Superficial Geology

East Hong Kong Island (USE). A prominent straight fault trace extends southeastwards from Causeway Bay
through Tai Tarn Country Park to Tai Tarn Harbour. In Mount Butler Quarry (3893 1435) the rock face
contains very close, vertical, parallel joint planes with a strike direction of 150°. An ENE-striking fault
extending from Repulse Bay to Chai Wan is displaced horizontally 100 m by the NW-SE fault (4048 1255).
Numerous photolineaments have been recognized in the granites of eastern Hong Kong Island, and many of
these may be fault traces. The Lei Yue Mun passage follows a marked NW-SE trending topographic feature,
but geophysical traverses across the passage have revealed no evidence for faulting. The NW-SE trending
faults dominate the circular histogram (Figure 17), the ENE trend being less well developed. The dykes are
widely dispersed, but no easterly trend is indicated (Figure 18).
Lamma Island, Ap Lei Chau and Ocean Park (15NW & SW). Significant lineaments trend NE through Sok
Kwu Wan (Picnic Bay) and Shek Pai Wan (3280 0677) on Lamma Island. ENE-trending lineaments cut Ap
Lei Chau and join an E-W feature that forms Aberdeen Harbour and the Wong Chuk Hang valley. This
lineament may represent an early fault, and is marked by dyke-like intrusions of quartz syenite at intervals
along the valley. The histogram of faults and photolineaments (Figure 17) does not readily identify dominant
trends, but the histogram of dykes (Figure 18) shows a marked E-W alignment corresponding to the rhyolite
dyke swarm of central and southern Lamma Island.
Stanley, D'Aguilar Peninsula and the Po Toi Island Group (15NE & SE). Faults and photolineaments in this
area are dominated by N-S and SSE trends on Hong Kong Island, and NE and ENE trends in the Po Toi
Island Group. Dyke trends are widely dispersed but are most commonly NE to NNE. The Po Toi—Sanmen
Islands Fault (Burnett & Lai, 1985) is inferred to pass several kilometres south-southeast of Po Toi Island,
striking in an ENE direction, but the present survey has found no evidence to support the existence of this
fault zone. On the southern part of Waglan Island faults with approximate strikes of 55° (4939 0497) show
minor sinistral displacements; other faults striking 175° have minor dextral displacements.
Joints
The equal area point plots and density contours for the joints of the district (Figures 19 & 20) give
only general indications of trends, and do not necessarily represent the joint trends at any specific
locality. Only systematic joints were measured, i.e. those that are planar and parallel or subparallel
and form joint sets. However, the density of sampling has been inconsistent because of
lack of exposure in some areas, for example inland on Lamma Island, and abundant exposure in
other areas, for example in East Kowloon, where numerous fresh cut slopes were available for
examination during the field survey.
Details
Granite outcrops, excluding Lamma Island. In all areas a girdle pattern of joint poles is developed, although
on 11SW this is resolved into two maxima, strike 170°/90° dip and strike 030°/90° dip. The joints are mostly
near vertical, and a break in the girdle at the north and south of the net indicates a marked absence of
E-W striking joints. In all cases the girdle is around the perimeter of the diagrams, indicating no significant
deviation from vertical. The weakest girdle development is found in the area of 11NW, where there is a
marked concentration around strike 160°/80°E dip. Only in 11SE is there a significant number of roughly
sub-horizontal joints.
Granite of Lamma Island. Two concentrations (strike 005°/85°W dip and strike 105°/85°S dip) are possibly a
conjugate set of shears, indicating the main compression direction as NE-SW. Few sub-horizontal joints are
present, but the sample is small. A girdle distribution is not developed in this area.
Volcanic rocks, excluding Lamma Island. In all areas girdles are well-developed, but unlike the granite areas
where joints are dominantly vertical, there is a marked deviation combined with a marked concentration.
The concentration of poles is consistently strike 010° to 030°/80°W dip. In all areas the contours can be
resolved into two clusters; a strong one strike 030°/80°W dip, and a weak one strike 120°/90° dip. In 11SW &
11SE a near-horizontal joint set is developed, which is absent elsewhere. The complex girdle pattern,
particularly in 11SW, may indicate a predominance of polygonal jointing (Whiteside, 1986).
Volcanic rocks on Lamma Island. Only a limited area of volcanic rocks occurs on Lamma Island, and the
marked joint concentration indicates joints striking 040°/80°N dip, i.e. similar to other areas. Neither a girdle
nor any sub-horizontal jointing is present on a very simple plot pattern.
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