GEOLOGY OF THE HAIYUAN FAULT ZONE, NINGXIA-HUI AUTONOMOUS REGION, CHINA, AND ITS RELATION TO THE EVOLUTION OF THE NORTHEASTERN MARGIN OF THE TIBETAN PLATEAU

The Haiyuan area, located along the northeastern margin of the Tibetan Plateau in north central China, provides a laboratory for studying how the plateau has grown in late Cenozoic time. Rocks in the area range from pre-Silurian (Precambrian?) to Recent; the pre-Silurian and Cenozoic rocks form the most extensive outcrops. The pre-Silurian rocks consist of amphibolite- and greenschist-grade metasedimentary and metaigneous 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 Eocene to Miocene red beds that were deposited over an extensive area in this part of 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 loess deposits, Quaternary rocks reflect deposition in local tectonic environments. The oldest Cenozoic structures in the Haiyuan area are folds and small thrust faults that generally strike N30-degrees-45-degrees-W and involve mostly pre-Quaternary rocks. These structures and all the Quaternary rocks are cut by the Haiyuan left-lateral strike-slip (left-slip) fault zone that generally trends N60-degrees-65-degrees-W and is nearly vertical. At the western end of the mapped area a fault zone, which strikes N75-degrees-90-degrees-W, forms a left-stepping transfer zone that connects with another segment of the Haiyuan fault zone, which continues N60-degrees-65-degrees-W west into Gansu Province. A small basin, the Salt Lake Basin, is marked by active faults in the area of the transfer zone and is interpreted as a pull-apart basin along the left-slip Haiyuan fault zone. At its eastern end the Haiyuan fault zone has an irregular surface trace; east of Luzigou an active fault striking N35-degrees-45-degrees-W 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 part of the Haiyuan fault zone. This younger fault connects through a left-stepping transfer zone Lo a parallel fault, the Xiaokou fault, that can be traced into the Liupan Shan about 60 km to the southeast. The Laohuyaoxian Basin is interpreted as a very young pull-apart basin in the area of the transfer zone. Matching different geological features across the Haiyuan fault zone 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 younger features are too imprecise to constrain slip rates through time. Surface ruptures that formed during the 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 extending to the northeast in time. In the Haiyuan area, deformation probably began in Pliocene time, compared with a likely earlier initiation to the southwest; thus deformation began about 40-45 m.y. after collision between India and Asia. Formation of the low ranges to the northeast of the Haiyuan area, however, may have developed at different times and deformation may not have propagated regularly to the northeast. A total displacement of 10.5-15.5 km on the Haiyuan fault zone indicates that this fault zone does not accommodate large-scale eastward lateral transfer of continental fragments in the northern Tibetan Plateau.