Geologica Acta: an international earth science journal ISSN: 1695-6133 [email protected] Universitat de Barcelona España ZHAO, G.; WANG, L.; CHEN, X.; TANG, J.; WAN, Z.; ZHAN, Y.; XIAO, Q.; CAI, J.; ZHANG, J.; WANG, J. The active fault belts in eastern Tibet margin inferred using magnetotellurics Geologica Acta: an international earth science journal, vol. 8, núm. 1, marzo, 2010, pp. 99-110 Universitat de Barcelona Barcelona, España Available in: http://www.redalyc.org/articulo.oa?id=50513113008 How to cite Complete issue Scientific Information System More information about this article Network of Scientific Journals from Latin America, the Caribbean, Spain and Portugal Journal's homepage in redalyc.org Non-profit academic project, developed under the open access initiative Geologica Acta, Vol.8, Nº 1, March 2010, 99-110 DOI: 10.1344/105.000001518 Available online at www.geologica-acta.com The active fault belts in eastern Tibet margin inferred using magnetotellurics 1 * 1 1 1 1 1 1 1 2 1 G. ZHAO L. WANG X. CHEN J. TANG Z. WAN Y. ZHAN Q. XIAO J. CAI J. ZHANG and J. WANG 1 Institute of Geology, China Earthquake Administration Beijing 100029, China 2 Earthquake Administration of Shandong Province Jinan 250014, China * corresponding author E-mail: [email protected] ABSTRACT A magnetotelluric (MT) sounding has been carried out in the eastern margin of the Tibetan plateau. The survey line is about 145 km long, trending in NEE direction and crossing the Daliangshan block in the eastern edge of the Tibetan plateau. The field measurements acquired effective data of 68 sites. Through data processing and a 2-D inversion with consideration of topography, a 2-D electrical structure model of crust and upper mantle was constructed. The structure reveals that there is a deep electrical boundary between the Daliangshan block in the west and Sichuan block in the east. West to the boundary, the crust has a relatively low resistivity with respect to the east and can be divided into three layers, the middle layer has low-resistivity with a minimum of 3-10 W·m, presumably associated with partial melt and/or salty fluids. Beneath the intersection area of the Anninghe fault, the Xianshuihe fault and the Longmenshan fault, which the MT profile crosses, the faults are separated into upper and lower sections. The upper section exhibits a nearly vertical low-resistivity zone in the upper crust, and the lower section manifests an electrical boundary in the lower crust and upper mantle. Other faults in the Daliangshan block are either nearly vertical low-resistivity zones or electrical boundaries. It is suggested that the formation of the low-resistivity layer in the middle crust is associated with the southeastward motion of the eastern margin of the Tibetan plateau, clockwise rotation of the Chuandian (Sichuan-Yunnan) block, and the westward obstruction from the Sichuan block in Huanan terrain. Seismicity, including the M 8.0 Wenchuan earthquake in the study area, is discussed. KEYWORDS Eastern margin of Tibetan plateau. Active fault. Electrical structure. Channel flow. Wenchuan Earthquake. 99 G. ZHAO et al. Active fault belts in eastern Tibet by MT INTRODUCTION In the eastern margin of the Tibetan plateau, there is a rhombic terrain, the Chuan-Dian (Sichuan-Yunnan) The eastern margin of the Tibetan plateau, block (CDb) encompassed by active faults with a NNW geographically covering the Sichuan (Chuan) and Yunnan trending long axis. East to the block, from north to (Dian) provinces in China, is one of the areas with the most south, are the Xianshuihe fault (XSHf), the Anninghe intensive crustal deformation and earthquakes in the world fault (ANHf), and the Zemuhe fault (ZMHf), adjoining (Fig. 1). Numerous studies have been done on active faults the Songpan-Ganzi block (SGb) and the Daliangshan and their relationships with seismicity of this area (Clark block (DLSb) in Huanan (South China) terrain (HNb). et al., 2000, Deng et al., 1994; Li et al., 2003; Lu et al., In the west, lie the Jinshajiang faults (JSJf) and the 1989; Peng and Zhou, 1991; Roger et al., 1995; Royden et Honghe faults (HHf) neighboring upon the Qaingtang al., 1997; Sun et al., 2003; Teng, 1994; Wang et al., 2007; block (QTb) and the Diannan (south Yunnan) block Xu et al., 1992; Zhang et al., 2003; Zhang et al., 2004; Zhu (DNb) (Fig.1) (Bureau of Geology, 1991; Deng et al., A et al., 2005; Zhu J et al., 2005; Zhu et al., 2004; Zhao et 1994; Ma, 1989; Wen et al., 2003; Xu et al., 2003, 2005; al., 2008, 2009). Zhang et al., 2003). 74 78 82 86 90 94 98 102 106 110 98 100 102 104 106 32 40 XSHf Eq TRb SGb QTb Chengdu Hetian QDb EDb EBf LMSf SCb 30 36 Golmud SGb Lanzhou LS Tibet Block QTb SM Eq JSJf CDb 32 DLSb ANHf LTHb Chengdu HYSf 28 ZMHf Lhasa CDb SCb 28 Himalayan XJHf New Delhi HNb DNb 26 24 India Plate HHf Dhaka DNb XJf 20 A B 24 32 � MT D C Eq faults 0 10 20km SCb MBf EBf DXLf LS 30 DLSb FYf 29.5 �� XSHf � � � � 77 ���� 74 �� 62 7273 75 �� � Xf � 55�� � ��� ��� WYf � 58 �� �� �� � 50 � 28 �� � CDb �� 21 ��� �� 45 HYf �66 � ��� ������ 41 ������������ ��� 32 39 1 7 SM LHf DLSf ANHf 26 XJHf 29.0 10mm/yr 24 102.5 103 103.5 98 100 102 104 106 FIGURE 1 Tectonic setting around the Shimian-Leshan MT line (SLp). A) Regional tectonics of the Tibet plateau (including Himalayan, LTHb, QTb, SGb, QDb and CDb) and its surrounding. B) Tectonics around CDb and DLSb (after Xu et al., 2005). Thick line shows Shimian-Leshan MT line (SLp). C) Location of MT line and the relation with some faults and blocks. D) GPS velocity field with respect to the HNb (after Wang et al., 2008). Cities (space circles): LS-Leshan, SM-Shimian and other full names (e.g., Chengdu city). Block names: TRb-Tarim block (hereafter “block” is omitted), QDb-Qidam, SGb- Songpan-Ganzi, QTb-Qiangtang, LTHb-Lhasa, DNb-Diannan, EDb-Erdos, SCb-Sichuan, HNb-Huanan (or South China), CDb-Chuandian, DLSb-Daliangshan (in between CDb and SCb). Fault names: XSHb-Xianshuihe faults (hereafter “faults” is omitted), ANHf-Anninghe, ZMHf-Zemuhe, XJf-Xiaojiang, JSJf-Jin- shajiang, XJHf-Xiaojinhe, HHf-Honghe, LMSf-Longmenshan, EBf-Ebian, HYSf-Huayingshan. DLSf-Daliangshan, WYf-Wanyuan, HYf-Hanyuan, FYf-Fengyi, DXLf-Daxiangling, LHf-Liujiang-Hongxi, MBf-Mabian, Xf-found in this paper. Exploded solid mark: Wenchuan earthquakes epicenter on May 12, 2008. Geologica Acta, 8(1), 99-110 (2010) 100 DOI: 10.1344/105.000001518 G. ZHAO et al. Active fault belts in eastern Tibet by MT The nearly NS striking Daliangshan block (DLSb) and the ZMHf. There were fewer stronger earthquakes that confined by active faults, about 100 km wide in the east- occurred during the last centuries in the LMSf (Wen et west direction, is situated east of the CDb as a part of the al., 2003; Chen et al., 2007; Xu et al., 2008; Zhang et al., HNb. West of the DLSb are the ANHf and the ZMHf, 2008). Investigating the crustal structure beneath the LMSf bordering on the CDb; and, in the east lie the Liujiang- and the neighboring regions will help in understanding the Hongxi faults (LHf) and the Ebian faults (EBf) next to the generation of the 2008 Wenchuan Ms 8.0 event. Sichuan block (SCb) in the HNb (Fig.1). The crustal structures on the east and west side of the Geological studies show that the western boundary faults Daliangshan block (DLSb) exhibit distinct features. The of the CDb are dominated by dextral strike-slip motion, DLSb is of a transitional zone in the eastern margin of the while the eastern boundary faults (XSHf, ANHf and ZMHf) Tibetan plateau. The crust is approximately 63 km thick exhibited tensile motion during early Pleistocene time and in the west and 45 km thick in the east. Both the average sinistral strike-slip motion by late Pleistocene time (Wang velocity (6.25 km/s) of crust and the velocity (7.75 km/s) of et al., 1996). South and north of the intersection area of the uppermost upper mantle in the west are evidently lower the XSHf and ANHf, active faults move quite differently. than those in the east (6.45-6.50 km/s and 8.00-8.20 km/s, In the north, the XSHf is of sinistral strike-slip with a rate respectively) (Wang et al., 2007). In the west, there is an of 12mm/yr, and in the south, the ANHf and the ZMHf 8-10 km thick low-velocity layer at depth 15 km, while have both sinistral strike-slip with a rate of 6~8 mm/yr and it does not exist in the east. A nearly NS-oriented intense eastward thrust motion (Shen et al., 2005; Xu et al., 2005; gradient zone of Bouguer gravity anomalies passes through Zhang et al., 2003). The study also indicates that the CDb the DLSb being from -200·10-5 m/s2 in the east of DLSb to experiences a southeastward motion and clockwise rotation -400·10-5m/s2 in the west for only about 100 km (Lou and simultaneously, and the total amount of the rotation since Wang, 2005). the Oligocene or the early Miocene is as much as 30-48º (Xu et al., 2003). Although a great number of studies have been done on active faults and seismicity in this area, as briefly presented Seismicity differs north and south of the intersection above, the deep structures of these faults remain unclear area.