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Mongolia SCRThe presence or absence of Neogene alkaline igneous rocks andrifting provides a guide to whether the Mongolia SCR shouldbe classified as an SCR or an ACR. The Mongolia SCR includesthe eastern half of Mongolia, most of northeastern China, andadjacent areas of Russia (Figure 2). The Mongolia SCR is a comparativelystable region. ACR crust surrounds this SCR; latersections will summarize the active nature of the North Chinablock on the south, a region of active faults and volcanism onthe west and north, and the eastern China continental marginon the east. The Tanlu fault separates the Mongolia and 2011China SCRs. The Mongolia SCR is moving eastward withrespect to a fixed Eurasia (Liu et al. 2007; Wang et al. 2011).The relative motion takes place on a belt of extensional andleft-lateral transtensional faulting north of the SCR, between itand the Siberian part of the Eurasian SCR. The belt of faultingcomprises the Hangay and Khubsugul extensional systems, theTunka fault system, the Baikal rift system, and the Stanovoyfault zone (Figure 2). Judging from the motions that Liu et al.used to compute Quaternary rates of fault slips, the MongoliaSCR appears to be moving eastward with respect to Siberia atmuch less than 1 mm/yr, and possibly as little as 0.1 mm/yr.Tomography shows that S-wave velocities at 50 km depthare similar across the Mongolia SCR, and the same is true for100 km depth (Feng and An 2010). From this it appears thatcrustal and lithospheric thicknesses vary little across the SCR.Seismicity is sparse and geodetically measured velocity andstrain are small (Broadbent and Allan Cartography 1994; Liuet al. 2007; Feng and An 2010) (see also the earthquake catalogsat http://earthquake.usgs.gov/earthquakes; last accessedJuly 21, 2011).The compilation map of Ren et al. (2002) shows normalfaults that bound Paleogene and older basins throughout theSCR, but no Neogene faults or basins. Active systems of northerlystriking normal faults and easterly striking left-lateralstrike-slip faults in western Mongolia do not appear to extendinto the SCR, except in its northwestern corner at the northeast-strikingnormal faults of the Hangay extensional system(Figure 2) (McCalpin and Khromovskikh 1995; Walker et al.2007; Yin 2010). Walker (2009) did not find active faults ineastern Mongolia and adjacent China, which include most ofthe Mongolia SCR.Figure 2 shows that Neogene volcanic rocks are less numerousper unit area within the Mongolia SCR than in more tectonicallyactive regions, such as the Indochina SCR and Chinaeast of the South China block and the Tanlu fault (Ren et al.2002; Liu et al. 2007; Yin 2010). The Neogene volcanic rocksin the Mongolia SCR are largely alkaline although older volcanicrocks range more widely in compositions (Whitford-Stark1987; Basu et al. 1991). Barry et al. (2003) cited computationsby McKenzie and Bickle (1988), which imply that generationof significant amounts of alkaline melts would require muchmore Neogene horizontal extension than appears to haveoccurred in most of the Mongolia SCR. Consequently, sincethe definition of the Mongolia SCR in 1994, new informationdoes not demonstrate extension younger than Paleogene exceptin the northwestern corner of the Mongolia SCR. The rest ofthe Mongolia SCR meets the criteria of Table 1 and retains itsclassification as an SCR.2011 China SCRNorth China BlockThe North China craton is the Chinese part of the Sino-Korean craton, with the remainder being the northern part ofthe Korean peninsula (for example, Zhang et al. 1984, Zhaoet al. 2009, Yang et al. 2010). I follow Kwon et al. (2009) incalling both cratons “blocks” because, as explained later, theyunderwent Mesozoic and Cenozoic metamorphism, extension,intrusion, and volcanism so that they are no longer cratoniccrust (Figure 2; note that the craton boundary is northwestof the boundary between North and South Korea). The Tanlufault splits the North China block into two parts. The largerpart of the block lies entirely west of the 1994 and 2011 ChinaSCRs, whereas the smaller part is within both versions of theSCR. The smaller part is of more interest here, but most of theinformation on the North China block comes from the activecrust of the larger part. Therefore, I will discuss the NorthChina block as a whole. The block is a triangular region innorthern China (Figure 2) that is made of early Precambriancrust (Zhang et al. 1984; Zhao et al. 2001; Kwon et al. 2009).The North China block is moving eastward with respect to afixed Siberia and the Mongolia SCR (Yin 2010). Geologic dataincluding slip rates of individual faults indicate eastward movementwith respect to Siberia of 1–2 mm/yr in the eastern halfof the North China block and 2–4 mm/yr in the western half(Liu et al. 2007). Geodetic data indicate rates consistent withthose of Liu et al. (2007) (Wang et al. 2011).The eastern part of the block is seismically active, whereasthe western part is less so (Liu et al. 2007). For example, the2008 version of the “Centennial” earthquake catalog ofEngdahl and Villasenor (2002) lists 17 earthquakes of magnitude6.0 or larger in the eastern part of the block but onlyone in the western part. Results of P- and S-wave tomographyshow a low-velocity zone that extends to 300–400 km depthbeneath the eastern part of the block (Zhao et al. 2009). S-wavetomography, deep seismic-reflection profiles, and receiver-functionimaging show that the crust thins eastward from approximately45 km in the western part of the block to about 30 kmin the eastern part (Li et al. 2006; Zheng et al. 2006; Chen etal. 2009; Feng and An 2010). Zheng et al. (2006) concludedthat most of the thinning took place in the lower crust and ina transitional zone between the crust and mantle. The thinningresulted from extension that began with widespread EarlyCretaceous eruption and intrusion of alkaline basaltic and graniticrocks (Ren et al. 2002; Wu et al. 2005; Zhu et al. 2010).The entire North China block underwent extension by normaland transtensional faulting of early Neogene age, whereas theeastern part of the block and the northern part of the Koreanpeninsula also underwent late Neogene alkaline igneous activity(Liu et al. 2001; Ren et al. 2002; Zheng et al. 2006; Yu etal. 2008; Yang et al. 2010; Yin 2010). Zhao et al. (2009) inter-Seismological Research Letters Volume 82, Number 6 November/December 2011 975