Reactivation of an Archean craton: Constraints from P‐ and S‐wave tomography in North China

Abstract

The unusual reactivation of the North China Craton (NCC) challenges the classical views concerning the strength and stability of cratonic lithosphere. By using teleseismic body‐waves recorded at 250 seismic stations, this paper presents high‐resolution North China Models of P‐ and S‐wave velocity based on finite‐frequency kernel tomography. Both P‐ and S‐wave velocity models reveal that: (1) an obvious N–S trending narrow low‐velocity region is located at the base of the lithosphere beneath the Central Block (CB) of the NCC, which extends to more than 500 km depth; (2) a region of high‐velocity extends to more than 250–300 km depth beneath the Western Block, in contrast to the much shallower high‐velocity zones beneath the CB and shallower high‐velocities beneath the Eastern Block. These features suggest that warm mantle material with a source at least as deep as the transition zone, possibly a mantle plume, may be responsible for the reactivation of the NCC. The Central Block may have behaved as a sublithospheric corridor for the warm mantle material due to its pre‐existing weakness.

Uppermost mantle structure of the North China Craton: Constraints from interstation Pn travel time difference tomography

“…Obvious fast velocities are found in most areas of the Western Block, which is consistent with previous Pn tomography at larger scales [18,19]. Seismic travel time tomography also indicated that the fast velocity anomaly beneath the Western Block extends to 200 km depth or deeper [15,16,27,28]. Given the thick lithosphere (160-200 km) [13,14], relatively low heat flow (~40 mW/m 2 ) [38], absence of volcanoes or strong earthquake activity, we infer that the lithosphere of the Ordos Block remains a stable Archean craton.…”

“…Similar characteristics also have been found in previous tomographic results of the lithospheric mantle [15,18,19,21,[27][28][29]. Local tomographic images from P and PmP travel time data also indicate obvious fast velocities at 42 km depth [30].…”

“…The characteristics of slow velocity were clearer in the S waves rather than P waves [27]. Since S wave velocities are more sensitive to variation of temperature compared with P wave velocities [37], the slow velocity anomaly could be primarily induced by high temperature.…”

“…In previous travel time tomography studies, slow velocity zones to 500 km depth, and extending in a N-S trend, also have been found at the base of lithospheric mantle in the Central Block [15,27]. The characteristics of slow velocity were clearer in the S waves rather than P waves [27].…”

“…Since S wave velocities are more sensitive to variation of temperature compared with P wave velocities [37], the slow velocity anomaly could be primarily induced by high temperature. Zhao et al [27] indicated that the Central Block may have behaved as a sublithospheric corridor for hot mantle material, which may have induced reactivation of the Archean NCC. If this inference is correct, the significantly slow velocities beneath Taihangshan Mountain and the YinchuanHetao Rift in our tomographic image suggest that the influence of the asthenosphere upwelling has already reached the uppermost mantle and crust-mantle boundary.…”