An unusually long eclogitic lower crustal body imaged by the Korean nuclear explosion

Abstract

<p>The Sino-Korean Craton (SKC), which consists of the North China Craton (NCC) in China and North Korea, is one of the oldest cratons on earth. Since the Paleozoic, the SKC has experienced multiple subductions of the peripheral plates and the northeastern SKC is located in a junction area. Its characteristics are being investigated by geophysical and geochemical methods, which provides insights into the formation and subsequent evolution of the continental lithosphere.</p><p>We interpret the crustal structure of the northeastern SKC with the refraction/wide-angle reflection perspective using North Korean Nuclear Explosion sources recorded by 40 permanent and 7 temporary broadband stations, which were operated by the China Earthquake Administration and the Institute of Geology and Geophysics, Chinese Academy of Science, respectively.</p><p>Primary reflection phases from a discontinuity at 30km depth have an apparent velocity of about 6.2 km/s. This phase is observed to 1200km ultra-long offset, which shows that the average crustal velocity is extremely low. Another spectacular observation is of extremely strong phases which we interpret as Moho to surface multiples of all main phases in the seismic sections. Clear upper mantle refractions (Pn) are observed with an apparent velocity around 8.05 km/s as first arrivals over the offset range 300-1000 km. All observations show that the crust of northeastern SKC is very thin (about 30km), it has a low average crust velocity (6.2km/s), and the velocity contrast at the Moho discontinuity is extraordinarily strong.</p><p>We detect the “Seismic Moho” discontinuity, which is marked by a very strong and sharp increase in velocity. We interpret this “Seismic Moho” as the top of a layer consisting of the lower crust in eclogite facies. This “Seismic Moho” does not coincide with the true Crust-Mantle Boundary, which is defined by a change from felsic/intermediate/mafic crustal rocks to the dominantly ultramafic rocks of the upper mantle in petrological terms.</p>