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preted the low-velocity zone in the mantle beneath the easternpart of the block in terms of warm mantle material that mighthave caused the rifting and alkaline igneous activity. Extensionis demonstrated in the upper crust by the mapped extensionalfaults, in the lower crust by the geophysical evidence, anddeeper than 70 km (McKenzie and Bickle 1988) in the mantleby the compositions of the igneous rocks. The Neogene extensiondemonstrates that the North China block and the rest ofthe Sino-Korean block are no longer a craton (Yang et al. 2008,2010). The extension also requires reclassifying this part of the1994 China SCR as active crust (Table 1).Eastern China Continental MarginThe eastern China continental margin comprises all of theYellow Sea, the East China and South China seas within200–600 km of the mainland Chinese coast, and the landeast of the North China and South China blocks (Figure 2;Zhou et al. 1995; Yin 2010). Bathymetric, geologic, and geophysicaldata show that the continental crust of the marginextends seaward approximately to the edge of the continentalshelf (GEBCO World Map Editorial Board 2006; Wang et al.2006; Yin 2010). Fault-slip data show that the margin is movingsouth-southwestward past the North and South Chinablocks (Figure 2; Yin 2010); geodetic data give the rate as 0.7mm/yr in the north and 1.8 mm/yr in the south (Wang et al.2011). The continental margin is more seismically active thanthe South and North China blocks, especially west of Taiwan(Liu et al. 2007; Wang et al. 2011).The compilations of Yin (2010), Ren et al. (2002), andSengor and Natal’in (2001) show the continental marginas having undergone Paleogene and older extension. S-wavetomography indicates thick sediments and thin crust and lithosphereunder the margin (Feng and An 2010). China east ofthe North and South China blocks contains numerous basinsbounded by normal faults; most of the basins are of Paleogeneage (Ren et al. 2002). In the same part of China and in offshorebasins on the continental shelf, abundant basaltic volcanicrocks of late Neogene ages are exposed from Hainan Islandnortheastward to the southern Tanlu fault (Figure 2; Ren et al.2002; Yin 2010). Many of the basaltic rocks are alkaline (Ho etal. 2003). In the South China Sea, early Neogene thermal subsidenceof several kilometers was followed by middle Neogenenormal faulting (Zhou et al. 1995). The alkaline basalts andnormal faulting indicate Neogene extension of the continentalmargin from Hainan Island to the southern Tanlu fault.Yin (2010) restricted the definition of the margin northeastof North Korea to offshore continental crust that has beenthinned by back-arc extension. However, three onshore areasbetween the coast and the Tanlu fault must also be consideredbecause they are in the 1994 China SCR, or adjacent to it andalong its tectonic grain (Figure 2):1. The northern Korean peninsula and the northern YellowSea are part of the 1994 China SCR. The northern peninsulaunderwent Neogene extension as described earlier.Thus, ACR of the Sino-Korean block and the easternChina continental margin surround the northern YellowSea on three sides (Figure 2). The Neogene extensionimplies that the northern Korean peninsula, and probablythe adjacent part of the Yellow Sea, should be reclassifiedas active crust (Table 1).2. Northeast of the North China block and east of the Tanlufault, northeastern China and adjacent Russia are notin the 1994 China SCR. This area contains numerousNeogene basalts, many of them alkaline (Liu et al. 2001;Ren et al. 2002). The alkaline basalts indicate Neogeneextension, and Sengor and Natal’in (2001) show latestCretaceous–early Neogene grabens in the same area as thebasalts. The evidence for Neogene extension implies thatthe area was correctly classified as active crust in 1994.3. The southern part of the Korean peninsula is part of the1994 China SCR. Yin (2010) reported that the peninsulaappears to have had little Paleogene or Neogene extension.The southern part of the peninsula is outside the Sino-Korean block. Summaries of Korean geology and tectonicsdo not show any evidence of foreland deformation ororogenic activity of Cretaceous or younger age (Table 1)(Exxon Production Research Company 1985; Chang1997; Kwon et al. 2009). However, the southeastern partof the peninsula exposes a few extensional or transtensionalfaults that bound Neogene basins, as well as sparseNeogene alkaline volcanic rocks (Exxon ProductionResearch Company 1985; Ren et al. 2002; Yang et al.2010; Yin 2010). Two small islands approximately 170 kmeast of the mainland and 120 km south of it also exposesparse Neogene alkaline volcanic rocks (Reedman andKim 1997). Kanter (1994) classified the southern island asbeing within the same northeast-trending belt of Mesozoiccontinental crust as the southern part of the Korean peninsulaand most of the Yellow Sea (Broadbent and AllanCartography 1994). The Neogene alkaline rocks exposedon the southern island, together with the Neogene alkalinerocks exposed along trend to the southwest in theeastern China continental margin (Figure 2), suggest thatsimilar Neogene alkaline rocks may be hidden beneath theYellow Sea. Thus, the evidence for Neogene extension issparse and whether the southern part of the Korean peninsulaand the Yellow Sea are SCR crust or ACR crustis arguable. Overall, I judge that neither area meets therequirements for classification as an SCR (Table 1).Regional geologic relations are consistent with the classificationof the southern part of the Korean peninsula as probable ACRcrust. As pointed out earlier in this section, all other areas eastof the Tanlu fault, including the northern part of the Koreanpeninsula, have undergone Neogene extension. As a result,500–600 km of the active crust of the North China block separatesthe southern Korean peninsula from the nearest SCR,which is the Mongolia SCR. This isolation allows the possibilitythat the southern Korean peninsula might be a small fragmentof comparatively intact ACR crust that is being carriedeast-southeastward by Neogene extension of the surroundingmore active continental crust. The possibility is consistent with976 Seismological Research Letters Volume 82, Number 6 November/December 2011