Late Mesozoic magmatic plumbing system in the onshore-offshore area of Hong Kong: Insight from 3-D active-source seismic tomography

We used active source wide-angle seismic data to determine a high-resolution P-wave crustal tomography beneath the onshore-offshore area of Hong Kong at the southern end of a broad belt dominated by the late Mesozoic intrusive and extrusive rocks in the coastal region of Southeast China. The active source data are much more precise than the natural earthquake data and so can be used to study the fine crustal structure. Our results reveal a localized high-velocity anomaly in the lower crust offshore between Hong Kong and Dangan Island, which may reflect basaltic underplating that is closely associated with formation of voluminous silicic eruptions and granitoid plutons in the onshore-offshore area of Hong Kong. Tilted high-velocity zones connecting with the localized high-velocity anomaly in the lower crust are clearly visible in the entire crust beneath Dangan Island and the calderas of Hong Kong. Taking into account the previous geochemical, petrologic and numerical modeling results, we think that the tilted high-velocity zones may be the results of mingling of mafic and felsic end members and extreme degree of crustal partial melt extraction necessary to generate a large amount of extrusive rocks in the calderas, reflecting cooled magma conduits as a manifestation of solidified Late Mesozoic magmatic plumbing system in the crust. Considering the petrologic and geochemical characteristics of the late Mesozoic granites and basalt in Southeast China, we suggest that subduction and dehydration of the paleo-Pacific plate might trigger the basaltic magma underplating and result in extensive crust-mantle interaction, which not only provided necessary heat energy to cause the crustal partial melting, but also added minor mafic materials to the newly generated granitic melts. This model explains our tomographic results as well as the intimate mingling of coeval mafic and silicic magmas in Hong Kong. Intersecting faults could play an important role in forming magma conduits and loci of fissure-like volcanic centers.