Download Chapter 6 Structure

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 northnorthwesttrending 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
300
400
500
200
500
0
10
200
TIN SAM
2 Km
CALDERA
0
30
0
40
HAU HOK
WAN
way
Run
CHEK LAP KOK
way
Run
Hong Kong
International Airport
LANTAU
200
SHA LO
WAN
0
Scale 1:60 000
1
TUNG
CHUNG
VALLEY
-40
-20
-40
-80
0
300
20
100
-140
-100
324
-60
-120
TUNG CHUNG
NEW TOWN
TUNG CHUNG
BAY
77
-60
-40
0
-2
0
-2
200
40
0
20
100
300
300
200
ISLAND
500
400
300
482
-20
721
200
300
0
60
500
400
PAK MONG
POR KAI SHAN
100
WONG LUNG HANG
LANTAU
0
20
0
40
0
30
200
KEI TAU KOK
-20
-40
0
30 POK TO YAN
529
TAI PO
-80
-600
-10
-40
TAI MO TO
d
se
po
Pr
o
Rockhead Contours (mPD)
la m
100
TAI HO
WAN
-40
c
Re
0
-8
ati on
0
-6
L im
0
-4
it
0
-6
TSZ KAN CHAU
THE BROTHERS
SIU MO TO
20
0
100
TAI HO
0
Palaeodrainage Network
0
-2 -40
0
10
Volcanic Layering
0
20
Caldera Boundary
-
0
MUI WO
300
40
-40
100
10
0
0
20
200
Sham Shui
0
Kok
16
HUNG FA
NGAN
0
-8
0
Concealed or Inferred Fault
10
-1
-1 00
-1 20
40
30
Fault
0
465
300
10
-20
0
0
-4
0
0
40
275
Reservoir
20
20
0
80
60
YI PAK AU
200
0
20
TAI CHE
TUNG
-60
-8
Golf Course
LO FU TAU
0
Photolineament
40
LEGEND
0
Figure 8 - Principal Structural Features of Tung Chung and Northshore Lantau Island
10
0
0
35
0
-40
10
814 000E
10
818 000E
10
0
200
200
-20
100
20
0
NI
M
SH
U
E
10
0
291
AN
W
818 000N
Discovery Bay
200
0
20
0
10
CHEUNG
SOK
YAM O
WAN
YAM O
0
TAI SHUI HANG
40 20
HA KOK TSUI
822 000N
Restoration of this sinistral displacement， along with a 250 m sinistral displacement on a subparallel northeasttrending 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 northeasttrending joints predominate.
In the granitic areas of the district， joint spacings are typically widely spaced (between 100 mm and 600 mm)，
whereas in the volcanic rocks， the joint spacing on surface outcrops， and from underground tunnels ranges
between very closely spaced and widely spaced (50 mm and 400 mm). Zones of very closely spaced joints
have been noted in rock outcrops in areas which coincide with major fault zones (e.g. the upper Tung Chung
valley and the coastal area between Tung Chung and Tin Sam). In certain locations (e.g. Kau Kei Tuk)，
exceptionally wide joint spacings (up to 2000 mm) have been recorded (Woods， 1993).
36
a
N
N
b
2%
4%
6%
8%
10%
8%
6%
4%
2%
No. of Joints measured=82
No. of Joints measured=68
N
d
c
2%
4% 6%
N
2%
4%
6%
No. of Joints measured=86
e
2%
No. of Joints measured=95
4%
6% N
No. of Joints measured=87
Figure 9 - Contoured Pole Plots of Joints Measured for All Rock TTpes on Tung Chung and Northshore
Lantau Island (after Woods， 1993). a) Sha Lo Wan to Hau Hok Wan， 的) Hau Hok Wan to Tin Sam，
c) Tung Chung to Lau Fau Sha， d) Lau Fau Sha to Kei Tau Kok， and e) Kei Tau Kok to Pak Mong
37
Rock Quality Designation (RQD) values for slightly weathered granite， feldsparphyric rhyolite and volcanic
rock are 55%， 47% and 56% respectively， and these values generally reduce with increasing weathering
grade. Average RQD values of 48%， 28% and 30% respectively are found in moderately weathered granite，
feldsparphyric rhyolite and volcanic rock. Fracture index values range from 5 to 15 in all rocks， and there is
no significant increase in fracture index with increasing weathering grade， or measurable variation among
the different rock types (Woods， 1993).
Joint persistence in granitic rocks observed from coastal exposures indicates that the dominant joint sets
(e.g. northeast- and eastnortheast-trending joints) may range up to to 30 m. Similar persistence has been
observed in shallowly-dipping sheeting joints， whereas minor random joints have a persistence of only a few
10s of mm. Joint persistence in volcanic rocks observed in the water tunnel from Ngong Ping to Shek Pik Au
averages 10 m (Woods， 1993).
Minor variations in joint regularity have been recorded among different rock types. For example， smooth and
planar joints are generally a feature of volcanic rocks， whereas joints in the granites and feldsparphyric
rhyolites are often irregular， and for gently dipping joints， often wavy.
38