA SEISMOLOGICA SINICA Vol.17 lake in this area retreated and disappeared and the bottom of the lake rose up above the water level. With the aeolian loess deposit, the second-order loess platform formed. However, to the time about 0.45 Ma to the present, the activity of Shenyusi-Xiaojiazhai fault became weak and the strong fault movement migrated to Xiekou-Dongda fault. Thus the hydatogenetic environment on the north side footwall of Shenyusi-Xiaojiazhai fault ended and the aeolian loess deposit started. Like Dabaopi-Niujiaojian fault, Shenyusi-Xiaojiazhai fault is still active and it can be seen in the section of Shenyusi gully that the paleosoil layer underneath the Late Pleistocene loess was dislocated up to 7~8 m by the fault (Figure 7). Figure 7 Shenyusi-Xiaojiazhai fault (viewing to the east) 3 Lateral migration of NW-trending fault activities from Baoji to Jingyang Similar to the case of Lishan uplifted block, the lateral migration of fault activities is common in the western part of Weihe basin, such as the activity migration of several NW-trending faults from Baoji City to Jingyang County is very typical (Figure 8). From the end of Tertiary to the beginning of Quaternary and with Qianhe fault as the border, the western zone began to rise, departed from the Tertiary hydatogenetic environment, accepted the Quaternary aeolian loess deposit and formed the third-order loess platform of Weihe basin at last (YUE, XUE, 1996; CHEN, et al, 2000). According to age scale of loess and paleosoil in Weihe basin (HAN, ZHAO, 2000; TONG, et al, 2000), the strong activity of fault laterally migrated to Qishan-Mazhao fault in the Early Pleistocene period (1.3~1.5 Ma BP). The area to the west of Qishan-Mazhao fault also rose, broke away from the hydatogenetic environment, accepted the aeolian loess deposit of Middle and Figure 8 Geological section from the mouth of Baoji Gorge to the east of Jinghe f1: Jinlinghe fault; f2: Qianhe fault; f3: Qishan-Mazhao fault; f4. Jinghe fault; P2, b21, ch13: Bore hole name No.2 FENG Xi-jie, et al: LATERAL MIGRATION OF FAULT ACTIVITY IN WEIHE BASIN 197 Late Quaternary, and formed the present second-order loess platform. To the late period of Early Pleistocene (0.9 Ma to the present), the controlling fault of the basin border laterally migrated to Jinghe fault on the east. Therefore, the region to the west of Jinghe fault and to the north of Weihe fault formed the first-order loess platform by breaking away from the hydatogenetic environment and accepting the aeolian loess deposit of the Middle and Late Quaternary period (Figure 8~9, Table 1). While the area to the east of Jinhe fault is still sinking with the hydatogenetic deposit. Figure 9 Top surface elevation of main bottom of loess platform from the mouth of Baoji Gorge to the east of Jinghe Table 1 Orders and their characteristics of loess platforms from Baoji city to Jingyang County Hp /m Covering strata Thickness /m 110~150 105~123 80~100 N 34 (S0~S34) 16 (S0~S16) 9 (S0~S9) Time Tertiary Middle and early period of Early Pleistocene Late period of Early Pleistocene Main bottom Lithological characters Mud and sandstone rock of river and lake facies Clay and sand of river and lake facies Clay, gravel and sand of river and lake facies Ht /m 790~665 690~507 530~370 Order of platform Third Second First 920~820 888~632 642~418 Note: Hp denotes elevation of platform surface; Ht denotes elevation of top surface; N denotes number of paleosoil layers 4 Lateral migration pattern of fault activity in Weihe basin If we divide the time period of lateral migration of fault activity in Weihe basin into the geological period a (corresponding to Tertiary period), geological period b (corresponding to the period from the end of Tertiary to Early Quaternary), geological period c (corresponding to Early and Middle Quaternary) and geological period d (corresponding to Middle and Late Quaternary), the lateral migration pattern of fault activity in Weihe basin can be described by Figure 10. In the geological period a, the fault Fa (such as F1 fault in the paper) was the dominant active fault and the controlling border in this region. The top wall of the fault suffered the denudation and the footwall accepted the deposit, then the hydatogenetic sediment series I formed. In the geological period b, the border-controlling role of fault Fa was substituted by fault Fb (such as F2, F4-1 and f2 faults in the paper). With the main fault activity migrated to fault Fb, its top wall was covered by the aeolian deposit series I and its footwall accepted the hydatogenetic deposit series II. In the geological period c, the border-controlling role of fault Fb was replaced by fault Fc (such as F3, F4-2 and f3 faults in the paper) and the main fault activity migrated to fault Fc, so the top wall of fault Fc was covered by the large-scale aeolian sediment series II and the footwall received the hydatogenetic deposit series III. In the geological period d, the border-controlling role migrated to 198 ACTA SEISMOLOGICA SINICA Vol.17 fault Fd (such as F4-3 and f4 faults in the paper), its top wall was covered by the aeolian sediment series III and its footwall accepted the hydatogenetic sediment series IV. Accompanied by the lateral migration of fault activity, the boundary area of Weihe basin rose step by step and separated from the hydatogenetic sediment environment. Figure 10 Sketch map of lateral migration pattern of fault activity in Weihe basin 1. Pre-Tertiary system; 2. Hydatogenetic sediment series I; 3. Aeolian sediment series I; 4. Hydatogenetic sediment series II; 5. Aeolian sediment series II; 6. Hydatogenetic sediment series III; 7. Aeolian sediment series III; 8. Hydatogenetic sediment series IV; 9. Faults 5 Conclusions Like other kinds of natural phenomena, the fault activity also has the variation of growth and death. The weak and strong alternating changes of fault activity not only occurred along their strike directions, but also the dip direction. From the end of Tertiary, the activity strength of many faults located along the boundary area of Weihe basin changed. With the gradual migration of fault activity to the center of Weihe basin, the range of ancient lake basin decreased greatly. The original area accepting the hydatogenetic deposit began to receive the aeolian loess sediment, and the loess platform and the river terrace covered by loess were formed. As a result of lateral migration of fault activity, Weihe basin formed many ground level landforms and their altitudes decrease step by step from the boundary to the center. In addition, these latest border-controlling faults can control the inner and outer geological functions, such as the moderate and strong earthquakes occurred in history and nowadays, as well as the landslides and collapses, which are distributed along the faults like a strip. Of course, although the previous strong active faults lost their border-controlling role in the late time, their activity did not stop. Meanwhile, the fault′s active sections and outcrops in the Late Quaternary can still be discovered in the field, however, the activity is weak. References Bureau of Geology and Mineral Resources of Shaanxi Province. 1989. Regional Geology of Shaanxi Province [M]. Beijing: Geological Publishing House, 1~398 (in Chinese). No.2 FENG Xi-jie, et al: LATERAL MIGRATION OF FAULT ACTIVITY IN WEIHE BASIN 199 CAO Bo-xun, LIU Shi-rong, ZHAO Bu-yi, et al. 1966. Preliminary study on Cainozoic group of Youhe region of Weinan, Shaanxi province [A]. 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