INTRODUCTION Among other geologic phenomena related to tectonically active continental regions, mass movements may affect large areas and cause great damage. Large landslides are commonly observed in mountainous regions and/or under wet climatic conditions. Outstanding examples include the Tsergo Ri landslide (100 km 3 ) in the Langthang valley in north-central Nepal, which occurred about 40 ka ago (Ibetsberger, 1996), the Said- marreh landslide (20–30 km 3 ) in southwestern Iran, which occurred in prehistoric times (Harrison and Falcon, 1938), and the Flims landslide (12 km 3 ) in Switzerland, which took place during an interglacial stage (Heim, 1920). Mass move- ments also are commonly triggered by ground shaking from earthquakes (e.g., Keefer, 1984; Jibson, 1996). Among the modern earthquake- triggered spectacular landslides are the rock avalanche (~0.1 km 3 ) that occurred in 1970 after the Mw 7.9 earthquake in the Cordillera Blanca in Peru, (Plafker et al., 1971) and the Bairaman land- slide (0.2 km 3 ) in New Britain, Papua New Guinea, that was triggered by an Mw 7.1 earth- quake on May 11, 1985 (King et al., 1989). Although landslides are commonly triggered by ground shaking from strong earthquakes, it is rare to observe a direct relationship between landslid- ing and coseismic rupture, as has been proposed, for example, for the Beni Rached landslide (15 km wide) that was associated with the El Asnam fault rupture of the October 10, 1980, M 7.3 earthquake (Philip and Meghraoui, 1983). On the basis of satellite images, aerial photo- graphs, and field observations, we present here the first results of an analysis of one of the largest continental landslides (estimated volume: ~50 km 3 ) whose geometry and mechanics appear directly associated with surface faulting during a strong earthquake. Until now, the structure cor- responding to the landslide has not been under- stood as a gravitational phenomenon. TECTONIC SETTING This gigantic paleolandslide occurred in the Gobi-Altay mountains in Mongolia, along the eastern segment of the Bogd fault, a strike-slip fault that ruptured most recently on December 4, 1957, during one of the strongest recorded intra- continental earthquakes (M 8.3) (Florensov and Solonenko, 1963) (Fig. 1). The surface rupture follows the northern edge of the two massifs of Ih Bogd and Baga Bogd, which stand 2500 m above the Valley of Lakes. Surface ruptures were also observed along a set of active thrust-related ridges, the Dalan Türüü and the Gurvan Bulag ridges, situated north and south, respectively, of the Ih Bogd massif, and the Hetsüü ridge situated north of the Baga Bogd massif. Florensov and Solonenko (1963) provided an outstanding description of ground-surface effects related to the seismic event. Kurushin et al. (1997) furnished an updated quantification of coseismic surface breaks, and preliminary results from a long-term slip rate analysis (Ritz et al., 1995) and from a paleoseismic expedition (Schwartz et al., 1996) showed that the Bogd fault is probably a low-slip-rate fault (~1 mm/yr) capable of generat- ing major earthquakes (M ≥ 8) separated by re- currence intervals of several thousands of years. One of the most impressive structures described by Florensov and Solonenko (1963) is the Bitut landslide (2.5 × 4 km 2 ), located in the central part of the Ih Bogd massif (Fig. 1). No equivalent grav- itational feature related to the 1957 event is known from the Baga Bogd massif. A new interpretation of remote images related to a detailed-scale mor- phologic study of this massif allowed us to show that an even larger landslide occurred in the past. MORPHOLOGICAL AND STRUCTURAL ANALYSIS OF THE BAGA BOGD LANDSLIDE A satellite image (Fig. 2A) allows recognition of the giant landslide as a morphologically distinc- tive structure; the landslide includes a zone of relief extending over an area of 20 × 15 km 2 and standing 150–200 m above alluvial fans deposited north of the Baga Bogd massif. It affects sedimen- tary rocks (Goshu Formation of Berkey and Morris, 1927) that appear in the ancient alluvial fans located along the northern front of the Gobi- Altay. The Goshu Formation is composed of homogeneous gray, poorly cemented, coarse- and fine-grained conglomerates and breccias with a rhythmic structure caused by changes in the size of the fragments from one horizon to the next, with alternations of thin horizons of argillaceous sands containing angular gravelly material (Florensov and Solonenko, 1963). The landslide has a well- preserved morphology and is limited laterally by two parallel and linear north-trending scarps. Two main morphological domains can be dis- tinguished (Figs. 2 and 3): (1) To the north, half of the slide mass is deformed into east-trending topo- graphic ridges that curve laterally on both sides. This pattern can be interpreted as frontal compres- Geology; March 1999; v. 27; no. 3; p. 211–214; 5 figures. 211 Gigantic paleolandslide associated with active faulting along the Bogd fault (Gobi-Altay, Mongolia) Hervé Philip* Laboratoire de Géophysique et Tectonique—CNRS, Université de Montpellier II, Place Eugène Bataillon, Jean-François Ritz* 34095 Montpellier Cedex 05, France *E-mail: Philip: philip@dstu.univ-montp2.fr; Ritz: ritz@dstu.univ-montp2.fr. Figure 1. Sketch map of 1957 Gobi-Altay earth- quake ruptures and loca- tion of Baga Bogd paleo- landslide. ABSTRACT On the basis of analyses of satellite imagery, aerial photographs, and field observations, we describe the occurrence of one of the largest paleolandslides (50 km 3 ) ever recognized in an intracontinental domain. The slide occurred along the active Bogd fault in the Gobi-Altay mountain range in Mongolia. Morphological and structural analyses of the relationships between the landslide and the area affected by active tectonics suggests that this gigantic mass movement was associated with surface faulting during a strong earthquake.