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Pre-Yensfaanlan StructureWithin the district rocks that pre-date Yenshanian magmatism, are regarded as structuralbasement. These include the Carboniferous sedimentary rocks, exposed on western Lantau Islandand at The Brothers islands, and correlated with the Mai Po Member of the Lok Ma ChauFormation in the northwestern New Territories (Langford et al, 1989).Pre-Yenshanian folding of basement strata is inferred from limited bedding measurements andyounging directions. Details of folds on East and West Brother islands are described below. OnLantau Island, the basement structure is complex, and probably includes tight folding, althoughthis has not been resolved. No consistent pattern of deformation was identified and it seems likelythat Yenshanian faulting and pluton emplacement have obscured any earlier trends.Yenshanian Structural DevelopmentThe earliest major structural event recorded in the district is reverse faulting, which placed thePalaeozoic basement over strata of the Jurassic Tsuen Wan Volcanic Group. The contactbetween the two is the Tai 0 Fault, exposed between Tai O and San Shek Wan on Lantau Island.Though generally shown as a reverse fault on the published map (Sheet 9), the structure wasmodified by later development of the Lantau Caldera (see below). The faulted contact betweenPalaeozoic sedimentary rocks and the volcanic rocks is locally vertical (e.g. 7790 5680).Foliation adjacent to the fault is poorly developed in a zone tens of metres wide.In the New Territories, Carboniferous rocks have been similarly thrust, along the San Tin Fault,over the Jurassic Tsuen Wan Volcanic Group. Foliation spatially associated with the fault occursin a very broad zone. The thrust, which was probably directed to the south or southeast, has beeninterpreted as Yenshanian in age by Langford et al (1989).Early Intrusive RocksReverse faulting of the basement was followed by an early phase of granite intrusion. Thesegranites, and minor granodiorite, are generally medium- or fine- to medium-grained, and arewidely distributed in the district. They intrude Tsuen Wan Volcanic Group rocks (e,g, on TsingChau Tsai Peninsula) and Carboniferous basement strata (e.g. at San Shck Wan), and truncatethe reverse fault in the basement.Rhyolite Dyke ComplexA voluminous rhyolite dyke complex developed in the district as an early response to a change toan extensional tectonic regime (Sewell et al, 1991). The feldsparphyric rhyolite dykes are thedominant lithology in northeastern Lantau Island, and are also common throughout the centraland eastern parts of the district. They are overwhelmingly oriented eastnortheast, and show littlevariation from this trend, which is an important indication of contemporaneous extensional, ortranstensional orientations. The erosively resistant dykes are evident on aerial photographs anddefine the eastnortheast structural grain apparent on satellite images.The dykes intrude Tsuen Wan Volcanic Group and other early intrusive rocks, They are rare,however, within the Lantau Formation, which probably largely post-dated (heir emplacement.They are not found within the Carboniferous rocks of the district, although this is probablybecause of their limited distribution to the north and west rather than any age relationship, Northof the district, feldsparphyric dykes intrude Carboniferous strata in the northwest New Territories(Langford et al, 1989). The dykes extend to the northeast of the district, but only as a relativelyminor component of the geology.96