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The Lantau CalderaThe clearest structural feature on the maps is associated with the distribution of the LantauFormation, (assigned to the Repulse Bay Volcanic Group), which occupies most of westernLantau Island. The outcrop of the Lantau Formation is fault bounded in all but the south, whereit conformably overlies volcanic rocks of the Tsuen Wan Volcanic Group. Bedding attitudes insedimentary intercalations are variable, but the regional outcrop pattern suggests that thesequence dips broadly northwards at 20-30°. The steeply faulted northwestern margin, againstCarboniferous rocks, has been discussed above. To the north, part of the faulted contact (west ofTung Chung) has been intruded by quartz syenite, but from age relationships it is inferred to havebeen originally faulted against granite. East of Tung Chung, and along the eastern contact,rhyolite dykes and granite are abruptly truncated by faulting against the outcrop of the LantauFormation. This largely fault-bounded outcrop pattern is interpreted as a primary volcanotectonicrelationship, and the entire structure is inferred to be a caldera (Chapter 4) or similarvolcanotectonic collapse structure. From stratigraphic and structural relationships, the caldera isyounger than other adjacent volcanic rocks.The internal structure of the caldera is markedly asymmetrical, and appears to reflect the patternof bounding faults. Hence, in contrast to the northward-dipping, and conformable succession inthe south, the internal structure is more complex northwards, although poor exposure limits thescope of this interpretation. Examples of this complexity include: abrupt changes in beddingorientation, lithology and degree of hydrothermal alteration; folding; local unconformities; andfaulting. All of these features are interpreted as primary, and related to the evolution of thecaldera, rather than to any later event.Associated Syenitic RocksFour small, fine-grained quartz syenite stocks and several minor dykes ring the Lantau Caldera.In the northwest, contact metamorphism of rocks adjacent to the caldera, indicate that syeniteintrusion post-dates the main caldera development.Late or Post-Yenshanian FaultingThe major faults included in the preceding discussion were generally identified and mapped fromdisplaced contacts in the solid geology. Numerous other faults in the district are mainly inferredrather than observed because of poor exposure. They are located on the basis of minor offsets,lineaments on air-photographs (and other geomorphological evidence), ground investigation data,geophysics, extrapolation from outside the district and, where possible, field observation. Faulttraces tend to coincide with Quaternary-filled valleys. However, as the published maps do notshow faults beneath Quaternary deposits, they tend to understate their importance. The faultspost-date the development of the Lantau Caldera, The structure of offshore areas (i.e. in thenorthwest and southeast of the district) is poorly known. Correlations with structures in the restof the Territory are tentative.The late faults of the district have been subdivided into a number of sets based on theirorientation, The sets arc apparently related, so are therefore described together. Theinter-relationships between sets, including their age-relationships are likely to be complex, so thatthe relationships suggested below should be considered tentative, Figure 10 presents a sketch ofthe solid geology, showing faults inferred from all available data.Northwest-trending FaultsA series of clear, northwesMrending photolineaments are regularly spaced along the length ofLantau Island, The Kap Shui Mm Channel Fault, between Lautau and Ma Wan islands,97