Appendix S4. Analysis of source area detrital zircon data
Catchments draining the present LJS exhibit a large and unique zircon age population (ca. 450
Ma, Fig. S2A), which has not been clearly identified in the NWQ. The very small ca. 250 Ma zircon age
population, present in catchments draining the LJS, was necessarily derived from recycled zircons in
recently eroded Cenozoic strata because all source rocks in the LJS are older than 250 Ma. The 450
Ma population is the greatest contributor to the LJS-derived zircons. Their sources are early Paleozoic
plutons and volcanic rocks that crop out along the modern LJS crest. The less abundant 700-1000 Ma
population represents recycled zircons contained in Silurian strata in the LJS (Lease et al., 2007).
These zircon age populations in the LJS are broadly consistent with those of 400-425 Ma, 425-480
Ma, and ca. 775 Ma zircon grains identified in the Qilian Shan (Fig. 1; Enkelmann et al., 2007, and
references therein).
Catchments draining the modern NWQ are characterized by a unique, ca. 250 Ma, large zircon
population, which is derived from Permian–Triassic plutons (Fig. S2B). The zircon populations of 796910 Ma, 1600-2100 Ma, and 2350-2600 Ma (Fig. S2B; Lease et al., 2007), may largely be derived from
Middle-Upper Triassic Songpan-Ganzi metasediments in the NWQ (Figs. S2B; Weislogel et al., 2010).
Zircon signatures of the Triassic turbidite strata and granitoids in the northern part of the SongpanGanzi complex (to the south of the Xunhua and Linxia basins in the NWQ) reflect abundant 210-300
Ma grains, fewer 400-480 Ma, very rare 700-1600 Ma, and some 1700-2200 Ma and 2300-2600 Ma
grains (Fig. S2D; Zhang et al., 2006; Weislogel et al., 2006; Chen et al., 2009; Weislogel et al., 2010;
Zhu et al., 2011). These age distributions further suggest that the ca. 250 Ma zircon population in the
Xunhua and Guide basins was sourced from Permian–Triassic plutons and Lower-Middle Triassic
strata in the NWQ, a ca. 450 Ma zircon population mainly from the LJS, a 700-1000 Ma zircon
population from the both the LJS and NWQ, and a bulk older zircon population, >1500 Ma from units
now in the NWQ thrust belt (Kroner et al., 1993; Weislogel et al., 2006; Enkelmann et al., 2007).