Cratonic nuclei often induce stress concentration and strain localization at their boundaries (Tommasi and Vauchez, 2001), but internally remain stable over very long geological periods. However, the unusual reactivation of the NCC challenges the classical views concerning the strength and stability of cratonic lithosphere. It is proposed that the North China Craton (NCC) (Figure 2.36) formed in the Paleoproterozoic by the amalgamation of two Archean blocks, the Eastern and Western Blocks (EB and WB), along the Central Block (CB) (e.g., Zhao et al., 2001). While there is a growing consensus that lithospheric rifting has occurred in the EB and extended to the CB (Xu et al., 2004; Zhao et al., 2008), evidence for the vertical and horizontal distribution of lithospheric reactivation still remains unclear. This study is the first to provide high-resolution imaging of both P- and S- wave velocities in an effort to understand the process of reactivation of an Archean craton.
Our results show that structures imaged by finite-frequency kernel methods (Figure 2.37) and ray-based methods are very similar, except that the kernel-based models yield higher root-mean-square amplitude of P and S wave velocity perturbation, as expected (Hung et al., 2004). The first-order features of our models, for both P- and S-waves, include: (1) A north-south trending narrow low-velocity region with dimension of 800 km north-south and 200-300 km east-west is located at the base of the CB lithosphere, and extends to more than 300 km depth. The northernmost and southernmost parts extend to more than 500 km depth. (2) A region of high-velocity extends to more than 250-300 km depth beneath the WB, in contrast to the much shallower high-velocity zones beneath the CB and EB.
Resolution tests show that the resolution is good down to 600km depth for anomalies 200 km even for data with 30 noise in the regions with good sampling coverage, and the downward smearing length is less than 50 km.
Finite-frequency kernel based P- and S-wave velocity images show that a N-S trending low velocity anomaly extends from beneath the CB to at least 500 km. High-velocity anomalies extend to more than 250-300 km depth beneath the WB, and to shallower depths beneath the CB and the EB. The imaged structure suggests that the presence of warm material with a source at least as deep as the transition zone is responsible for reactivation of the NCC. The pre-existing weak zone within the CB of the craton likely acted as a sublithospheric corridor for the warm mantle material.
This work was completed during L. Zhao's visit to Berkeley Seismology Lab sponsored by K.C. Wong Education Foundation, Hongkong. We thank M. Xue, M. Obrebski, H.Y. Yuan , R. Porritt, W.W. Xu, L. Chen and C. Paffenbarger for constructive help.
Tommasi, A., and A. Vauchez, Continental rifting parallel to ancient collisional belts: an effect of the mechanical anisotropy of the lithospheric mantle, in Earth Planet. Sci. Lett., 185, 199-210, 2001.
Xu, Y., et al., Contrasting Cenozoic lithospheric evolution and architecture in the eastern and western Sino-Korean craton: constraints from geochemistry of basalts and mantle xenoliths, in J. Geology, 112, 593-605, 2004.
Zhao, G., et al., Archean blocks and their boundaries in the North China Craton: lithological, geochemical, structural and P-T path constraints and tectonic evolution, in Precambrian Res. 107, 45-73, 2001.
Zhao, L., et al., Insight into craton evolution: Constraints from shear wave splitting in the North China Craton, in Phys. Earth Planet. Inter., 168, 153-162, 2008.
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