GNGTS – Atti del 23° Convegno Nazionale / 06.09

P. Boncio (1), X. Li (2) and T. Marcelli (1)

(1) Università G. D’annunzio, Dip. Scienze della Terra - Laboratorio di Geodinamica e Sismogenesi, Chieti scalo, Italy (2) Institute of Chemical Technology, Hubei,

NEW GEOLOGIC DATA IN THE EPICENTRAL AREA OF THE 1695 LINFEN EARTHQUAKE (-WEIHE GRABEN SYSTEM, NORTHERN CHINA)

The Shanxi-Weihe Graben System (SWGS) consists of Tertiary-to-Quaternary extensional grabens extending for more than 1000 km, from to Xian, at the eastern side of the continental Ordos block. Normal faulting or normal faulting in association with strike-slip movements is responsible for basin formation and evolution. Several large earthquakes (M > 6.0) occurred in historical times, documenting the present activity of the faults (Chinese Working Group of the Project 206, 1989). While the active tectonics of the southern (Weihe basin) and northern (Datong-Yangyuan-Yanqing basins) SWGS is covered by the international literature (Zhang et al., 1998; Pavlides et al., 1999; Shaoping and Guizhi 1999 and references therein), there are not detailed works on the central SWGS, in particular on the Linfen basin. Two large historical earthquakes struck the northern portion of the basin: the September 25, 1303 Hongdong earthquake (I = XI, M = 8.0) and the May 18, 1695 Linfen earthquake (I = X, M = 7 3/4). We present the results of a structural field survey carried out during the September-October 2002 campaign along the Luoyunshan fault: a normal fault bordering to the NW the Linfen extensional basin. The Linfen basin is bordered toward NW by a large normal fault (Luoyunshan fault) extending for about 100 km and delimiting at the footwall the Luliang Shan Mountains of the Loess Plateau. Archean basement rocks overlaid by Cambrian-to- Permian carbonate and siliciclastic deposits compose the footwall block. The extensional basin at the hanging wall is filled by Miocene-Pliocene fluvial-lacustrine deposits overlaid by aeolian, colluvial and fluvial accumulations of Quaternary Loess. The continental deposits reach their maximum thickness (1600-2000 m) near the town of Linfen, in the central-northern portion of the basin. By integrating surface and subsurface geology data, it is possible to subdivide the Luoyunshan fault in 3 main fault segments. The northern and central segments (40 and 35 km long) strike on average NE-SW and dip toward SE; the southern segment (~ 24 km long) strikes on average WNW-ESE and dips toward SSW. This segmentation pattern has a strong control on the geometry of the sedimentary basin, as well as on the morphology of the hanging wall and footwall blocks. At a closer view, the NE-trending northern and central segments result from the linkage of lower-rank segments ranging in length from 6 to 10 km. These segments are arranged in a left-stepping en échelon fashion and are separated by breached relay zones. The WNW-trending southern segment is separated from the central segment by a sharp bend. Its evolution is probably related to the reactivation of a pre-existing discontinuity inherited from the pre-Wutai tectonic period. The kinematics along the northern and central segments is mainly normal dip slip, with subordinate right-lateral oblique component; the average slip vector is SE- trending or SSE-trending. Along the southern segment, the kinematics is more complex and both SE-trending and SW-trending slip vectors have been observed on faults affecting both the Quaternary deposits and the pre-Quaternary substratum. GNGTS – Atti del 23° Convegno Nazionale / 06.09

In several places, the fault cut late Quaternary (Late Pleistocene to Holocene) deposits and morphologies. This suggests that the fault can be considered as active. Preliminary reconstructions along geologic cross sections, allowed us to determine average long-term throw rates of 0.2-0.33 mm/yr for the Quaternary (last 2.4 Ma, according to the Chinese literature) and of 0.4-0.48 mm/yr for the late Quaternary (last 125 ka).

REFERENCES

Chinese Working Group of the Project 206; 1989: Atlas of active faults in China. Seismological Press, Xian Cartographic Publishing House, 121 pp. Pavlides S.B., Zouros N.C., Zhongjing F., Shaoping C., Tranos M.D. and Chatzipetros, A.; 1999: Geometry, kinematics and morphotectonics of the Yanqing–Huailai active faults (northern China). Tectonophysics, 308, 99-118. Shaoping C. and Guizhi Y.; 1999: Segmented variations in tectonic geomorphology of Datong- Yangyuan fault zone, NW , China. Journal of Balkan Geophysical Society, 2, 46-62. Zhang Y.Q., Mercier J.L. and Vergély P.; 1998: Extension in the graben systems around the Ordos (China), and its contribution to the extrusion tectonics of south China with respect to Gobi- Mongolia. Tectonophysics, 285, 41-75.