Extremely weak Lg attenuation reveals ancient continental relicts in the South China block

Abstract

Whether the South China block (SCB) occupied an external or internal position in the Rodinia supercontinent remains controversial. The external model favors an accretion orogenic amalgamation setting, whereas the internal model assumes postcollisional extensional rifting in the Neoproterozoic. Recent geochronological data suggested that the SCB was assembled from several ancient nuclei or microcontinental fragments with various origins and geological histories during the early Neoproterozoic, and supported the external position model for the SCB in Rodinia. However, whether these ancient continental nuclei are hidden in the crust requires direct constraints such as those from seismic tomography. In general, these early ancient continental relicts preserved in the present-day crust tend to be characterized by high strength, high velocity, and weak attenuation. Compared with velocity, seismic attenuation tends to be more sensitive to ancient rigid continental fragments. Here, we constructed a high-resolution broadband crustal Lg attenuation model for the SCB and explored the existence and scale of the ancient continental fragments potentially hidden in the deep crust. Four extremely weak attenuation anomalies in the SCB indicate potential locations of these ancient continental relicts in the crust; one is in the Sichuan basin and the other three are beneath the Cathaysia block. The three ancient continental relicts in the Cathaysia block, which are closely corresponding to early surface lithological records, provide seismic evidence supporting an external position for the SCB in the Rodinia supercontinent.