TECTONICS, VOL. 12, NO. 4, PAGES 801-813, AUGUST 1993 AN INDENTATION MODEL FOR THE NORTH AND SOUTH CHINA COLLISION AND THE DEVELOPMENT OF THE TAN-LU AND HONAM FAULT SYSTEMS, EASTERN ASIA An Yin Department of Earth andSpace Sciences, University of California, Los Angeles, California Shangyou Nie Paleogeographic Atlas Project, Department of Geophysical Sciences, University of Chicago, Chicago, Illinois Abstract.Passive continental margins aregeometrically irregular asa consequence of either triple-junction evolution or thedevelopment of transfer zones in detachment faultsystems, whereas active continental margins aresmoothly arc-shaped dueto subduction of plates on theEarth's spherical surface. We propose that this basic difference in boundary geometry has played an important rolein thelatest Paleozoic-early Mesozoic collision of NorthandSouth China. In particular, we suggest thatprior to collision, the activesouthern margin of theNorthChinaBlock(NCB) was contiguous across the Qilian Shan, Qinling, DabieShan, Shandong peninsula of east central China to theImjingang area of central Korea. The passive northern margin of the South ChinaBlock (SCB), in contrast, hada more irregular shape, such that itsnortheastern segment in northern Jiangsu andeastern Anhuiprovinces of China extended some 500 km farther north than its western counterparts in northern Sichuan, southern Shaanxi, and northern Hubeiprovinces. Collision of theNCB andtheSCB began by indentation of thenortheastern SCB intotheeastern NCB in thelate Early Permian andlasted until theLate Triassic-Early Jurassic. Theindentation produced the left-slip Tan-Lufaultin northeastern China andtheright-slip Honam shear zone in southeastern Korea and caused the northward displacement of theShandong and theImjingang metamorphic belts. Thismodel predicts that collision along theDabie and Qinling metamorphic beltoccurred significantly later than along theShandong belt,whichis consistent with radiometric anddepositional constraints on thetime of collision. The proposed model accounts for the abrupt termination of the Tan-Lufaultat its south end andthedrastic decrease in slip along the Tan-Lu faultnorth of theShandong metamorphic belt. Themodel also predicts the distribution and ages of metamorphism along thesuture andtheobserved local but intense Triassic deformation (=Indosinian orogeny) in northeastern China and northern Korea, which was previously an enigmatic feature in this region. INTRODUCTION TheEurasian continent consists of a collage of microplates (Figure 1). Majoramalgamation of the continental blocks (microplates withcontinental basement) occurred during thecollision of theNorthChina Block(NCB) andtheSouth China Block (SCB)[Sengor, 1987]. Although intense research has been conducted formore than two decades since the concept of plate tectonics was first applied toChinese geology [Li, Copyright 1993 by theAmerican Geophysical Union. Paper number 93TC00313. 0278-7407/93/93TC-00313 $10 1975],many controversies regarding thenature of thecollision remain. It has been debated whether the collision occurred during themiddle Paleozoic [Zhang et al., 1984; Mattauer et al., 1985;Liu and Hao, 1989] or the Triassic [Sengor, 1985; Hsu et al., 1987; Lin and Fuller, 1990; Zhao and Coe, 1987; Wanget al., 1992]andhowthecollision between theSCBand theNCB wasaccomplished [Okay andSengor, 1992; Yin and Nie, 1992]. Related to theproblem of theNorth and South China collision is the originof the Tan-Lu fault [Xu et al., 1987],a prominent left-lateral strike-slip faultalong theeastern margin of the Eurasian continent. The Tan-Lu fault offsets the collisional belt between theNorthandSouth China blocks by about 500 km (Figure 2), however, displacement decreases sharply north of theShandong suture zone anddiminishes to zeroimmediately south of DabieShan [Xu et al., 1987; Okay and Sengor, 1992] (Figure 2). Recently, a Triassic right-lateral strike-slip fault,theNE trending Honam shear zone, has been recognized in South Koreain theOgcheon belt (Figure 2) [Kim andLee, 1984; Yanai et al., 1985].Although Cluzelet al. [1991a]suggest thatthe shear zone juxtaposes the SCB to the north andthe NCB to the south, andhas a minimum displacement of 200 km, the development of thisshear zone has notbeen discussed in the context of the NCB-SCB collision. In thispaper, we firstreview theregional geologic setting before and during the collision and the extent and geometry of theNorth andSouth Chinablocks prior to collision. We then show thatthecollision occurred diachronously, starting from theeast in Shandong andnorthern Koreain lateEarlyPermian and progressing to thewest in Qinling in thelatest Triassic to earliest Jurassic. Finally,we propose a tectonic model that explains thediachroneity of thecollision andthe development of theleft-slip Tan-Lu faultin eastern China and theright-slip Honam fault in southeastern Korea. GEOLOGIC SETTING North and South China Blocks The locus of the North China-South China collision is well constrained in China along the Qinling, Dabie, and Shandong belts (Figure 2). Theeastern extension of the Shandong belt in Korea is not clear, because ophiolites, representing the existence of an ocean(s), have not been found in Korea[Reedman and Um, 1975; Lee, 1987]. However, the presence of ophiolites is a necessary but not a sufficient condition forthe existence of a suture, because ophiolites are commonly accreted to continental margins prior to collision. It is also possible that certain segments of anactive continental margin have noaccretionary ophiolites sothatthecollisional suture is marked only by deformation and metamorphism. On the basis of lithostratigraphy, the Korean peninsula is divided by theImjingang and Ogcheon belts into three tectonic blocks (Figure 2). The blocks are, from north to south, the Nangrim- Pyeongnam (NPB),the Gyeonggi (GB),and the Ryeongnm•l (RB) [Reedman andUrn, 1975;Lee, 1987;Cluzel et al., 1990]. Despite the lack of ophiolites in Korea, the Imjingang belt has been correlated withthe Shandong suture zone [e.g., Ernst, 1988; Hsuet al., 1990]. Following Cluzel et al. [1990], weuse the Honam shear zone as the boundary between the NPB and the GB (Figure 2). Thus, thetraditionally defined Ogcheon belt [e.g., Reedman and Urn, 1975] consists of rocks belonging to both theNPB andGB. The Nangrim-Pyeongnam and Ryeongnam Blocks have Cambrian fauna of North China affinity [Kobayashi, 1966]. In contrast, theGyeonggi Blockin central Korea consists of Cambrian strata bearing South China affinity fauna