TECTONICS, VOL. 10, NO. 6, PAGES 1091-1110, DECEMBER 1991 GEOLOGY OF THE HAIYUAN FAULT ZONE, NIlNGXIA-ItUI AUTONOMOUS REGION, CHINA, AND ITS RELATION TO THE EVOLUTION OF THE NORTHEASTERN MARGIN OF THE TIBETAN PLATEAU B.C. Burchfiel, • Zhang Peizhen, TM Wang Yipeng, 3 Zhang Weiqi, 4 Song Fangmin, 3 Deng Qidong, 3 Peter Molnar, • and LeighRoyden • Abstract. The Haiyuanarea,located alongthe northeastern margin of theTibetan Plateau in north central China, provides a laboratory for studying how the plateauhas grown in late Cenozoic time. Rocks in the area range from pre-Silurian (Precambrian?) to Recent; the pre-Silurian andCenozoic rocks form the most extensiveoutcrops. The pre-Silurian rocks consist of amphibolite- andgreenschist-grade metasedimentary andmetaigneous rocks unconformably overlain by Silurian and Devonian red beds. All of these rocks are intruded by granodiorite of unknown age. Cenozoic rocks consist of 2.6- 3.0 km of Eoceneto Miocenered beds thatwere deposited over an extensive areain thispartof China. Pliocene conglomerate contains clasts from all older formations and is interpreted to have been derived from highlands developed during the beginning of Cenozoic deformation in the Haiyuan area. Except for the widespread 1oess deposits, Quaternary rocks reflect deposition in local tectonic environments. The oldest Cenozoicstructures in the Haiyuan area are folds and small thrust faults thatgenerally strike N30ø-45øW and involve mostly pre-Quaternary rocks. These structures andall theQuaternary rocks are cut by the Haiyuanleft-lateral strike-slip (left-slip) fault zone that generally trendsN60ø-65øW and is nearly vertical. At the western endof the mapped areaa fault zone, whichstrikes N75ø-90øW, forms a left-stepping transfer zone thatconnects with another segment of the Haiyuan faultzone, which continues N60ø-65øW west into Gansu Province. A smallbasin,the Salt Lake Basin,is marked by activefaults in the areaof the transfer zoneand is interpreted as a pull-apart basin alongthe left-slipHaiyuan fault zone. At its eastern end the Haiyuan fault zone hasan irregularsurface trace;eastof Luzigouan active fault striking N35ø-45øW branches to the south. This southern branch appears to be a younger fault and now accommodates most of the left-slip deformation that formerly occurred on the easternmost partof theHaiyuan fault •Department of Earth, Atmospheric andPlanetary Sciences, Massachusetts Institute of Technology, Cambridge. 2Nowat Institute of Geology, StateSeismological Bureau, Beijing,People's Republic of China. 3Institute of Geology, State Seismological Bureau, Beijing, People's Republicof China. 4Ningxia Seismological Bureau, Yinchuan, Ningxia-Hui Autonomous Region,People's Republicof China. Copyright 1991 by the American Geophysical Union. Paper number 90TC02685. 0278-74 07/91/90TC-02685 $10.00 zone. This youngerfault connects througha left-stepping transfer zone to a parallel fault, the Xiaokou fault, that can be traced into theLiupanShan about 60 km to the southeast. The Laohuyaoxian Basinis interpreted as a very young pull-apart basin in the area of the transfer zone. Matching different geological features across the Haiyuan faultzone yields a total left-slip offset of between 10.5 and 15.5 km, and the best constrained offsets yield 12.9-14.8 km. If left slip began near the end of the Pliocene time or earliest Pleistocene time, it indicates an average slip rate between 5 and 10 mm/yr. Progressively smaller offsets can be determined on progressively younger geological features, but dates for these youngerfeaturesare too imprecise to constrain slip rates through time. Surface ruptures that formedduringthe 1920 Haiyuan earthquake (M = 8.7) show mostly left-slip displacement with magnitudes of more than 10 m in some places. Active faulting in the region suggests the Tibetan Plateau may be extendingto the northeast in time. In the Haiyuanarea,deformation probably began in Pliocene time, compared with a likely earlierinitiation to the southwest; thus deformation began about40-45 m.y. after collision between India andAsia. Formation of thelow ranges to thenortheast of the Haiyuan area, however,may have developed at different times and deformation maynothave propagated regularly to the northeast. A total displacementof 10.5-15.5 km on the Haiyuan fault zone indicatesthat this fault zone does not accommodate large-scale eastward lateral transfer of continental fragments in the northern Tibetan Plateau. INTRODUCTION The Tibetan Plateau, with its high elevation and thickcrust, appears to have been built by crustalshortening beginning when India collided with southern Eurasiain early Cenozoic time. At present, however, muchof the active deformation of the plateau occursby normal and strike-slip faulting. An important question is: How does thechange from shortening to strike-slip deformation occur?Clearlya good place to examine thisis on the margins of the plateau, wheresuch changes have occurred recently. The Haiyuanareaof theNingxia regionis such a place. The Ningxia region is characterized by numerous active faults and containsthe transitionfrom strike-slip and thrust faulting within the broadnortheastern marginof the Tibetan Plateau to normal faulting that dominates the tectonics of northeastern China (Figures 1 and 2). Thrust faulting dominates the northern boundary of the western Qinling (Maxian fault zone)in the south andin thenorth andnorthwest trending mountains in theeastern partof theregion (Figure 2). All of these active faultscontribute to the continuing uplift of the northeastern margin of the Tibetan Plateau. The thrust faults in the Yanton Shan, Daluo Shan, and Niushou Shan lie northeast of the main topographic frontof theplateau andmay foreshadowfuture northeastward migration of the plateau [Zhang et al., 1990]. The Helan Shan in the northeastern part of the regionis bounded by normal faults,and the Yinchuan valley is underlain by a graben with at least 1609 m of Quaternary sediment andmore than 6000 m of Cenozoic rocks, some of whichwerepenetrated by a drill hole 10-20km north of Yinchuan [Ma et al., 1982]. This area of active extension is only partof the regional areaof extension thatsurrounds three