Four major magmatic (volcanic–plutonic) episodes occurred
in Hong Kong during the Middle and Late Jurassic, and Early Cretaceous
(Chapters 5 & 6),
a timespan of about 25 million years. Each episode records the emplacement
of hundreds of cubic kilometres of rhyolitic magma, and possibly
more, within the area of Hong Kong alone. However, individual episodes
were separated by periods of a few million years during which time
there appears to have been little or no magmatic activity.
There were marked changes with time in the bulk chemistry of the
magmas, the locations and orientations of individual faults that
acted as magmatic pathways, and the form and locations of the plutons,
dykes and related volcanic centres. These changes are described
in the following magmatic and volcanotectonic reconstructions and
they are considered in relation to the evolution of the local and
regional tectonic regimes.
The dominant regional tectonic influence behind magmatism and volcanotectonism
in southeastern China during Late Triassic to Cretaceous times has
largely been ascribed to westerly subduction of the Kula–Pacific
Plate beneath the Eurasian Plate (Huan
et al., 1982; Guo
et al., 1983). This convergence was accompanied
by movement on large-scale northeast-trending and subsidiary northwest-trending
strike-slip faults, the development of broad open folds, and widespread
magmatic activity along the continental margin (Yanshanian Orogeny,
190–70 Ma).
The first magmatic episode in Hong Kong comprised the Tsuen Wan
Volcanic Group and ‘I-type’ Lamma Suite. Geochemical
evidence (Sewell
& Campbell, 1997) suggests that these magmas were
derived from a mafic igneous-type (‘I-type’) source
rock (protolith) with a late Archaean crustal signature (Darbyshire
& Sewell, 1997).
The magmas may have been generated above an active subduction zone.
Following this initial pulse of magmatism, a second thermal event,
but still part of the first magmatic episode, involved remelting
of these Lamma Suite ‘I-type’ granitoids. This produced
the Lamma Suite ‘A-type’ granitoids. The heat responsible
for this melting event may have been generated by an elevated mantle
geotherm in response to crustal extension (Campbell
& Sewell, 1997).
The second magmatic episode took place some 12 million years later,
but was preceded by a minor thermal and magmatic event at 152 Ma
(Davis
et al., 1997). The second episode occurred in
response to the onset of major crustal extension and addition in
the Late Jurassic (148–146 Ma). It culminated in the production
of ‘A-type’ magmas (e.g. Needle Hill Granite) that may
also represent anhydrous melting of an earlier ‘I-type’
granitoid. These Late Jurassic magmas show mixed isotope signatures
reflecting the involvement of both late Archaean and Mesoproterozoic
crustal components in the source region (Darbyshire
& Sewell, 1997). The change in isotope signature
was associated with the southeastward migration of magmatic activity
across Hong Kong from an area presumed to be underlain by late Archaean
crust to one underlain by dominantly Mesoproterozoic crust.
The third and fourth magmatic episodes, at 144 to 142 Ma and 140
Ma respectively, were associated with renewed pulses of tectonism,
resulting in the development of major tensional structures. These
enabled increasingly alkalic (K2O-rich) magmas to rise along fault
zones of crustal dimensions. There is evidence that during these
two younger episodes of magmatism, there were two distinctly different
sources supplying magma to the surface (Sewell
& Campbell, 1997). One source was characterised by
magma with a low Zr/TiO2 ratio, similar to that of the calc-alkaline
Middle Jurassic magmas (granitic/rhyolitic source). The other source
generated magma with a relatively high Zr/TiO2 ratio, as is typical
of the later alkalic (K2O-rich) magmas (trachytic/monzonitic source).
Given the close proximity of these two contrasting magma-types,
it is likely that they were derived from different depths in the
crust. Of the two, the more alkalic magmas were probably sourced
from deeper crustal levels where the mantle influence was stronger.
However, the differing magmas appear also to reflect areas underlain
by basement of contrasting age with Archaean to the northwest and
Proterozoic to the southeast. As the focus of magmatism continued
to migrate to the southeast, the late Archaean component in the
magmas diminished and was replaced by a dominantly Mesoproterozoic
source component. Magmatism also persisted along the main boundary
zone between the two source terranes and ultimately provided the
main conduit for the passage of strongly K2O-rich magmas to the
surface (Sewell
& Campbell, 1997). Later Cretaceous tectonic events
were accompanied by sporadic intrusions of mafic to intermediate
dykes, suggesting the growing influence of a mantle-derived component
in the source region.
The magmatism and associated tectonism is consistent with a change
in tectonic setting from one of crustal convergence to strong divergence.
This divergence was transtensional to some extent, being accompanied
to varying degrees by strike-slip faulting. The overall setting,
therefore, seems to have evolved from essentially a continental
volcanic arc environment, associated with westerly subduction, to
a back-arc environment. Strong transtensional stresses operative
in this back-arc environment controlled the loci of magmatism and
ultimately the composition of eruptive magmas. The southeastward
shift in the magmatic loci may represent an overall oceanward migration
in the position of the subduction zone as more igneous material
was accreted to the continental margin.
Mesozoic structure in Hong Kong was dominated by the
mainly northeast-trending Lianhuashan Fault Zone (Figure
2.3), within which the main fault trends are variably
northeast- to east-striking. Several faults are thought
to have influenced the individual episodes of volcanism
and intrusive magmatism (Figures
7.1, 7.2,
7.3
& 7.4).
These faults have been identified on the basis of the
distribution of plutons, subvolcanic dykes and stocks,
high viscosity lavas and sills, and calderas. The most
important of these faults are described below.
As volcanism evolved, the locations of the main eruptive centres
and their plutonic equivalents migrated from northwest to southeast,
indicating that the focal point of principal extension also migrated.
There is evidence in the changing orientations and characteristics
of volcanic centres and their controlling structures with time,
that during the Mesozoic volcanic period as a whole, changes occurred
in the regional stress field. These tectonic transformations strongly
influenced the nature, location and geochemistry of volcanism and
are likely to have been ultimately controlled by the evolving plate
tectonic configuration at the time.
Chek Keng Fault
This east–west, varying to eastnortheast–westsouthwest-trending
fault, includes features in a zone up to 1200 m wide
that can be traced for 28 km. The fault was one of the
most important and long-lived controls of volcanism
in Hong Kong. For example, it appears to have formed
the southern margin of the Long Harbour Caldera (Figure
7.3), the northwestern margin of the Sai Kung Caldera,
and was the fissure from which the Clear Water Bay Formation
rhyolite flows were erupted (Chapter
5). In addition, it is also the northwestern and
southeastern limits of the Kowloon Granite and the Sha
Tin Granite respectively, and appears to have controlled
the emplacement of dykes of several ages and compositions,
including quartzphyric rhyolite and monzonite (Chapter
6). The fault may also have been the source from
which rhyolites were emplaced, and conglomeratic fans
derived, within the Lai Chi Chong Formation and its
underlying strata. Finally, the fault may have acted
as a conduit during the eruption of the High Island
Formation, and been one of the bounding structures during
its associated volcanotectonic collapse (Figure
5.28).
Tin Ha Shan Fault
This broad zone of faulting, intrusion and extrusion (up to 1400
m wide) (Figure
7.4), varies in strike from east to eastnortheast
and extends for more than 14 km. The fault passes through
Clear Water Bay in the eastern New Territories and across
the Chai Wan area on eastern Hong Kong Island.
The fault is interpreted as a persistent control of volcanism
that acted as a conduit to the surface for magmas related to several
formations. These include the block-rich pyroclastic facies and
eutaxites (Silverstrand Member) of the Che Kwu Shan Formation and
welded tuffs of the High Island Formation, both occurring in linear
outcrops within the fault. Lavas in the Mang Kung Uk and Pan Long
Wan formations can also be traced to sources along the Tin Ha Shan
Fault. The fault also influenced emplacement of dykes of varying
composition and grain size, including microgranite, quartzphyric
rhyolite and monzogranite, and coincides with the southeastern limit
of the Kowloon Granite.
Tolo Channel Fault
This comprises discrete faults within a zone up to
750 m wide, and can be traced in a northeasterly direction
for at least 30 km through the central New Territories
(Figures
7.1 & 7.2).
For much of its course, the fault lies offshore on the
northern side of Tolo Channel where it appears to define
the northwestern limit of the Long Harbour Caldera.
Northeast of Ma On Shan, the fault comprises two main
structures which may have formed the bounding margins
of a graben during deposition of the Tolo Channel Formation.
The northerly of the two faults is locally exposed along
the northern coast of Tolo Channel. Southwest of Ma
On Shan, one of the two faults extends to Lai Chi Kok
where it has also been referred to as the Lai Chi Kok
Fault (Lai
& Langford, 1996). The other fault continues
more westerly towards Tsuen Wan and broadly coincides
with the northern boundary of the Sha Tin and Needle
Hill plutons, and the northern limit of a swarm of feldsparphyric
rhyolite dykes on Tsing Yi. There may have been a further
westerly extension of the fault along the north coast
of Lantau Island, beyond the inferred ‘East Lamma
Channel fault’ (see below). The more southerly
of the two structures has less obvious influence on
the distribution of magmatic rocks and may be an entirely
post-volcanic feature.
‘Jordan Valley fault’
A fault is inferred to underlie the northeast-trending
Jordan Valley. Mylonite has been observed locally in
outcrop along the trend of the fault but there is some
doubt as to the lateral continuity of this structure.
Hence the structure is referred to informally as the
‘Jordan Valley fault’. The likely continuation
of the ‘fault’ farther northeast runs along
the southeast margin of the Sai Kung Caldera (Figure
7.3), and may also have formed the northwestern
margin of the caldera that ponded the Ap Lei Chau Formation.
The fault also coincides with the northern limit of
the Silverstrand Member of the Che Kwu Shan Formation.
Hence, it may have been active during the emplacement
of the member, acting as a topographic barrier to its
northward spread.
Faults north and south of Lantau Island
A pair of faults, trending eastnortheast–westsouthwest
are tentatively inferred to have lain just offshore
to the north and south, respectively, of Lantau Island.
The interpretation of the fault to the north is complicated
by the later development of the Sha Tau Kok Fault and
for this reason is not formally named. However, this
fault (Figure
7.2) may have been the northerly limit of the zone
of major extension within which both the Lantau Caldera
developed (147–146 Ma) and, at the same time,
the Lantau Dyke Swarm was intruded. Subsequent intrusions
of quartzphyric rhyolite, monzogranite and lamprophyric
dykes also suggest further control of emplacement of
magma by the same zone. The southerly of the two faults
is thought to lie mainly along the southern coast of
Lantau Island, but passing through Chi Ma Wan and possibly
continuing through Hei Ling Chau, and to the south of
Chau Kung To. The zone marks the approximate southerly
limit of the Lantau Caldera and Lantau Dyke Swarm. Substantial
monzogranite intrusions also lie along this trend. Both
of the faults, to the north and south of Lantau Island,
can be regarded loosely as westerly extensions of the
Tolo Channel Fault.
‘East Lamma Channel fault’
A fault is inferred to underlie the East Lamma Channel
which trends northnorthwest–southsoutheast along
the west side of Hong Kong Island (Figures
7.3 & 7.4).
The inferred structure is considered to influence the
location of present-day channels between Ma Wan and
Tsing Yi, and Lamma Island and Hong Kong Island. Furthermore,
the fault has been suggested (Campbell
& Sewell, 1997)
as having been a volcanotectonic structure on the grounds
that the Ap Lei Chau Formation and Kowloon Granite do
not occur to its west. However, the presumed trace of
the fault lies entirely offshore and the existence of
this fault has yet to be demonstrated conclusively by
borehole or other means. Hence, it is referred to informally
as the ‘East Lamma Channel fault’
Crustal extension
Emplacement of magma within the crust, and volcanism in general,
are both facilitated by crustal extension (Cas
& Wright, 1987 and references therein). The corollary
of this is that volcanic rocks can be used to interpret the location
and orientation of extensional zones within the crust.
Considerable crustal extension occurred as a consequence
of the Middle Jurassic to Early Cretaceous volcanism
in Hong Kong. Virtually all of the central and southern
parts of Hong Kong comprise magmatic and volcanic rocks
of that age, although remnants of pre-existing crustal
rocks may occur at greater depth. Specifically for example,
on Lantau Island (Figure
5.27), approximately 75% of the width (northnorthwest–southsoutheast)
of the eastern part of the island, in a zone 8 km wide,
comprises easterly and eastnortheast-trending dykes
of the Lantau Dyke Swarm. These were intruded during
the second magmatic episode. Further westsouthwest,
the generally eastnortheast-trending Lantau Caldera
(Figure
5.27) also occurs within the zone, and developed
at broadly the same time. Similarly, to the eastnortheast,
the eastnortheast-trending Sha Tin and Needle Hill plutons,
also part of the second magmatic episode, lie within
the zone (Figure
5.27). Arguably, the Sai Kung and Long Harbour calderas
of the third magmatic episode may also lie within an
easterly extension of the same zone of extension.
Most of Lantau Island can therefore be regarded as silicic crust,
several kilometres wide, and probably of substantial thickness too,
created during the Late Jurassic (Campbell
& Sewell, 1997). The
granites, dykes and volcanic rocks, respectively, represent the
deeper to shallower spectrum of levels of crustal development within
a broadly extensional zone. The section of silicic crust exposed
on Lantau Island is analogous in many respects to the layered structure
of oceanic crust created at extensional mid-oceanic ridges: the
granites on Tsing Yi and in Sha Tin are comparable to the lower
part of layer 3 of oceanic crust (gabbroic magma chamber); the feldsparphyric
dykes are equivalent to the sheeted dyke complex of upper layer
3; and the pyroclastic rocks occupy the same position as pillow
lavas in layer 2. The late stage volcaniclastic and epiclastic deposits
seen in the upper parts of the Lantau Volcanic Group can even be
compared with layer 1 sediments.
Similar extension appears to have been associated with both the
Repulse Bay and Kau Sai Chau volcanic groups. The extension was
focussed respectively along the east- to eastnortheast-trending
Tin Ha Shan Fault, and the east- to eastnortheast-trending Chek
Keng Fault.
Rhyolitic volcanism and granitic magmatism, indicative of extension,
ceased in Hong Kong after the eruption of the High Island Formation
(Kau Sai Chau Volcanic Group) at c.140 Ma. However, Li (2000) has
shown that ‘A-type’ granitic magmatism, and within-plate
basaltic magmatism, continued throughout the Cretaceous in southeast
China, to the northnortheast of Hong Kong. Further magmatic episodes
occurred at 129–122 Ma, 109–101 Ma and 97–87 Ma.
Li (op. cit.) has interpreted this magmatism as indicating a dominantly
extensional environment in the region.
A notable feature of the later Cretaceous magmatic episodes is
that they were largely concentrated in a fault-bounded zone to the
west of the Haifeng Fault, and the Lianhuashan Fault Zone as a whole.
Hence, the trend that has been identified in Hong Kong of volcanism
and magmatism migrating southeasterly with time, appears to have
been extended on a regional scale into the Late Cretaceous.
Crustal transtension
Selective extension of the crust can be controlled by large-scale
strike-slip movements, with transtension generated at dilational
jogs, or bends along the fault. Thus, for a dextral fault, a Z-shaped
bend would be potentially transtensional whereas an S-shaped bend
would be transpressive, and vice versa for a sinistral fault. Z-shaped
inflection patterns are typical of the Tai Mo Shan area, and these
appear to have controlled the locations of granodiorite intrusions
related to the first magmatic episode (Middle Jurassic). This suggests
the likelihood of a dextral shear component on the main eastnortheast-trending
faults at the time of Tsuen Wan Volcanic Group volcanism.
The best example in Hong Kong of obliquity of the orientation of
extension relative to major bounding faults is that
shown by the Lantau Dyke Swarm associated with the second
magmatic episode (Figure
5.27). Individual dykes within the swarm are commonly
easterly-trending whereas the zone as a whole trends
mainly eastnortheast. This sense of obliquity can be
interpreted in terms of a pure shear model, as indicating
a dextral strike-slip component of movement along the
bounding margins of the zone, whereas essentially north–south
extension occurred within the zone.
