REGIONAL TECTONIC SETTING OF HONG-KONG - IMPLICATIONS OF NEW GRAVITY MODELS

Hong Kong lies on the SE margin of the Cathaysia Block and straddles t he NE-trending Lianhuashan Fault Zone, one of the dominant structural features of SE China. Isotope signatures of the Mesozoic granites have indicated that the zone overlies a major crustal discontinuity. The p roposed gravity model of Hong Kong consists of a heterogeneous upper c rust underlain by middle to lower crust composed of a felsic Archaean segment, approximately 25 km wide, flanked by more mafic Proterozoic c rust. The southern boundary of the felsic segment dips steeply to the north, whereas the northern boundary is subvertical. Euler gravity ano malies define the fundamental faults in the upper crust, and many of t hese can be correlated with faults that have been mapped at surface. T he middle to lower crustal discontinuities are considered to be associ ated with a major shear zone within the Cathaysia Block that has simil ar dimensions to transcratonic structures in other Precambrian shield areas. The geological development of Hong Kong and neighbouring SE Chi na has largely been controlled by periodic reactivation of this deep c rustal shear zone since the late Neoproterozoic.