Characteristics of late quaternary activity along the Southern Border Fault Zone of Weihe Graben Basin

Quaternary International, Vol. 25, pp. 25-31, 1995. Copyright © 1994 INQUMElsevier Science Ltd Printed in Great Britain. All rights reserved. 1040-6182/95 $29.00

Pergamon 1040-6182(94)E00032-8

CHARACTERISTICS OF LATE QUATERNARY ACTIVITY ALONG THE SOUTHERN BORDER FAULT ZONE OF WEIHE GRABEN BASIN Zhang Anliang,* Yang Zhongtang,t Zhong Jin:~ and Mi Fengshou~

*Seismological Bureau of Shaanxi Province, Xi'an, 710068, P.R. China t Xi'an Institute of Geology and Mineral Resources, Chinese Academy of Geological Science, Xi'an, 710054, P.R. China ~.Seismological Bureau of Shaanxi Province, Xi'an, 710068, P.R. China

The Weihe graben basin located in the centre of Shaanxi, China, is a tectonic depression on the southern border of Ordos massif, a subtectonic unit in the Sino-Korean paraplafform. In the light of new information exhibited from geological mapping on the scale of 1:50,000, the active history and the feature of the Southern Border Fault Zone of Weihe graben basin have been researched. The fault zone may be subdivided into two segments: the piedmont fault of Huashan Mt. in the east sector and the northern border fault of Qinling Mts in the west sector. The research shows that new activity of the piedmont fault of Huashan Mt. is obvious since the Late Quaternary. A great number of relics of earthquake deformation, such as the newest fault scarps, bedrock fractures, loess crevices, landslides and mountain creep, are distributed along this fault and its two flanks. This fault is most likely the causative fault zone of the Huaxian great earthquake in 1556 (M = 8). The analysis of two palaeoseismic profiles shows the earthquake recurrence interval is 20130-2500 years for the Huashan Mt. piedmont fault, whereas 2000--4000 years for the northern border fault of Qinling Mts.

INTRODUCTION

HISTORY OF T H E FAULT M O V E M E N T

The Weihe graben basin is located in the centre of Shaanxi, China. It is a tectonic depression situated in the southern border of the Ordos massif in the Sino-Korean paraplatform (Fig. 1). The development of the Weihe graben basin was strictly controlled by its southern and northern faults. The northern fault is called the southern border fault of Beishan Mt. The southern fault is named the Huashan-Qinling piedmont fault, and is termed the Southern Border Fault Zone of Weihe graben basin in this paper.

S.B.F.Z. extended eastward for over 310 km from Baoji to Tongguan and is the largest and most intensely active fault in this area. It has a decisive role in forming and developing the Weihe basin. In general, the east segment of this zone is the piedmont fault of Huashan Mt., and the west segment is named the northern border fault of the Qinling Mts. The south wall of S.B.F.Z. is the Qinling-Huashan hilly country, composed of Proterozoic with minor Paleozoic strata, as well as the numerous Mesozoic granites. Although

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FIG. 1. Sketch showing regional neotectonism of Weihe graben basin. (1) Active fault and buried fault; (2) isopoch line of Quaternary stratum (m); (3) area of upwardly-displaced bedrock; (4) hot spring and its temperature; (5) boundary of the basin; (6) shock of M > 8; (7) M > 7; (8) M > 6; (9) M > 5.: (1) Sino-Korean paraplatform; (2) Qinling-Qilian fold system.

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FIG. 2. Distribution of the Southern Border Fault Zone of Weihe graben basin and its segments. (1) Alluvial fan terrace; (2) loess table land; (3) basement hilly country; (4) segmental faults and their coding; (5) segmental boundary; (6) exploratory trench.

Taibai Mt. (about 3767 m above sea level) is the highest mountain in the Qinling Mts, the palaeogene period planation surface developed at the top of it. The north wall of S.B.F.Z. is Weihe graben basin. It is about 400 m above sea level and is infilled by a 6000 m thick accumulation of Neogene strata. Studying of landforms and strata indicates that the S.B.F.Z. has undergone tectonic movement many times since it formed in Cenozoic time. The development of S.B.F.Z. could be subdivided as follows. The germination of S.B.F.Z. could be traced to the period before the Early Tertiary. In Late Mesozoic time, the Weihe basin was still connected with the Qinling landmass, as a whole upwarping denudation area. At the beginning of Cenozoic time, the regional tectonic stress field began to change from a compressive SE-NW pattern into an extensional type due to the intense tectonic movements of the Qingzang Plateau plate and the Pacific plate (Wang Yipeng, 1979). The pioneer compression zone traced former shear fractures in sublatitudinal, northwest and northeast directions and finally formed the original fault, i.e. the S.B.F.Z. An intensely uneven vertical movement of the fault zone occurred in the period from the Tertiary to the beginning of the Early Pleistocene epoch, in which there existed three phases. (1) At the beginning of the Early Tertiary, faultblock mountains and fault depressions were formed by means of uneven vertical movements. For example, the Paomaliang first-grade planation surface, 3300 m above sea level in Taibai Mt., represents the oldest denudation surface. (2) In the Oligocene-Early Miocene, a movement led to disintegration of the Palaeogene planation surface and formed a second-grade planation surface, 2600-3000 m above sea level. (3) In the Mid-Late Pliocene, the mountain moved up again and formed a third planation surface, 1500-1800 m above sea level. By means of drilling and seismic prospecting in the fault depression at the side of the foot wall, the Cenozoic fluviolacustrine deposits have been estimated at 6000 m in thickness. This shows that the fault was an intensely active normal fault in the Cenozoic era. In the Early Pleistocene, the S.B.F.Z. inherited the mode of the Tertiary activity. The fourth-grade denudation-cumulation surface, 1000-1300 m above sea

level, was formed in the early-middle stage of the Early Pleistocene. As intermittent faulting continued in the late stage of this period, the initial antecedent drainage and meanders were developed on the fourth-grade denudation-cumulation surface. In the Middle Pleistocene, the regional uplifting of mountains was becoming predominant, whereas faulting was somewhat less. The fourth- and fifth-grade river terraces are more than 170 m above the river-beds, and valleys are 'V'-shaped and 200-300 m deep. In the Late Pleistocene-Holocene, the fault developed dip-slip movement. One to three terraces formed, along with alluvial fans and 30 m high cliffs along the boundary of the mountains and basin. ACTIVE FEATURES OF SEGMENTAL FAULTS IN S.B.F.Z. It is common knowledge that the fault zone can be subdivided into several segments. The geometric and kinematic features of any segment are usually inconsistent with those of the others. The S.B.F.Z. has variety along its strike in mobile intensity and textual distinction, and has distinctively segmental features. It can be divided into two major segments: the Huashan piedmont fault in the east sector and the Qinling northern border fault in the west sector. Each of these could further subdivide into many sub-segments. Their main characters are listed in Table 1 and Fig. 2. SURFACE RUPTURE CHARACTERISTICS ARISING FROM THE GREAT EARTHQUAKE AND RELICS OF THE PALAEOEARTHQUAKES

Relics of earthquake deformation in the Huashan Piedmont Fault A great earthquake (M = 8) happened at Huaxian county, Shaanxi province on 23 January 1556. The mechanism of this earthquake has been researched by many seismologists. Recently, the authors found many deformation relics along the piedmont belt of Huashan Mountain, in which the following major types were included (Fig. 3): (1) Fault scarplets and clay fissures. The fault scarplets

Late Quaternary Activity of Weihe Graben Basin

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FIG. 3. Distribution of the deformationrelics of the Huaxian great earthquake of 1556. (l) Seismic fault, ground fissure; (2) collapseand slumping; (3) sand (or clay) vein.

are well developed in the area from Duyu valley to Shiti valley, and are generally 3-8 m high and tens to hundreds of metres long. They were located several to tens of metres away from the basement fault surface, and they cut through the new alluvial and talus fans. The original landforms of scarplets often suffered from later deformation. The scarplet directly connected with the fault in Huaxian town cut across the Yangshao cultural layer containing numerous pieces of painted pottery and tiles, dated about 4000 years ago. It is clear that the occurrence of the scarplet is much later than 4000 BP. (2) Collapses and fissures of bedrock. These are mainly distributed around the Shiti valley. This area is about 1.53 km away from the piedmont fault zone. The collapse body is 900 m in length, with rolled stones filling the valley along

the sides of the mountain. Fissures of bedrock are often seen in the collapse area. They are 0.2-2.3 m wide, 40-100 m deep (visible depth) and up to 200 m long. In the light of the field observations of the collapse of bodies clogging the valley, and the lack of cover by proluvim, residuum and talus material, we infer that the formation time of the collapse was relatively later. The age of the collapse body had been examined by means of lichenometry. It is about 400 years old (Xie Xinsheng e t a l . , 1991). This result is basically consistent with the great Huaxian earthquake (M = 8) which happened 436 years ago. Thus, the collapse may have resulted from this earthquake. (3) Ground fissures developed in the east of Huaxian county. The gully wall east of Shaohua Middle School filled with ash sand, mild clay and charcoal slacks. 14C dating

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FIG. 4. Distributionof palaeoearthquakerelics in Taiyi palace area, Changancounty. (1) Fault; (2) fissure; (3) collapse; (4) remnantpeak and cliff; (5) dammedlake; (6) waterfall; (7) landslides; (8) fault scarp.

Late QuaternaryActivityof Weihe GrabenBasin indicate an age of 1400 ___ 125 BP. According to historical records, although several seisms happened in this area, such as the Weihua quake (M = 6) in 793 and the Chaoyi quake (M = 7) in 1501, this surface rupture had not been formed. As only a single great earthquake had taken place in this area within 1500 years of the present, these ground fissures might be relics from the great Huaxian earthquake of 1556. Neotectonic activity of the Huashan piedmont fault is obvious (Zhang Anliang et al., 1989). This, with the fact that the various relics and fissures of bedrock are typical seismic deformations in the meizoseismal area of a strong quake developed in the Huaxian area, and the fact that they temporally conformed to the great Huaxian earthquake of 1556, indicate that the Huaxian piedmont fault could be considered as the causative fault of the great Huaxian earthquake (M = 8).

29

nearly 1000 m long dammed lake with a stone dam 200 m high by 300 m wide, totalling more than 3700 m 3 in volume. Some larger stones are 60 × 30 × 30 m in dimension. The Ganqiu pool lies on the middle-upper reach of Taiyi stream, and is 1800 m long, 260-900 m wide and about 400 m in height, with a volume of about 200,000,000 m 3. Collapse fractures of bedrock can be commonly seen in the area where the collapses mentioned above occur. They extend from the hill top to hillside or to the bottom of gullies, with widths of 0.5-2.5 m, and lengths of tens to hundreds of metres. Some form remnant peaks and cliffs. These morphostructures mentioned above are commonly developed in fault areas. The fault is the active fault that controlled the topography and landforms of the area, so the landslide collapse and fissure probably resulted from activity of this fault. Consideration of the scale and features of the collapse, which coincided with the rupture deformation of many macroseisms in the World, suggests that the collapse is very likely a trace of an ancient earthquake. In addition, according to historical records, Emperor Wudi of the Weatern Han Dynasty built a palace near the Shuiqiu pool in Yuanfeng Second year (109 B.C.) in order to offer a sacrifice to Taiyi God (a man in a Chinese mythological tale). Hence, the development of Shuiqiu pool has lasted at least 2000 years. In addition, dating the Shuiqiu pool and the Gangqiu pool by lichenometry shows that the age is about 3000 years (Xie Xingsheng et al., 1991). For this reason, the collapse body distributed in Taiyi valley would be produced by an earthquake, which would be the great Guanzhong earthquake of 1189 B.C. (2) New faults and fault scarps. Many faults cutting Pliocene series and Holocene strata have been tound around the hanging wall of piedmont faults. The faults are small in

Relics of seismic deformation in the northern border fault of the Qinling Mts The mobility of the northem border fault of Qinling Mts is outstanding in the Quaternary especially since human activity began, but no strong earthquake has been reliably recorded. Recently the authors found some relics of earthquake deformation (Fig. 4). (1) Collapse of bedrock. This is mainly distributed in the Kuyu-Shiti valley area, 20 km long and 7 km wide. The area is 1-3 km away from the piedmont of Qinling Mts. Collapse and rolled stones from the mountain masses mainly filled in the sides of the gullies in Kuyu-Shibian valley area and formed many pools, such as Shuiqiu pool and Ganqiu pool, which are famous for their landscape since the Han and Tang Dynasties in Chinese history. The Shuiqiu pool is located at the lower reach of Taiyi stream, and is a

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FIG. 6. A palaeoseismologicalsectionat Taipinggullyon the northern piedmontof QinlingMts. (1) Clayeysoil;(2) sabuloussoil; (3) greyyellow soil; (4) light red soil; (5) middle-finesand; (6) sand and gravel; (7) colluvialdeposit; (8) mycelium;(9) fauit tectonite; (10) loesslike tectonite; (11) numberof strata; (12) numberof colluvial wedges; (13) ~4Csamplesite and its age; (14) cultural relics. scale. Most of them are normal faults spreading in step and en echelon. Some of them appear as the horst-graben type, with displacement ranging from 1 to 3 m. They are very similar to fractural features developed in the meizoseismal region of the Huaxian earthquake. In addition, the new fault scarps are well developed at many sections. The young proluvial lands and fans have been cut by these fault scarps, and they are in general 5-7 m in height. Formation of these fault scarps might have arisen from the stick-slip process of S.B.F.Z. Therefore, they would be the relics of a palaeoearthquake (3) Moniliform landslide. This developed only in the small basin which lies at the northern end of the Taiyi bedrock collapse area. As there were no records in historical documents, it was inferred that the age of its development would be ancient. R E C U R R E N C E I N T E R V A L OF G R E A T

EARTHQUAKES Ancient Earthquake Profile of the Huashan Piedmont Active Fault The Shama gully palaeoearthquake section is located at the western side of a pluvial fan at Taoxia, Huayin county (Figs 2 and 3). As the Huashan piedmont fault extends through this area, the fault scarp with a height of 5-8 m and about several kilometres long was formed. The profile can be divided into two by the main fault (F). The foot wall is composed of Early Quaternary sands and gravels, and the hanging wall of the main fault is formed by the Holocene formation with seven layers totalling 4.5 m in thickness (Fig. 5).

The research on the sectional structure shows that there are three groove-like accumulational wedges on the hanging wall. According to the relationship between the main fault (F) and sub-reverse faults (Ft-F4), it is deduced that four earthquakes occurred. Dating of earthquake events by the 14C method indicates that the first event dates to 4910 _+ 180 BP, the second event to 4000-4500 BP and the third event 2500-3000 BP. The fourth event was possibly the great Huaxian earthquake of 1556 A.D. It is very clear that the interval of great earthquakes for the Huashan piedmont fault is about 2000-2800 years within the Holocene.

Ancient Earthquake Profile of the Northern Border Fault of Qinling Mts The Taiping gully ancient earthquake profile, sited at the scarp in front of the diluvial tableland, on the east bank of Taiping stream in Huaxian county (trench site is shown in Fig. 2), is 9 m in length and 7 m in height. The northern border fault of Qinling Mts cross through this area. There are five sub-parallel faults dipping northwards (i.e. F1-F3) and a fault dipping southwards (F6) in the profile (Fig. 6). The section can be divided into two by Fs. The Holocene formation on the hanging wall of F 5 is subdivided into 12 lithological units, and that on the foot wall of F5 has been divided into five units, Analysis of the profile survey shows that there are four sedimentary wedges on the hanging wall: the earthquake colluvial wedges A and B, the earthquake grooves C and the infilling wedge D (Fig. 6). Therefore, it is inferred that four earthquakes had taken place in this area. The ages of the seismic events by 14C determaction are as follows: the first event occurred prior to 13,065 _+ 183 BP, the second event

31

Late Quaternary Activity of Weihe Graben Basin

occurred between 13,065 ± 183 and 11,747 + 167 BP, the third event occurred between 11,747 + 167 and 8763 +_ 141 BP and the fourth event occurred between 8763 ± 141 and 3000 BP. Therefore, the interval of great earthquakes for the northern border fault of the Qinling Mts is about 20(0)°4000 years (Zhang Anliang et al., 1990).

CONCLUSION (1) The Southern Border Fault Zone of the Weihe graben basin has undergone an intensive activity during the Quaternary. It is a causative fault of the middle-great earthquakes in this area and the seisms have been strictly controlled by this Fault Zone. (2) The recurrence interval of great earthquakes would be about 2000-2500 years at the eastern segment of the S.B.F.Z., while it is about 2000--4000 years at its western segment.

ACKNOWLEDGEMENTS We are grateful to Mr Wang Yipeng and Mr Dang Qidong for their help in fieldwork and given directions for study.

REFERENCES Li Yongshan (1982). Investigation of the Huaxian great earthquake damage and on the palaeoearthquake traces. Papers of Prehistoric Earthquake and Quaternary Geology, pp. 30--41. Science and Technique Press of Shaanxi (in Chinese with English abstract). Wang Jinming (1980). Ground ruptures during the large earthquake of 1556, Huaxian county, Shaanxi, Acta Seismologica Sinica, 2(4), 430--437 (in Chinese with English abstract). Wang Yipeng (1979). Intraplate earthquake and Meso-Cenozoic stress field in China. Seismology and Geology, 1(3), 2-10 (in Chinese). Xie Xingsheng et al. (1991). The Lichnometry in North China, pp. 81-87. Chinese Seismic Press (in Chinese). Zhang Anliang et al. (1989). Deformation relics of the 1556 Huaxian great earthquake and the study of palaeoseismicity on the frontal fault zone of Huashan Mountain. Seismology and Geology, 11 (3), 73-81 (in Chinese with English abstract). Zhang Anliang et al. (1990). A palaeoseismological profile across piedmont fault zone at Talpingkou on northern segment of Qinling Mountains. Sesimology and Geology, 12(4), 333-334 (in Chinese).