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(A)70°N100°W 60°W70°N(B)100°E140°E50°N50°NMO140°ECH30°N*30°N10°NIO10°N100°E0 2000Kilometers100°W0 2000Kilometers60°WEarthquake epicenterMainland and coastline of continent*Boundary of stable continental regionLarge earthquakes of New Madrid seismic zone▲ ▲ Figure 1. Comparison of sizes and seismicities of North America (A) and China (B) stable continental regions (SCRs). Coastlinesand SCR boundaries after Kanter (1994) and Broadbent and Allan Cartography (1994). Epicenters are of earthquakes of magnitude 6.0or larger on any magnitude scale (Wheeler, in preparation). Both maps use Lambert azimuthal equal area projections and the samescale to aid visual comparison (Broadbent and Allan Cartography 1994). The North America SCR covers 24,007,000 km 2 and generated35 reported earthquakes of magnitude 6.0 or larger over the four-century historical record, whereas the smaller China SCR covers7,118,000 km 2 and has generated 27 such earthquakes over its 15-century historical record (Johnston et al. 1994). Chinese earthquakesappear more numerous in the figure because more of the North American epicenters overlap one another at the scale of the figure. Forease of discussion, I will refer to the single large SCR of part B as the 1994 China SCR, and to its three components as the Mongolia SCR(MO), the 2011 China SCR (CH), and the Indochina SCR (IO).igneous rocks of alkaline compositions are spatially associatedwith continental rifts (Bailey 1974; Neumann and Ramberg1978; Keller and Hoover 1988; McKenzie and Bickle 1988;Wilson 1989). The spatial association is generally accepted asimplying that extension produces alkaline melts (Wilson 1989).Furthermore, common crustal rocks have melting temperatureswell below those of basalts. This implies that melting inWheeler, Figure 1the mantle generates alkaline basalts. Petrological modelingcalculations of McKenzie and Bickle (1988) and of Barry et al.(2003) imply that the melting of peridotite, a common mantlerock, yields alkaline basaltic melts at depths exceeding 70 km. Inlaboratory experiments, the initial melting of peridotite undermantle pressures and temperatures produces small amounts ofalkaline basaltic melts (Jaques and Green 1980; Olafsson andEggler 1983; Takahashi and Kushiro 1983). Both the meltingexperiments and the petrological calculations show that additionalmelting shifts the composition of basaltic melts away fromalkaline toward less-alkaline basaltic compositions. Most riftrelatedalkaline igneous rocks are alkaline basalts. In addition,some rifts also contain alkaline volcanic and intrusive rocks ofgranitic compositions. Alkaline igneous rocks are known in severalparts of a rift that contains the New Madrid seismic zone,the most active seismic zone in the CEUS (Figure 1) (see summaryof these alkaline rocks in Wheeler 1997). Importantly forthe present study, basaltic rocks dominate in Southeast Asia(Whitford-Stark 1987; Yin 2010), for example in the Baikalrift system of the study area (Wilson 1989; Figure 2 this paper).Thus, volcanic rocks of alkaline basaltic composition imply asmall amount of extension within the upper mantle.Exposed or shallow normal or transtensional faults demonstrateextension of the upper crust and its seismogenic zone.The larger the extensional fault slips, the more likely it is thatbrittle extension penetrates into or spans the seismogenic zone.Where both rifts and alkaline basaltic volcanic rocks are presentand are of similar ages, they indicate that extension affectsthe upper crust, upper mantle, and therefore perhaps the middleand lower crust as well. Rifts without known alkaline volcanicrocks demonstrate brittle extension of at least the uppermostcrust, perhaps including the seismogenic zone. However,alkaline volcanic rocks without recognized, coeval extensionalfaults are more problematic. Absent known extensional faults,alkaline rocks might indicate that incipient extension in theupper mantle has not extended far enough upward to affect theseismogenic zone.Seismological Research Letters Volume 82, Number 6 November/December 2011 973