Hong Kong lies on the SE margin of the Cathaysia Block and straddles the 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 proposed gravity model of Hong Kong consists of a heterogeneous upper crust underlain by middle to lower crust composed of a felsic Archaean segment, approximately 25 km wide, flanked by more mafic Proterozoic crust. The southern boundary of the felsic segment dips steeply to the north, whereas the northern boundary is subvertical. Euler gravity anomalies define the fundamental faults in the upper crust, and many of these can be correlated with faults that have been mapped at surface. The middle to lower crustal discontinuities are considered to be associated with a major shear zone within the Cathaysia Block that has similar dimensions to transcratonic structures in other Precambrian shield areas. The geological development of Hong Kong and neighbouring SE China has largely been controlled by periodic reactivation of this deep crustal shear zone since the late Neoproterozoic.

Widespread Mesozoic igneous rocks in Hong Kong form part of the giant Mesozoic igneous province in Southeastern China. The Tuen Mun Formation in the Northwestern New Territories is the oldest volcanic unit known in Hong Kong. This formation contains a lower unit composed of dominantly volcanic breccia, minor dacite, rhyolite, and tuff, and an upper unit of basaltic andesitic to andesitic lava. The volcanic rocks from the lower unit contain zircon grains with a mean U-Pb age of 163.4 ± 0.9 Ma, synchronous with the adjacent Tsuen Wan Volcanic Group in the region. Good inter-correlations between different elements in the Harker diagrams indicate that the volcanic rocks with high SiO 2 (e.g. >65 wt.%) are essentially physical mixtures of intermediate magmas and various xenoliths of country rocks, characterizing the palaeo-volcanic plug facies. Preservation of large amounts of xenoliths of sandstone and marble in the volcanic rocks, which is revealed both in field and under microscopic observation, further supports this hypothesis. The original andesitic magma, represented by the intermediate volcanic rocks with SiO 2 <65 wt.%, exhibit enrichments of LILEs and LREEs compared with N-MORB, but have negative Nb-Ta-Ti anomalies in the primitive-mantle normalized diagram, similar to those derived from metasomatized mantle wedge in an arc environment. Large ranges of SiO 2 , MgO, and CaO contents, and negative Eu and Ce anomalies are consistent with significant fractional crystallization of likely olivine/pyroxene and plagioclase. We propose that during the Late Jurassic subduction of the Palaeo-Pacific Plate underneath the coastal continental arc in Southeast China generated prolonged and cyclic volcanism in the northern part of Hong Kong.

Hong Kong lies within a major NE-trending fault zone that has been reactivated during several tectonic episodes since the Palaeozoic. Three main faults sets are recognized in Hong Kong: a dominant NE-trending set, an ENE- varying to E-trending set and a subordinate NW-trending set. Over the last 1000 years, within a distance of 350 km of Hong Kong, there have been about 40 earthquakes with magnitudes of over 4.75, and of these 11 had magnitudes of over 6.0. Microseismic events in the last ten years are diffuse but may be associated with a major NE-trending fault and a fault intersection. Thermoluminescence (TL) dates of fault gouge suggest that there have been three episodes of recent fault activity in Hong Kong; these occurred at approximately at 100 000, 190 000 and 270 000 years BP. TL dating of alluvial sediments also indicates fault activity in the Late Pleistocene.

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