Quantifying Crustal Thickness in Continental Collisional Belts: Global Perspective and a Geologic Application

We present compiled geochemical data of young (mostly Pliocene-present) intermediate magmatic rocks from continental collisional belts and correlations between their whole-rock Sr/Y and La/Yb ratios and modern crustal thickness. These correlations, which are similar to those obtained from subduction-related magmatic arcs, confirm that geochemistry can be used to track changes of crustal thickness changes in ancient collisional belts. Using these results, we investigate temporal variations of crustal thickness in the Qinling Orogenic Belt in mainland China. Our results suggest that crustal thickness remained constant in the North Qinling Belt (~45-55 km) during the Triassic to Jurassic but fluctuates in the South Qinling Belt, corresponding to independently determined tectonic changes. In the South Qinling Belt, crustal thickening began at ~240 Ma and culminated with 60-70-km-thick crust at ~215 Ma. Then crustal thickness decreased to ~45 km at ~200 Ma and remained the same to the present. We propose that coupled use of Sr/Y and La/Yb is a feasible method for reconstructing crustal thickness through time in continental collisional belts. The combination of the empirical relationship in this study with that from subduction-related arcs can provide the crustal thickness evolution of an orogen from oceanic subduction to continental collision.