The geologicalstructure of the district is complex,and the volcanicrocks have beenseverely disrupted by igneousintrusions and subsequentfaulting. lies closeto the intersectionof three regional fault sets that are thoughtto have beenactive in the mid-Tertiary (Lai, 1977). The main faults in the district are shownin Figure9. Someof thesefaults also form photolineaments

Faults The areais cross-cutby three sets of faults: one trending NW, one trending E, and one trending N. The north-trendingset has offsetthe earlierNW-trending faults. A prominentnorth-trending fault throughthe district is delineatedby a 5m-thick quartz vein which is encounteredin boreholesand is exposedon the north-facing slopes. The other faults are characterisedmainly by thin «50 mm) shearzones within the bedrock. The shear zones may contain highly-brecciatedmaterial comprising clay-size particles up to gravel-sizerock fragments. Moy'ementof solutionsalong thesefault planesmay have enhancedweather- ing and promotedthe developmentof the soft to fInD silty-clay gougematerial. Lineamentsdetermined from aerial photographicinterpretation are generallysubparallel to the main fault trends. Field relation- ships indicate that someshearing has also occurred subparallelto dyke margins. Recrystallisedmylonite (pseudotachylite)is sometimesobserved along dyke-volcaniccontacts, indicating fault movement(Plate 13). The senseof movementof the fault setsis difficult to judge, althoughthe north-trendingset appearsto be mainly dextral strike-slip(Figure 9).

Faults in offshore areasare inferred from boreholedata, geophysics and regionalgeology considerations. Boreholesdrilled as part of the site investigationhave revealed a broad (100 m) zone of microfractured porphyritic dacite ( Formation) in the westernpart of the Tsing Yi Channel. This fault zone is tentatively namedthe Tsing Yi ChannelFault and may correspondto southwestern

Plate 13 -PseudotachyliteExposed at the Margin of a Mafic Dyke at Sai Tso Wan(27892237)

31 extensionof the TsuenWan -Chinese University fault shownon the 1:20 000 publishedgeological map. In the Channel,west of Tsing Vi, a major NW-trending fault is inferred from geophysicaldata (B.G.S., 1993).This fault is an extensionof the LammaChannel Fault to the southeast.

Joints Joint orientationsare plotted on an equalarea projection in Figure 10. The dataindicate the development of at leastthree subverticalsets of joints and a low-angle setdipping west at about30°. The joint setsare equally developedin all major rock types. The dominantjoint sethas an orientationof 70/235 (dip/dip direction). Jointing is generallyclosely to mediumspaced in weldedtuff and granitoid rocks,whereas it becomesvery closelyto closelyspaced in mafic dykes. Within granitoid rocks,joints are frequently iron stainedor coatedwith kaolin, whereasin weldedtuff, joint facesare commonlychloritised.

,

N

)

d No. of Joints measured = 1140

Pole No. of Concentration ~ ~ 1-2% 11-23 + ~ 2-3% 23-34 ~ 3-4% 34-47

IIIIll 4-5% 47-57

Figure 10 -Contoured Pole Plot ofJoints Measuredfor All Major Rock Typeson Tsing Yi :

33