Chapter 6 Structure
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Chapter 6 Structure The major structural features of the district include folds in Carboniferous strata surrounding The Brothers islands， regional scale Post-Yanshanian basement faults， Late Jurassic to Early Cretaceous faults related to volcanic and plutonic activity， photolineaments and joints. The eastnortheast-striking Lantau Dyke Swarm also exerts a strong geomorphological influence， especially in the eastern part of the district where the rhyolite dykes are interspersed with less resistant granitic material. Many of the larger valleys within the district are interpreted as being underlain by major faults. As found elsewhere in Hong Kong， these faults are commonly associated with zones of weaker material which is more susceptible to weathering. However， some lineaments seen on aerial photographs may be joint related rather than fault related. Sometimes there is no field evidence of fault or joint control of the photolineaments， and， in the case of volcanic and sedimentary rocks， many of the lineaments are more closely related to the strike of bedding. Folds The metasedimentary rocks forming The Brothers islands are considered to belong to the southern continuation of an outcrop of Carboniferous strata extending from Man Kam To in the northern New Territories to Tuen Mun in the western New Territories. This zone has been intruded by the Middle Jurassic Tai Lam Granite. The variations in bedding attitude of the metasedimentary rocks on The Brothers islands， together with their outcrop pattern， indicate that the strata are tightly folded into an asymmetric plunging syncline with the fold axial plane striking toward the northeast. Marble encountered in offshore boreholes to the south and east of East Brother island is correlated with the Yuen Long Formation in the western New Territories and stratigraphically underlies the graphitic siltstones and sandstones of the Lok Ma Chau Formation. The tightness of the folding and proximity to the granitic intrusion suggests that the fold might be an annular structure， formed as a result of diapiric emplacement of a granite pluton. Faults The major faults of the district can be broadly divided into those which are associated with Late Jurassic to Early Cretaceous volcanism and those which are related to post-Yanshanian movement (Yanshanian Orogeny: 190 - 80 Ma， Jahn et al.， 1976). Faults associated with the former generally reflect extension on easterly， and eastnortheast-trending structures， whereas those related to the latter mainly reflect transtension on major northeast-， and northwest-trending structures (Campbell & Sewell， 1997). The most prominent volcanic-related structure of the district is a major fault marking the boundary of the Lantau Caldera (Langford et al.， 1995; Campbell & Sewell， 1997; Sewell et al.， 2000) (Figure 8). This fault extends for approximately 5.7 km in a northwesterly direction from Tai Ho in the east to Sha Lo Wan in the west， and is offset by both the Shek Pik Fault and the Por Kai Shan Fault. In the east， the northern boundary of the caldera intersects the eastern boundary marked by a north-trending fault. In the west， the caldera boundary is partly intruded by the Tong Fuk Quartz Monzonite. Details of the caldera collapse are uncertain， but the collapse is thought to have occurred during the main episode of eruption of the Lantau Volcanic Group (c. 146 Ma). In the east， the bounding fault is sometimes marked by the presence of a thick (up to 5 m wide) quartz vein， and is easily distinguished in the field and on aerial photographs by a prominent break in slope， the volcanic rocks being more resistant to erosion. Minor syndepositional faulting associated with soft sediment deformation within the volcanic succession is marked by variably oriented faults on Por Kai Shan (1398 1633). Some of these have been inferred from rapid pinching out of intercalated sedimentary units. Post-Yanshanian faults dominate the structure of the district forming prominent northeast- and northnorthwest- trending features. The most pronounced feature is the northeast-trending Shek Pik Fault. The Shek Pik Fault， which is inferred to be a segment of the Sha Tau Kok Fault farther north， transects the district in a southeastward direction from offshore northern Lantau Island to Shek Pik， passing onshore just to the south and east of Tung Chung. The Shek Pik Fault offsets the northern boundary of the Lantau Caldera by c. 3.1 km in the vicinity of Tung Chung and provides an excellent example of sinistral displacement (Sewell et al.， 2000). 34 Figure 8 - Principal Structural Features of Tung Chung and Northshore Lantau Island ChungandNorthshore ofTung Features 8-PrincipalStructural Figure LEGEND 814 000E 818 000E Photolineament Fault Concealed or Inferred Fault Caldera Boundary SIU MO TO 822 000N Volcanic Layering HA KOK TSUI CHEUNG Palaeodrainage Network THE BROTHERS SOK Rockhead Contours (mPD) TAI MO TO YAM O WAN Scale 1:60 000 -80 01 2 Km -60 -20 TSZ KAN CHAU -40 0 -20 40 20 0 -40 20 -60 40 YAM O 291 60 -80 80 100 -100 Sham Shui 200 -120 100 -140 Kok 200 -160 200 TAI CHE Runway TUNG 200 100 n Limit atio 200 m -40 la 100 ec R 35 CHEK LAP KOK -80 -60 100 -40 YI PAK AU Hong Kong Proposed -20 International Airport 200 Discovery Bay Runway -40 818 000N -40 TAI HO 300 465 -20 -40 -60-80 PAK MONG -20 WAN 400 -100 400 300 KEI TAU KOK LO FU TAU -60 200 100 40 200 100 200 -40 -100 -80 77 -60 -120-140 -20 HAU HOK 0 SHA LO 200 -40 TAI HO WAN WAN -40 NIM SHUE WAN -40 -20 100 TIN SAM -20 TUNG CHUNG 0 BAY 300 Reservoir TAI PO 100 300 POR KAI SHAN 482 TUNG CHUNG 100 200 100 Golf Course NEW TOWN 300 POK TO YAN 200 100 200 529 200 300 100 HUNG FA 275 300 400 NGAN 300 400 100 200 TAI SHUI HANG TUNG 500 200 300 CHUNG 200 200 VALLEY 500 600 100 100 721 400 400 300 300 200 100 LANTAU ISLAND 200 200 LANTAU400 CALDERA 500 200 500 300 100 324 WONG LUNG HANG MUI WO Restoration of this sinistral displacement， along with a 250 m sinistral displacement on a subparallel northeast- trending fault extending from Wong Lung Hang to Pak Mong (see Fletcher et al.， 2000)， realigns the northern caldera boundary， and also reveals a northward extension of the eastern caldera boundary fault at Kei Tau Kok， east of Tung Chung. Rockhead contours for the offshore area north and east of Tung Chung have revealed a prominent ridge extending north from the small promontory at Kei Tau Kok. Gravity profiles for the offshore area have also revealed a sharp east-west change in gradient across this ridge， separating metasedimentary rocks， including marble， in the Tung Chung area from granite farther east. Therefore， it is likely that the western margin of the ridge marks the offshore extension of the eastern caldera boundary fault， which currently forms the axis of Tai Ho Wan valley. Restoration of the sinistral displacements also suggests the presence of some Post-Yanshanian faulting. For example， the northnorthwest to northwest-trending faults forming Tung Chung valley and Wong Lung Hang valley have not been displaced by the Shek Pik Fault and， therefore， must represent a period of younger fault movement. Rockhead contours， together with gravity (Kirk et al.， 2000) and boomer surveys， for the offshore area along the north Lantau coast have revealed areas of deep weathering which appear to coincide， in part， with the offshore extension of the Shek Pik Fault. The rockhead contours and data on superficial deposits also suggest a series of westnorthwest- and northwest-trending buried drainage lines which have been captured by a buried northeast-trending valley. The northwest-trending drainage lines appear to be partly controlled by northwest-trending faults described above， whereas the westnorthwest-trending drainage lines are subparallel to the feldsparphyric rhyolite dyke orientations. The buried valley which marks the offshore extension of the Shek Pik Fault along the north Lantau coast， diverges to the north adjacent to Ha Kok Tsui before intersecting a major buried drainage channel trending westward north of The Brothers islands. Gravity and boomer surveys， together with borehole data， suggest that to the north of Ha Kok Tsui， the buried valley follows the boundary between the Carboniferous metasedimentary rocks and Middle Jurassic to Early Cretaceous igneous rocks. The buried drainage channels are marked by deposits of marine sand which have been partly dredged for reclamation projects. Photolineaments Laterally continuous photolineaments within the district have mostly been mapped in the volcanic area around Pok To Yan (1360 1591). A northeast-trending photolineament in the southeastern corner of the district follows a small valley， but there is no field evidence that this valley is fault controlled. Farther west， on the flanks of Pok To Yan， two relatively short (200 m) photolineaments have been mapped which coincide with narrow (<20 m) zones of hydrothermal alteration. On the southwestern and western flanks of Pok To Yan， several northeast-trending photolineaments have been mapped and these generally appear to be subparallel bedding contacts within the volcanic succession. Joints A large number of joint orientations have been measured within the district. Many were obtained during an engineering geological study of the Tung Chung area undertaken by Woods (1993). The available joint data are summarized in Figure 9. The dominant subvertical joint orientation in granitic rocks of the district is northnortheast to northeast， with a conjugate eastsoutheast-trending set. A subordinate northnorthwest-trending subvertical joint set is also present along with a flat-lying to low angle joint set. In the water supply tunnel from Ngong Ping to Shek Pik Au， close to the Tung Chung valley， the dominant subvertical joint orientation in the volcanic rocks is northwest trending， whereas between the Tung Chung and Wong Lung Hang valleys， eastnortheast- and northeast- trending joints predominate.