Basin Research (2000) 12, 307±327
Normal fault displacement characteristics, with
particular reference to synthetic transfer zones,
Mae Moh mine, northern Thailand
C. K. Morley*.and N. Wonganan*².
*Department of Petroleum Geoscience, University of Brunei
Darussalem, Bandar Seri Begawan, 2028, Negara Brunei
Darussalem
²Department of Geological Sciences, Chiang Mai University,
Chiang Mai, 50200, Thailand
ABSTRACT
Normal faults in middle Miocene sedimentary rocks of the Mae Moh mine have their
geometries and displacement patterns well constrained by outcrop and subsurface data.
Seventeen of the largest faults are described and analysed. The 17 faults have very different
displacement pro®les, with differences between the pro®les being explicable in terms of the
linkage of initially separate faults. Lateral tip gradients show a large range from 0.035 to 0.6.
Fault displacement±length (D±L) relationships plot with considerable scatter. Following
previous studies, data points with relatively high D±L ratios are attributed to displacement
transfer between overlapping faults; faults with relatively low D±L ratios display linkage of two
or more faults. In transfer zones conservation of displacement between the faults ranges from
high (> 70%) to low (10±40%). This appears to depend upon whether the faults propagated
relatively early (high displacement transfer) or late (low displacement transfer) into overlapping
con®gurations. Where displacement transfer is low, extension appears to be conserved in a
broader zone on adjacent mappable faults, with little increase in ductile deformation.
INTRODUCTION
Normal fault propagation has been investigated in a
number of outcrop-based studies centred on natural
exposures and coal mines (e.g. Barnett et al., 1987.; Walsh
& Watterson, 1988., 1992.; Peacock & Sanderson, 1991.;
Cartwright et al., 1995.; Huggins et al., 1995.). These
studies have documented a number of features concerning
fault displacement patterns, propagation and linkage.
Some of the key observations include:
10Relay zones between synthetic faults are very common
features in fault zones that evolve through different
geometries with increasing strain (Morley et al., 1990.;
Peacock & Sanderson, 1991., 1994.). In some cases a
lithological heterogeneity can be identi®ed which was
responsible for causing relay features to develop (Huggins
et al., 1995.). Relay zones can form either between two
separate faults or by bifurcation of a single fault (Larsen,
1988.; Peacock & Sanderson, 1991., 1994.; Huggins et al.,
1995.).
Correspondence: C. K. Morley, Department of Geology &
Petroleum Geology, University of Aberdeen, Aberdeen AB24
3UE, UK.
# 2000
Blackwell Science Ltd
20Commonly, the aggregate pro®les of overlapping faults
are similar to the smooth bell-shaped displacement
patterns found on a single fault trace. However, aggregate
throw often decreases in the vicinity of relay zones
suggesting throw is transferred onto other structures,
such as more ductile features within or beyond the relay
zone (Walsh & Watterson, 1991.; Nicol et al., 1996.). This
drop in throw between transfer zones has been observed
at many scales, ranging from small outcrops (Peacock &
Sanderson, 1991.) to rift segments (Morley, 1988.).
30Two main models for fault development have been
considered: isolated radial growth (e.g. Barnett et al.,
1987.; Walsh & Watterson, 1987.; Schlische, 1991.; Cowie &
Scholz, 1992a, b.; Schlische & Anders, 1996.; Fig. 1a.) and
linkage of segments (e.g. Davison, 1994.; Cartwright et al.,
1995.; Schlische & Anders, 1996.; Fig. 1b.). The models are
not mutually exclusive, but serve to highlight a
continuum of variations where faults may grow dominantly by either lateral propagation from a central
starting region, or by linkage of numerous initially
separate faults. Identifying such differences in fault
development can be dif®cult; for example, Nicol et al.
(1996.) recognize that faults can grow by merging of two
307
Map view
Two-way lateral propagation
i
0
ii
iii
Line of section
i
1
12
0
ii
3
12
0
Hanging wall strike cross-section
onlap of basin
onto margin as
faults propagate
iii
a
strike distance
a
b
strike distance
Displacement
a
Displacement Displacement
C. K. Morley and N. Wonganan
a
b
c
strike distance
i
ii
ii
iii
Displacement
i
strike distance
Displacement
b
Hangingwall strike cross-section
iii
Displacement
strike distance
strike distance
i
ii
iii
ii
Hangingwall strike cross-section
iii
= new displacement
increment below
dashed line
Displacement
i
strike distance
Displacement Displacement
c
strike distance
Fig. 1. Schematic illustration of some basic ways in which fault systems may develop and how displacement (and synkinematic
sedimentary basins) may progressively build on the faults. (a) Isolated radial propagation: an isolated fault grows by progressively
increasing in length by lateral propagation and increasing in displacement. Commonly the location of maximum displacement will
remain ®xed (e.g. Barnett et al., 1987.). (b) and (c) increase in fault length by radial propagation and linkage with other faults. In
(b) fault linkage is gradual with respect to the building of fault displacement. Hence the displacement pro®le of the fault is
irregular, re¯ecting displacements on the earlier isolated faults (e.g. Schlische & Anders, 1996.). In (c) the other extreme is
illustrated where fault linkage occurred very early in the propagation history so the fault displacement pro®le is smooth and
dominated by the displacement on the fault post-linkage (e.g. Morley, 1999.). Although the ®nal displacement±strike distance
pro®les of (a) and (c) are similar, the way displacement built with time is different. If the faults are associated with synkinematic
sedimentary rocks then onlap of the sedimentary section onto the prerift section and a progressive increase in width of the basin
should be apparent for radial propagation (e.g. Schlische, 1991.), while a ®xed basin width should be apparent for (c).
308
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Blackwell Science Ltd, Basin Research, 12, 307±327
Normal fault displacement characteristics
antecendent faults (e.g. Fig. 1b.), but suggest faults which
demonstrate this behaviour are relatively uncommon.
This is attributed to further growth on the fault
overprinting the earlier displacement patterns (Fig. 1c.).
40The interaction of overlapping faults can affect the
length±height dimensions of faults and the way displacement varies approaching the fault tip (tip gradient). This
has led to the identi®cation of restricted fault geometries
where adjacent faults have interacted. Conversely,
idealized unrestricted faults grow in isolation from
other faults (e.g. Huggins et al., 1995.; Nicol et al.,
1996.). Typically, tip gradients of restricted faults increase
in regions of overlapping faults and the displacement
pro®les tend to be asymmetric. The simple relationships
can be numerically modelled (Willemse et al., 1996.).
Cartwright et al. (1996.) stress the importance of more
studies to determine whether faults tend to grow by
mixed radial growth and linkage of segments or isolated
radial growth. However, outcrop examples of medium- to
large-scale fault systems that provide enough information
to tackle fault displacement problems are dif®cult to ®nd.
The Canyonlands of Utah are one such area where several
studies have been conducted (as reviewed in Moore &
Schulz, 1999.).
This study investigates well-documented normal faults
in the Mae Moh mine of northern Thailand (Fig. 2.). The
aim of this paper is to provide more data on fault
displacement patterns, describe how displacement is
distributed between contemporaneous fault sets, and to
investigate evidence for fault linkage. The results are
compared with previous natural and theoretical studies of
fault development.
G E O LO G Y O F T H E M A E M O H M I N E
The Miocene Mae Moh Basin is 16.5 km long by 9 km
wide. It overlies folded and faulted Triassic limestones,
sandstones and shales of the Lampang Group (Fig. 3.).
The regional basement tectonic grain is north-east to
south-west. The Mae Moh Basin is a structurally complex
remnant of a larger Cenozoic depositional basin that has
been dislocated by extensive syn- and post-depositional
deformation, and by erosion. Neogene rifting resulted in
about 20% west±east extension of the basin along a series
of subparallel, normal faults (Fig. 3.). The Central Subbasin is over 900 m deep and has the geometry of a highly
faulted graben at the level of mineable coal.
The Tertiary stratigraphy has been divided into three
formations by Corsiri and Crouch in an unpublished
report for EGAT (Electricity Generating Authority of
Thailand) in 1985 (Fig. 3.). The lowest is the Huai King
Formation, it is up to 320 m thick and is dominated by
¯uvial and alluvial sandstones, with some claystones and
conglomerates. The overlying Na Khaem Formation
(< 420 m thick) contains the main coal-bearing units,
interbedded with lacustrine and deltaic claystones,
mudstones and minor sandstones. The youngest unit is
the Huai Luang Formation, comprising a sequence up to
400 m thick of red±brown and grey claystones, mudstones
and siltstones with occasional sandstone, conglomerate
and gypsum-bearing horizons. Watanasak (1990.) determined an early Miocene to middle Miocene age for
samples from one well in the mine, but the formations
encountered in the borehole are not described. Vertebrate
fossils from the Na Khaem Formation indicate a middle
MYANMAR
Chiang Rai
THAILAND
THAILAND
GULFOF THAILAND
N
200 km
Nan
Chiang Mai
Lampang
N
Mae Moh Basin (Fig. 3)
Phrae
50 km
Normal fault,
Tertiary basin
Uttaradit
Fig. 2. Location map for the Tertiary
rift basins in northern Thailand.
# 2000
Blackwell Science Ltd, Basin Research, 12, 307±327
Predominantly
granitic basement
Town or city
309
C. K. Morley and N. Wonganan
Mae Moh Mine
Quaternary
F 17
F 15
400 m.
N
Late MiocenePliocene
HUAI LUANG FMN
P
400m
Extension direction
determined from
fault kinematic data
Lignite J zone
Middle
400m
Miocene
NA KHAEM FMN
Lignite K zone
Lignite Q zone
F 14
F 16
Lignite R zone
Lignite S zone
F 13
Early
150m
Miocene
HUAI KING FMN
F 10
Northern Basin
4
F 12
F 11
F9
F3
Western
Basin
F2
F5
Central Ridge
Central
Basin
F7
Pleistocene
volcanics
S65°E
S55°E
F1
F6
Quaternary
over Tertiary
basin
Measurement directions
for summing extension
along faults
F4
Permo-Triassic
sedimentary rocks
N
Southern
Basin
0
N50°E
N70°E
N80°E
E-W
F8
Quaternary
over Triassic
3 km
Fig. 3. Location map of the normal faults in the Mae Moh basin used in this study. Inset map ± fault map of the Miocene±
Recent sedimentary ®ll of the Mae Moh basin. Both maps and stratigraphic log are based on unpublished maps and reports by
EGAT.
Miocene age (Ginsburg et al., 1988.). Watanasak (1990.)
attributes the major coal seams (Na Khaem Formation) in
the mine to the middle Miocene.
In outcrop individual fault zones tend to be narrow and
zones of drag on either side of the fault are also narrow.
Some claystones show plastic elongation, which suggests
the rocks ranged from poorly to moderately lithi®ed at the
time of faulting. Today the rocks are moderately well
lithi®ed. Seismic re¯ection data show that the Miocene
310
section expands towards major rift bounding faults, and
hence faulting was syndepositional.
Geotechnical pit maps (unpublished EGAT 1: 1000,
February 1999, sheet numbers 9, 151 and 253) show
approximately 50 faults have been identi®ed in the study
area (Fig. 3.). Of these 17 were suf®ciently long
(< 300 + m) to be crossed by at least four cross-sections,
and hence their displacement characteristics could be
studied using map and borehole data. As illustrated in
# 2000
Blackwell Science Ltd, Basin Research, 12, 307±327
Normal fault displacement characteristics
Fault 3
North
0
400 m
Throw (m)
10
0
40
Fault 1
30
Fault 4
Fault 2
15
0
0
Fault 1
0
Distance
300
Subsurface depth (m)
(0 = sea level)
12.0
300
200
100
0
Distance
60
10.0
200
40.0
37.5
35.0
32.5
30.0
27.5
25.0
22.5
20.0
17.5
15.0
12.5
10.0
7.5
5.0
2.5
0.0
8.0
100
6.0
4.0
2.0
0
0.0
140
Fault 6
Fault 5
50.0
0
45.0
40.0
Meter.
35.0
300
30.0
130.0
120.0
110.0
100.0
90.0
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
0
25.0
200
20.0
15.0
100
10.0
300
5.0
0
0.0
Fault 4
90
200
100
Fault 7
90
Fault 8
0
90.0
80.0
0
70.0
300
60.0
50.0
200
75.0
70.0
65.0
60.0
55.0
50.0
45.0
40.0
35.0
30.0
25.0
20.0
15.0
10.0
5.0
0.0
-5.0
40.0
300
30.0
100
20.0
200
10.0
0
0.0
100
120
Fault 9
40
Fault 10
35.0
32.5
30.0
27.5
25.0
22.5
20.0
17.5
15.0
12.5
10.0
7.5
5.0
2.5
0.0
0
110.0
100.0
0
90.0
80.0
70.0
S
300
200
100
N
Blackwell Science Ltd, Basin Research, 12, 307±327
200
60.0
50.0
100
40.0
30.0
20.0
10.0
0.0
# 2000
300
Fig. 4.
311
C. K. Morley and N. Wonganan
Fault 11
36
35
Bq
Bq
G1
X-SECTION Ne36
G1
G1
G2
G3
G2
G3
Fault 12
50
Bq
G1
G2
G3
G2
G3
200 meters
100
0
200 meters
X-SECTION Ne35
0
33
100.0
34
BK
200
Bq
Bq G1
60.0
G3
G3
N
Bk
70.0
G2
G2
300
X-section scales
300
80.0
N
G1
G1
X-SECTION Ne34
90.0
BQ
BQ
S
S
G1
G2
100
G3
G2
G3
50.0
40.0
200
30.0
0
20.0
100
X-SECTION Ne33
60
Fault 13
45.0
40.0
35.0
30.0
25.0
20.0
15.0
10.0
5.0
0.0
Fault 14
80
10.0
0.0
32
31
Bk
Bk
Bq
G1
G1
X-SECTION Ne32
Bk
G2
G2
G3
G3
Bq
Bk
0
Bq
G1
G2
G3
G1
G2
0
G3
55.0
50.0
45.0
X-SECTION Ne31
S
40.0
35.0
300
Meter.
30
30.0
25.0
200
200
20.0
Bk
100
Bk
Bq
X-SECTION Ne30
0
Bq
G1
G1
10.0
100
Bk
5.0
Bq
55.0
29
45.0
35.0
Bk
25.0
Bq
15.0
G1
0
G2
G2
G3
5.0
G3
50
Fault 16
Fault 15
X-SECTION Ne29
50
28
Bk
Bk
0
Bq
G1
G2
G3
X-SECTION Ne28
300
200
75.0
65.0
15.0
G2
0.0
G3
G2
G3
N
300
45.0
42.5
40.0
37.5 Bk
0
35.0
Bq
32.5
G1
30.0
27.5
S
25.0
22.5
300
20.0
17.5
15.0
200
12.5
10.0
7.5
100
5.0
2.5
0.0
0
27
G2
G3
N
40.0
35.0
26
N
X-SECTION Ne27
Bk
100
Bq
0
G1
G2
G3
100 m.
140
45.0
30.0
25.0
20.0
15.0
10.0
5.0
0.0
Fault 17 X-SECTION Ne26
FAULT 16
120.0
110.0
0
100.0
90.0
80.0
70.0
60.0
300
50.0
40.0
200
30.0
20.0
100
10.0
312
# 2000
Fig. 4. continued
Blackwell Science Ltd, Basin Research, 12, 307±327
Normal fault displacement characteristics
36
35
Bq
Bq
G1
G1
Bq
G1
G1
G2
G3
G2
G3
G2
G3
G2
G3
200 meters
X-SECTION Ne36
200 meters
X-SECTION Ne35
X-section scales
34
BK
BQ
BQ
X-SECTION Ne34
33
Bk
G1
G1
G2
G2
G3
Bq
Bq G1
G3
G1
G2
G2
G3
G3
X-SECTION Ne33
32
31
Bk
Bk
Bq
G1
G1
X-SECTION Ne32
Bk
G2
G2
G3
G3
Bq
Bk
Bq
G1
G2
G3
G1
G2
G3
X-SECTION Ne31
30
29
Bk
Bk
X-SECTION Ne30
Bk
Bk
Bq
G1
Bq
G1
G2
G3
G2
G3
Bq
Bq
G1
G2
G3
G2
G3
X-SECTION Ne29
28
27
Bk
Bk
Bq
Bk
G1
G2
G3
Bq
X-SECTION Ne28
N
G1
G2
G3
26
X-SECTION Ne27
Bk
Bq
G1
G2
G3
100 m.
X-SECTION Ne26
FAULT 16
Fig. 5. Example of data used to build the diagrams in Fig. 4.for fault 16. The short cross-sections across the fault have been
extracted from regional cross-sections. Cross-sections are well constrained by boreholes spaced every 50±100 m, which penetrate
depths down to 450 m from the surface, and by 1 : 2000 geological maps updated every 3 months during pit excavation.
BK = base K seam, BQ = base Q seam.
Fig. 4. Summary diagrams of the displacement characteristics of the faults used in this study. The diagrams used for each fault
are (1) maximum displacement vs. strike distance, (2) map view of the fault trace, (3) contours of fault displacement, vertical
scale in metres above sea level; horizontal scale = strike distance along fault (see Fig. 3.for location). Fault contours are based on
the offset of horizons extracted from geological cross-sections spaced every 100 m, boreholes and geological maps. Displacement
values used in the contours (black diamonds) are located at the mid point between hangingwall- and footwall-cutoffs for each
horizon. Note the contours of some faults (e.g. 6, 8, 15 & 17) close outside the data ®eld. The actual point of maximum
displacement may lie below the mapped fault area, so that faults may have propagated from below and not outwards from the
contoured part of the fault.
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Blackwell Science Ltd, Basin Research, 12, 307±327
313
C. K. Morley and N. Wonganan
Common post-linkage
geometries
Early fault and fracture geometries
prior to linkage
En-passant (Kranz 1979)
(convergent)
Fault 6
En-passant (divergent)
Faults 8, 9, 10, 17
En-echelon
Northern tip of fault 14
Approaching/underlapping
Faults 5, 16
Overlapping
Faults 5-4
Faults 17 13;12-11; 9-8; 8-7; 7-6; 3-2; 2-1
Fig. 6. Examples of common fracture
linkage patterns based on observations
of the growth of natural cracks in
various materials including rock,
concrete and plaster. Although some
fractures have formed under tension,
and others under shear the fracture
patterns are commonly similar. Faults
in the Mae Moh mine representative
of some of the different patterns are
labelled on the ®gure.
Faults 12-13
Hooked
Fig. 4., maximum displacements on the faults studied
range from 6.49 m (fault 3) to 122.5 m (fault 6), and their
trace lengths range from 280 m (fault 3) to 1400 m (fault
5). The open cast pit has been mapped every few months.
Particular attention is paid to identifying normal faults
due to their effects on slope stability. Consequently, as
rock layers were removed maps of the fault geometry at
different levels were produced. These maps together with
borehole data (with boreholes drilled on a 100-m grid)
enable fault geometries down to 300±400 m depth to be
well constrained. Using the borehole and map data,
314
EGAT constructed geological cross-sections across the
mine every 100 m. These sections are digitized and have
been used to generate structure maps almost akin to a 3D
seismic data set. The mine data therefore provide a rare
opportunity to study fault displacements in detail from
outcrop and subsurface information (Fig. 5.).
F A U L T G EO M E T R I E S
The map view pattern of faults in the portion of the
central basin described in this study shows a system of
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Blackwell Science Ltd, Basin Research, 12, 307±327
Normal fault displacement characteristics
0.7
Lateral displacement gradient
0.6
0.5
0.4
0.3
0.2
0.1
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Faults (southern tip black, northern tip grey)
Fig. 7. Values of lateral tip gradients for each fault tip of the 17 faults investigated in this study (black = southern tip,
grey = northern tip). Tip gradients calculated using the ®rst displacement maximum (Cartwright & Mans®eld, 1998.). Error bars
show variation in tip gradient assuming the location of the fault tip is only known within t 25 m.
dominantly west-dipping, left-stepping faults arranged in
a system of synthetic transfer zones (Morley et al., 1990.;
Fig. 3.). Some large east-dipping faults also exist but are
less frequent than west-dipping faults in the study area.
Synthetic transfer zones comprise en echelon or
stepping fault systems in map view, with similar dip
directions. When the faults are unconnected in the
horizon under study, structure contours between the
faults tend to be reoriented from regional trends to strike
subperpendicular to the faults. This zone between two
overlapping, synthetic fault tips which die out towards
each other is known as a relay ramp (e.g. Goguel, 1962.;
.Grif®ths, 1980.; Larsen, 1988.). With increasing displacement relay ramps may evolve from simple ramps to
breached ramps affected by oblique faults (e.g. Childs
et al., 1995.). Not all synthetic transfer zones comprise
relay ramps; oblique-slip transfer faults bounding rhomb
blocks or strike-slip faults may link the stepped normal
faults (Gibbs, 1984.; Patton et al., 1994.; Morley, 1995.).
Synthetic transfer zones can also develop between two
parallel, completely overlapping faults (Morley et al.,
1990.). In this study the synthetic transfer zones appear to
be entirely composed of relay ramps.
Following the methods of previous studies investigating displacement patterns of normal faults (e.g. Barnett
et al., 1987.; Peacock & Sanderson, 1991.; Cartwright et al.,
1995.) fault displacements were contoured (Fig. 4.). From
the contoured data maximum displacement±distance
pro®les were derived. The maximum displacement
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Blackwell Science Ltd, Basin Research, 12, 307±327
value could come from anywhere on the fault plane
(Fig. 4.). Hence the maximum displacement±distance
pro®les differ from outcrop studies where the alongstrike variations in displacement on a single exposed
horizon are used, but are similar to previous studies of
fault displacements using mine data or 3D seismic data
(e.g. Barnett et al., 1987.; Needham et al., 1996.).
For outcrop studies the choice of how to measure
displacements tends to be simple since only one horizon is
usually used. However, outcrop horizons have arbitrary
paths through fault planes; for one fault the offset horizon
might be exposed running along the low-displacement
edge of a fault, while in another location it might be offset
around the displacement maxima of a fault. Variations in
tip gradients on horizons will also depend on whether the
horizon was prekinematic (i.e. has the potential to
measure the maximum displacement of the fault) or
was synkinematic (i.e. can only have recorded part of the
fault displacement history). Consequently, given the
potential inconsistencies inherent in comparing data
measured from single horizons, the contoured fault
plane data (i.e. maximum displacement) were used.
This method will tend to have data sets with higher tip
gradients, or more frequently occurring high tip gradients
than outcrop studies since only a fraction of the faults
exposed in outcrop, if any, will display horizons that run
through the fault displacement maxima.
Fault displacement contour maps were constructed
from the intersection and offset of marker horizons (base
315
C. K. Morley and N. Wonganan
of coal K seam, base of coal Q seam, G1, G2, R and G3)
with faults on structural cross-sections made by the mine
geologists (Fig. 5.). Commonly, the faults comprise zones
of two or more splays, the contours representing the sum
of displacement across the splays. The fault tip-lines are
approximately positioned within about 25 m accuracy,
due to the limited resolution of the cross-sections and
maps.
The majority of pro®les in this study exhibit a similar
pattern of displacement for the shallower and deeper
levels of the fault, but there are some exceptions. Faults 1,
14 and 15, for example, would have fairly symmetric
pro®les if displacements from the upper 100 m of the fault
were used (Fig. 4.). It is the deep displacement maxima at
about 300 m depth that cause the asymmetry. For faults 6,
8 and 9 the shallow displacement geometry would not
convincingly show evidence for the linkage of several
faults (displacement maxima) that the deeper geometry
shows (Fig. 4.).
The summary diagram of fault displacements (Fig. 4.)
shows that a variety of geometries are present. Some faults
have a symmetric, smooth displacement pro®le with
maximum displacement in approximately the centre of
the fault (faults 2, 3, 4, 7, 13), while others have smooth,
asymmetric displacement pro®les (faults 1, 11, 12, 14, 15).
The longer faults tend to be composed of multiple peaks
and troughs and a smooth displacement pattern is not
apparent. Seven of the 17 faults examined have
displacement pro®les composed of multiple peaks and
troughs assumed to represent linkage of two or more
faults (faults 5, 6, 8, 9, 10, 16, 17) as discussed, for
example, in Peacock (1991.) and Childs et al. (1995.).
Fault trace geometries often provide hints of how the
separate faults are linked. Subtle features like slightly
0.6
0.5
Tip gradient
0.4
0.3
0.2
0.1
0
0
0.04
0.08
0.12
0.16
TIP GRADIENTS
Cartwright & Mans®eld (1998.) have discussed different
ways of measuring displacement gradients at the tips of
faults to overcome the problem of irregular displacement
pro®les. Here the tip gradient is de®ned as the value of the
®rst displacement maximum divided by the distance from
the fault tip to the ®rst displacement maximum following
Cartwright & Mans®eld (1998.). The ®rst point method
also used by Cartwright & Mans®eld (1998.) is not used
here because the uncertainty surrounding the exact
location of the fault tips in the Mae Moh mine
(c. t 25 m) adds considerably more error to the ®rst
point method than the ®rst maximum method.
Tip gradients in the Mae Moh mine range from 0.035
to 0.6 with a mean gradient for the fault population of 0.23
(Fig. 7.). The tip gradients are considerably higher than
those observed in the Canyonlands (Cartwright &
Mans®eld, 1998.; Moore & Schulz, 1999.), where the
data range between 0.0164 and 0.25 with a mean of 0.072.
There is a general increase in displacement gradient with
an increase in the dimensionless fault displacement±
length ratio (Fig. 8.) but the data are highly scattered. In
general the data points plot at or above the reference line
based on the ideal `C' type displacement pro®le for
unrestricted faults (Nicol et al., 1996.; their Fig. 13). The
dominance of plots in the laterally restricted ®eld re¯ects
the high tip gradients on some faults. A cross-plot of
lateral tip gradients against fault trace length shows no
positive correlation (Fig. 9.). However, cross-plots of fault
displacement amount and tip gradient show a highly
scattered, but progressive increase in the displacement
gradient with increasing fault displacement (Fig. 10.).
Faults 14 and 15 show asymmetry on the maximum
displacement graphs (Fig. 4.). The values which cause
asymmetry are located at around 300 m depth on the fault
displacement contours (Fig. 4.). Passing up section the
maximum displacement contours for higher levels of the
fault tend to be more symmetric. The asymmetric high tip
gradients can be found in the mine outcrops associated
with relatively steeply inclined relay ramps with dips
perpendicular to the fault strike of 10±20u and as
transverse anticlines (Schlische, 1995.).
0.20
Dmax /L (fault segment)
Fig. 8. Graph of displacement gradient against dimensionless
maximum fault displacement±length ratio, for 17 faults in the
Mae Moh mine.
316
oblique segments and more obvious features such as fault
splays commonly coincide with areas of total displacement
minima (Fig. 4.). Figure 6.is a summary of some common
linkage geometries associated with fracture arrays in many
brittle materials (rocks, concrete, plaster), a number of
which can be recognized in the fault patterns of Mae Moh
mine.
DISPLACEMENT±LENGTH
RELATIONSHIPS
A number of early papers investigating fault population
characteristics examined the displacement and length
# 2000
Blackwell Science Ltd, Basin Research, 12, 307±327
Normal fault displacement characteristics
0.6
0.5
Tip gradient
0.4
0.3
0.2
Fig. 9. Cross-plot of displacement
gradient against fault length. Note the
shortest segment lengths only show
small tip gradients. This may re¯ect
problems with undersampling
displacement variations along the faults
since the cross-sections are spaced
every 100 m.
0.1
0
0
250
0.6
0.5
Tip gradient
0.4
0.3
0.2
0.1
0.0
20
40
60
80
100
120
Maximum fault displacement
Fig. 10. Cross-plot of displacement gradient against maximum
fault displacement, with best ®t line through data.
# 2000
750
1000
1250
1500
1750
Segment length
characteristic of fault populations to determine a relationship between fault displacement and fault trace length.
Walsh & Watterson (1988.), Marrettt & Allmendinger
(1991.) and Cowie & Scholz (1992b.) showed that faults
increase their displacement (D) as they increase their
surface trace length (L). Displacement is measured as
offset of marker horizons measured along the dip of the
fault plane.
The cross-section and map data at Mae Moh coal mine
cover the entire basin, and reveal the complete strikelength of major faults and smaller faults. Measurements of
displacement and trace length of 17 faults are shown in
Fig. 11.. The minimum measurement is 280 m in trace
length and 6.49 m of displacement. The maximum
measurement is 1360 m in trace length and 117.16 m of
0
500
Blackwell Science Ltd, Basin Research, 12, 307±327
displacement. Distribution of the data is scattered, and
hence the best-®t line is rather poorly determined; linear
regression through the data plotted on log±log axes gave a
best-®t line given by D = 0.0255 L1.1652 with a best-®t
ratio (R2) of 0.4111.
INTERPRETATION OF THE DATA
Interpretation of the fault displacement data centres
around two main aspects: (1) the displacement patterns of
individual faults and (2) how the faults have interacted
during the growth of the fault system. These aspects are
discussed below.
The individual patterns of faults are highly varied, both
in general geometry and in the tip gradients. This is
apparent in the plots of D±L ratios for faults which show
highly scattered data (Fig. 11.). Scholz & Cowie (1990.)
proposed a linear displacement and trace length (D±L)
relationship. Dawers et al. (1993.) supported Cowie &
Scholz (1992a.) in demonstrating that the relationship
between displacement and trace length is linear. They
proposed that D = 0.011 L for short faults and
D = 0.008 L for longer faults. Walsh & Watterson
(1988.) proposed D = kL2, while Marrettt & Allmendinger (1991.) proposed a D = kL1.5 relationship. More
recently, Davison (1994.) and Cartwright et al. (1995.)
suggested that at any instant in the evolution of a fault
population, individual faults would be at different stages
of development on their own particular step-wise growth
curve. Some would be at a stage of radial propagation,
while others would be at different stages of overlap,
interaction and linkage, probably with different length
and numbers of segments involved. These faults would
have D and L values lying beneath the growth line, with
growth paths and positions re¯ecting their own speci®c
histories of linkage. The variability in the timing and
length scale of linkage to be expected in any natural fault
317
C. K. Morley and N. Wonganan
Log displacement (m)
1000
Envelope of data from Nicol et al. (1996),
Timor Sea
Envelope of data from Cartwright et al.
(1995), Canyonlands, Utah
M
M
100
M
M
M
M
M
M
10
M
D = 0.0255L1.1652
2
R = 0.4111
Faults in study area
with multiple displacement
maxima
Faults in central basin
outside study area
Faults in study area
1
10
100
1000
10 000
Log trace length (m)
system would create a scatter in D and L data. Moreover,
the magnitude of the scatter would be largely a function of
the number and length of segments involved during a
typical cycle of linkage. Measurement biases such as the
problems of rough distribution of displacement on the
fault surface, the sampling of fault trace length, the
measurement of fault displacement and the complications
due to complex fault geometry will also affect D±L plots
(Cowie & Scholz, 1992a, b.). Peacock (1991.) and Peacock
& Sanderson (1991., 1996.) showed that with increasing
interaction between faults the D±L ratio increases and
causes data to plot above an `ideal' line.
In the Mae Moh mine some faults have unusually high
D±L ratios (up to 1 : 6) and high tip gradients (0.6), while
others display relatively low D±L ratios (as low as 1 : 27;
Fig. 11.) and low tip gradients (0.08±0.09). The low D±L
faults are combination faults where linkage of two or more
faults has occurred (similar to those described by
Cartwright et al., 1995.). For example, faults 5 and 16
show displacement pro®les with two maximum peaks of
the same magnitude (Fig. 4.). Other combination faults
show differences in magnitude of the maximum displacement peaks (particularly 9 and 17) but still show they were
amalgamated from the originally separate faults (Fig. 4.).
The high D±L ratio, high tip gradient faults cause
scatter above the best-®t line (Fig. 11.). Some of the high
D±L ratio faults appear to conserve displacement with
just a few other faults, which results in asymmetric
maximum displacement±length plots (Fig. 12;. faults 11,
12, 13, 17) as previously described by Peacock (1991.) and
Peacock & Sanderson (1991., 1996.). High D±L faults are
mostly of intermediate length; they have built anom318
Fig. 11. Log±log plot of maximum
displacement vs. maximum fault
length. There is a poor ®t of the data;
a least squares regression through the
Mae Moh data points has a low
correlation coef®cient. Many of the
data plot above trends found in
previous studies (Cartwright et al.,
1995.; Nicol et al., 1996.) indicating
many of the faults have unusually high
displacement±length ratios.
alously high displacements for their lengths which is
re¯ected in high tip gradients. These high tip gradients
indicate one or a combination of the following factors has
operated (e.g. Burgmann et al., 1994.; Cowie & Shipton,
1998.): (1) the faults zones have undergone strain
softening, where it was easier to keep deforming the
same region than propagate into undeformed rock; (2) the
adjacent rock volume was more resistant (stronger) to
fault propagation in comparison with other parts of the
mine; (3) stresses at fault tips produced areas of stress
shadows and enhancement which promoted or inhibited
the growth rate of faults (Cowie, 1998.).
Figure 12.shows the distribution of fault tip gradients
that have been split into three main categories: low±
moderate, tip gradients up to 0.15; high, gradients from
0.16 to 0.30; and very high with gradients of 0.31±0.6. A
few patterns are present. The low±moderate tip gradients
are mostly associated with east-dipping faults antithetic to
the dominant west-dipping faults. Where low±moderate
tip gradients are found on west-dipping faults they tend
to occur where the fault tips strongly overlap adjacent
faults. High and very high tip gradients tend to be found
in relay ramps between closely spaced west-dipping
faults. They are also found at the southern end of faults
14, 15 and 17, which form a left-stepping pattern. On
1 : 2000 geological maps of the mine made by EGAT the
absence of faults to the south of the high tip gradient
terminations suggest basement control on fault distribution. Hence the distribution of tip gradients indicates that
at least mechanisms 2 and 3 above acted to cause high tip
gradients. High tip gradients can be associated with
irregular displacement faults (Fig. 1b.) where the linkage
# 2000
Blackwell Science Ltd, Basin Research, 12, 307±327
Normal fault displacement characteristics
F 15
F 17
F 14
F 16
F 13
F 10
F 12
Fault tip displacement gradient
F 11
F9
0-0.15
F3
0.16-0.30
F5
F2
F8
F7
0.31-0.6
N
F1
F6
F4
400 m.
Fig. 12. Map of faults in the study area showing the magnitude of the fault tip gradients. Boxes represent the distance along the
fault between the fault tip and the ®rst displacement maximum point.
of two or more faults results in much higher tip gradients
when calculated to the ®rst displacement maximum,
compared with the average displacement gradient to the
centre of the fault. This pattern is seen in faults 5, 6, 9, 10
and 16.
The fault map from Mae Moh mine shows that most of
the faults overlap to some degree with their neighbours
(Fig. 3.). The close proximity and overlapping nature of
some faults of similar dip direction suggests they are
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Blackwell Science Ltd, Basin Research, 12, 307±327
related and form part of a linked fault system. How the
fault system developed can be tested by constructing
aggregate throw pro®les for various fault segments (e.g.
Peacock & Sanderson, 1991.; Dawers & Anders, 1995.;
Willemse et al., 1996.; Cartwright & Mans®eld, 1998.). For
a strongly linked system that approximates radial
propagation the aggregate throw pro®le is expected to
show a simple bell-shaped curve (Schlische, 1991.;
Schlische & Anders, 1996.; Morley, 1999.). Thus the
319
C. K. Morley and N. Wonganan
Transfer zone
160
Displacement (m)
Smoothed, projected
displacement pattern
Difference between
actual displacement
and ideal displacement
in transfer zone
total displacement
17
11
12
13
Displacement (m)
0
0
2000
1000
Distance (m)
TZ
TZ
60
1
3
2
0
1100
Distance (m)
Transfer zone
Displacement (m)
160
9
6
7
0
0
Difference between
actual displacement
and simple
projected displacement
in transfer zone
8
1000
Distance (m)
2000 m
Fig. 13. Displacement±distance graphs for faults involved in three transfer zones. Two fault sets (1-2-3 and 11-12-13-17) show a
high degree of displacement conservation between adjacent fault sets, while faults 6-7-8-9 show a low degree of displacement
conservation between adjacent fault sets.
sum of the various fault segments creates the simple
displacement pattern of a much larger fault (Fig. 1a or 1c.).
A system that indicates fault linkage by numerous
propagating fault segments will show an irregular
aggregate pro®le of numerous displacement maxima
and minima (Fig. 1b.). A variety of these patterns are
found in the Mae Moh mine.
Figure 13.shows the individual and total displacement
curves for three sets of faults in synthetic transfer zones.
Two patterns of displacement pro®le are recognized,
namely: (1) the simple and regular pattern exhibited by
fault group 11-12-13-17 and fault group 1-2-3 (Fig. 10.),
and; (2) the irregular pattern displayed by faults 6-7-8-9.
The ®rst pattern is demonstrated by simple stepping
faults, with each fault displacement graph commonly
displaying asymmetry. For faults 11-12-13-17 the two
highest displacement faults (11 and 17) have their highest
320
tip gradients in the transfer zone (Fig. 13.). In contrast, the
second pattern is exhibited by overlapping faults which
individually have several displacement maxima (faults 6, 8
& 9). In the transfer zone between faults 6 and 9 the
maximum total fault displacement for an idealized
symmetrical bell-shaped displacement pro®le would be
projected at about 140 m, while the actual total displacement is only 40 m (Fig. 13.).
Previous studies (e.g. Peacock & Sanderson, 1991.;
Huggins et al., 1995.) have proposed that a decrease in
aggregate throw across a relay zone could occur due to (1)
interaction of independently nucleated faults, and/or (2) a
signi®cant proportion of displacement is accommodated
by ductile strain within the relay zone and/or (3) transfer
of displacement to a neighbouring fault group. The high
degree of aggregate throw conservation within fault group
11-12-13-17 and low degree of aggregate throw conserva# 2000
Blackwell Science Ltd, Basin Research, 12, 307±327
Normal fault displacement characteristics
tion between faults 6, 7, 8 and 9 indicates that
considerable variation exists in the roles played by the
three factors above even locally within fault systems.
Building on the variations in transfer zone geometry
above and in other papers (notably Morley et al., 1990.;
Peacock & Sanderson, 1991.; Cartwright et al., 1995.; Nicol
et al., 1996.) a general model for the evolution of strain in
transfer zones is discussed below.
STRAIN EVOLUTION IN TRANSFER
ZONES
Transfer zones are regions where strain is transferred
along strike from one structure that dies out onto one or
more other contemporaneous structures. Hence even
though individual structures are discontinuous, strain
continuity is preserved. The general features of extension
in transfer zones are described in several papers (e.g.
Rosendahl et al., 1986.; Morley et al., 1990.; Gawthorpe &
Hurst, 1993.; Faulds & Varga, 1998.). One aspect of
displacement transfer between two faults that forms part
of the classi®cation of Morley et al. (1990.) is the degree of
overlap of faults in the transfer zone. Morley et al. (1990.)
mentioned that for approaching or underlapping stages
Main stages of displacement
associated with faults 6-7-8-9
(Fig. 6di.), where laterally there is a gap between the faults,
there cannot be the opportunity for displacement
`transfer' between the faults (assuming plane strain and
more-or-less pure extension). Consequently, the distribution of extensional strain associated with the faults must
change with time as the fault geometries evolve and
recon®gure to permit progressively greater displacement
transfer between the faults with time. This evolution is
schematically illustrated in Fig. 14.. Strain distribution
around two propagating faults can be divided into various
types. First the area between the two faults is referred to
as the internal area of the transfer zone (I) (for synthetic
faults this would be the relay ramp), beyond which lies the
external area (e). Strain can be accommodated by other
large faults (ef), or by structures, which at the level of
observation are relatively small-scale or ductile (ed, i.e.
numerous small faults, pressure solution, cleavage,
compaction, deformation bands, etc., e.g. Nicol et al.,
1996.; Fig. 6c and div.).
As fault systems and transfer zones evolve, the way the
total strain is distributed across the extending area will
change, as indicated by numerical modelling (e.g. Cowie,
1998.). During the early stages of fault development many
fault tips will have approaching con®gurations, where
Main stages of displacement
associated with faults 11-12-13-17
Approaching
Joined
Overlapping
ed+ef
Id
ed+ef
Pd
Id
Td = Id+ed+ef
tends to be Id+ed<ef
Td = Id+ed+ef
tends to be ed+ef<Id
Td= Pd+Id+ed+ef
tends to be:
Pd<ed+ef<Id
Td= Pd+Id+ed+ef
tends to be:
ed+ef<Id<Pd
Td=Pd
Transfer of displacement (Td)
Displacement between the
principal faults (Pd)
Internal ductile deformation (Id)
External ductile deformation (ed)
External fault displacement (ef)
Fig. 14. Schematic diagram illustrating how strain in transfer zones is likely to evolve as faults pass through different stages of
development ranging from approaching or underlapping, to overlapping and ultimately linkage. Td is total displacement on the
system.
# 2000
Blackwell Science Ltd, Basin Research, 12, 307±327
321
C. K. Morley and N. Wonganan
there will be little or no displacement transfer between
neighbouring faults. Hence strain must be distributed
relatively widely by internal and external ductile strains
and on other larger faults. When the two principal fault
tips begin to overlap more strain can be concentrated on
the faults and ductile strains between (internal) the two
faults, and less on external structures. Finally, with
signi®cant fault overlap, or linkage by cross-faults, all
displacement transfer can occur between the two faults,
although even at this stage displacement transfer does not
have to be 100%. Thus transfer zones might be expected
to narrow with time.
The amount of displacement transfer between two
faults can be measured as the difference between a simple,
projected displacement curve (representative of 100%
displacement transfer) and the actual sum of displacement
(e.g. Childs et al., 1995.; Fig. 13.). For descriptive purposes
the difference in displacement conservation on principal
faults in the zone of transfer between the simple projected
displacement curve and the actual sum of displacement is
referred to here as high (70±100%), moderate (40±70%),
low (10±40%) and insubstantial (< 10%). Displacement
conservation between the principal faults in the transfer
zones relative to an idealized smooth total displacement
curve is extremely variable, ranging from high (fault
group 1-2-3, 100%; fault group 11-12-13-17, 75%) to low
(fault group 6-7-8-9, 31%).
In Fig. 15(a.) fault group 6-7-8-9 displays considerable
evidence for early nonlinked faults with high displacements, following the schematic pattern illustrated in
Fig. 1(b.). From the highly jagged displacement pro®les in
Fig. 15(a.) it is inferred that the post-linkage displacements
on the faults are relatively small. If extension conservation
was high during the post-linkage phase then the 40 m
displacement in the transfer zone (Fig. 13.) may approximate the maximum displacement on the fault postlinkage. Then 100 m of displacement occurred prior to
linkage. The early faults built high tip gradients that have
been modi®ed and commonly lessened by later fault
propagation and linkage.
In contrast, the displacement pattern for fault array
17-13-12-11 (Fig. 15b.) shows little evidence for displacement on earlier, shorter faults, except for fault 17. In
general, the displacement patterns match those schematically illustrated in Fig. 1(a.) or 1c.. It is dif®cult to infer
the early linkage geometry except for fault 17. It is
suspected that the faults propagated to near their current
length before building most of their displacement. Such
faults are characterized by high tip gradients.
It was argued above that the amount of extension not
conserved within transfer zones should be transferred to
other structures within or beyond the transfer zone. To
determine whether this occurs largely on other large faults
or through ductile structures it is necessary to examine
how summed displacement across all known faults varies
perpendicular to the regional extension direction.
Figure 16.is a displacement±distance plot where displacement is the sum of the extension across all the mapped
322
faults in Fig. 3.. The problem with constructing this type
of ®gure is that extension has to be summed in the
extensional transport direction. Two ways can be used to
determine the transport direction. First, fault striation
data can provide palaeostress and palaeostrain data
(Fig. 3.). The extension direction from these data is
approximately E±W. Second, the method of summing
extension across the fault might be expected to give an
approximation of the transport direction, if it is assumed
that summing extension in the wrong direction is likely to
produce greater variability in the summed extension
values than summing them in the correct direction. In
Fig. 16. the least variability occurs when extension is
summed in the N80uE direction. The average extension
across the map in Fig. 3.in the N80uE direction is 150 m,
with a range from 130 m to 215 m (Fig. 16.).
In Fig. 16. the location of the transfer zone between
faults 6, 7, 8 and 9 lies in a region of the graph for N80uE
which has consistent values close to 150 m, despite the
100 m local drop in displacement between faults 6 and 9
(Fig. 13.). This plot indicates the loss of displacement
between adjacent faults (6-7-8-9) is compensated by
displacements on other mapped faults (in particular faults
1, 2 and 5). There appears to be no extra ductile strain
required to help conserve displacement. This inference is
supported by outcrops in the coal mine around faults
6-7-8-9 that display remarkably few faults with displacements in the order of centimetres to millimetres between
the mappable faults. In some sections tens-of-metres-long
minor faults are absent.
The anomalously high value of 215 m in Fig. 16., line
N80uE, coincides with the transfer zone between faults 11
and 12 where displacement conservation is high. To have
65 m of extra extension on this part of the system requires
a drop in extension on a fault or faults beyond the faults
studied. The alternative solution, that ductile strain varies
considerably within the study area, is not feasible because
except at local, high strain features, such as rollover
anticlines and drag synclines, there are very few (i.e. less
than 1 per metre length of outcrop) low-displacement
(centimetre to millimetre) faults in any of the numerous
outcrops in the mine. The high strain suggests the faults
were able easily to build displacement at the expense of
displacement on fault systems a considerable distance
away (probably some 400±600 m west of the study area).
The high strain might represent an example of healingreloading feedback control suggested by Cowie (1998.)
where fault tip con®guration increased stresses around the
en echelon fault zone which lead to enhanced fault
activity.
Relative to the currently observed displacement some
fault zones show little displacement through the early
stages of fault development of approaching to weakly
overlapping fault tips. Such faults are likely to be
associated with high displacement transfer zones. Transfer zones that could be classed as moderate and low will
tend to re¯ect fault zones that built relatively substantial
amounts of displacement during the approaching to
# 2000
Blackwell Science Ltd, Basin Research, 12, 307±327
Normal fault displacement characteristics
a
160
b
a
Displacement (m)
Early
d
f
g
e
c
Early displacement
b
e
c
a
f
g
d
00
2000 m
1000
Distance (m)
Intermediate
Fault 9
Fault 8
Fault 6
Fault 7
160
Total displacement
Displacement (m)
Present day
Fault 10
f
b
a
g
e
c
6
9
d
7
8
00
1000
Distance (m)
2000 m
= latest displacement
increment
b
Early
Present day
Fault 11
Fault 12
Fault 13
Fault 17
400 m
Displacement (m)
160
speculative structure
contour shape- intended
to highlight changes
in fault linkage with time.
17
11
13
12
0
0
1000
Distance (m)
2000
Fig. 15. Illustration of how fault arrays 6-7-8-9 (a) and 17-13-12-11 (b) may have developed, illustrating a contrast in the way
displacement has built on fault systems in the Mae Moh mine. The displacement±strike distance diagrams are those illustrated in
Fig. 13.; fault locations can be found in Fig. 3.. (a) The fault array displays evidence for early nonlinked faults with high
displacements, following the schematic pattern illustrated in Fig. 1(b.). From the highly jagged displacement pro®les it is inferred
that the post-linkage displacements on the faults are relatively small. The early faults built high tip gradients which have been
modi®ed and commonly lessened by later fault propagation and linkage. (b) The displacement pattern for fault array 17-13-12-11
shows little evidence for displacement on earlier, shorter faults (unlike a above), except for fault 17. In general, the displacement
patterns match those schematically illustrated in Fig. 1(c.). It is dif®cult to infer the early linkage geometry except for fault 17. In
contrast to (a) above it is suspected that the faults built little displacement until the fault systems developed to near their current
length. The subsequent building of displacement is characterized by high tip gradients.
# 2000
Blackwell Science Ltd, Basin Research, 12, 307±327
323
C. K. Morley and N. Wonganan
Displacement (m)
400
N50°E strike
300
200
100
100 m
0
Distance
Displacement (m)
400
N70°E strike
300
200
100
0
Distance
Displacement (m)
400
N80°E strike
300
200
100
0
Distance
Displacement (m)
400
90°E-W strike
300
200
100
0
Distance
Displacement (m)
400
S65°E strike
300
200
100
0
Distance
Displacement (m)
400
S55°E strike
300
200
100
0
Distance
Fig. 16 Summed displacement±distance graphs for all the faults in Fig. 3.. Displacement values (Fig. 4.) were marked along the
faults (and estimated for the smaller faults using cross-sections and geological map data). A series of lines spaced every 100 m
were superimposed on the map. The displacement value at the location where the line cut a fault was taken. Then the values for
all faults cut by the line were summed to the value marked by a black dot on the ®gure. By repeating the measurements on each
line across the map the resulting curves show how well or poorly extension is conserved in a particular direction. The
calculations were repeated for different line orientations between S55uE strike and N50uE strike (Fig. 3.). It is assumed that the
direction with the best conservation of extension (i.e. least variation between maximum and minimum values) lies perpendicular
to the bulk transport direction of the extension system (i.e. N80uE in this case). Note that the values always fall off towards the
last few points at the ends of the graph; these are not reliable values and re¯ect undersampling of faults at the map edges.
324
# 2000
Blackwell Science Ltd, Basin Research, 12, 307±327
Normal fault displacement characteristics
weakly overlapping stages. It is apparent that the faults in
groups 11-12-13-17 and 1-2-3 associated with high
displacement transfer zones have relatively simple
symmetric or asymmetric displacement pro®les while
fault group 6-7-8-9 associated with a low displacement
transfer zone is characterized by irregular displacement
pro®les and strong evidence for multiple fault linkage.
Figure 15. illustrates how fault zones 11-12-13-17 and
6-7-8-9 may have evolved using the fault geometries and
displacement patterns as a guide, following the assumptions about how fault displacements may have accumulated shown in Fig. 1..
SEDIMENTARY BASINS
The faults discussed in this study cannot be demonstrated
to have a signi®cant effect on sedimentation. However,
the observations about fault development can be applied
to larger faults that do impact sedimentation. There have
been several models published concerning the growth of
normal faults and their impact on the development of
sedimentary basins (e.g. Schlische & Olsen, 1990.;
Schlische, 1991.; Anders & Schische, 1994; Schlische &
Anders, 1996.; Gawthorpe et al., 1997.; Gupta et al., 1998.;
Morley, 1999.; Fig. 1.). One aspect of these models is
whether signi®cant boundary fault propagation and
linkage occurs largely prior to signi®cant basin development (Morley, 1999.) or whether linkage occurs during
basin development (Schlische & Anders, 1996.; Gupta
et al., 1998.). These different mechanisms can result in
different basin ®ll and isopach patterns, and variations in
the way sediment entry points into rifts change with time
(Schlische, 1991.; Gupta et al., 1998.; Morley, 1999.).
Examples of the three principal paths of fault development illustrated in Fig. 1.no doubt exist, but published
data on the displacement characteristics of fault systems
are too limited to make conclusions about their relative
importance. The Mae Moh data are one example where
faults representing the different models can be found in
the same area of contemporaneous faults (e.g. Fig. 15.).
Apparently local factors (e.g. local stress ®eld variations at
fault tips/degree of overlap between faults, pre-existing
basement fabrics, variations in fault zone material
properties) can strongly in¯uence which path is followed.
CONCLUSIONS
Normal faults exposed in the Mae Moh mine represent a
fault population that developed during the early to late
Miocene formation of a synrift sedimentary basin. Of the
17 faults investigated, four have simple symmetric
displacement pro®les, six have smooth asymmetric
displacement pro®les and seven show irregular displacement pro®les with multiple displacement highs and lows
indicative of fault linkage. Similar types of pro®les have
been described for other fault systems (e.g. .Ellis &
Dunlap, 1988.; Peacock & Sanderson, 1991., 1994.). Hence
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Blackwell Science Ltd, Basin Research, 12, 307±327
at an advanced stage in the fault evolution there remains
considerable evidence for fault growth by linkage as well
as radial propagation of faults. Signi®cantly, the four
longest faults (5, 9, 16, 17) all have irregular displacement
pro®les.
The scale range of faults examined in this study was
insuf®cient to establish the D±L relationship for the
system; however, it is still possible to look at the relative
D±L relationships of individual faults. The fault
displacement patterns found in this study support
previous work (e.g. Peacock & Sanderson, 1991., 1996.;
Peacock, 1991.; Cartwright et al., 1995., 1996.) that suggests
fault displacement±length plots signi®cantly deviate from
the ideal cohesive-zone models of fault growth (e.g. Cowie
& Scholz, 1992a.; Scholz et al., 1993.) due to the effects of
fault interaction and linkage. The scatter is large, with
variations in displacement for faults of 1000±1500 m
length almost spanning an order of magnitude (Fig. 11.).
The scatter can be partially attributed to varying degrees
of fault linkage. Low D±L faults tend to show multiple
linkage of similar displacement magnitude faults (e.g.
faults 5 and 16) and the shorter faults in the data set. Due
to problems of resolution it is uncertain whether the
shorter length, low D±L faults represent multiple linkage
faults or faults that failed to build displacement because
they were in stress shadows as described by Cowie (1998.).
Even the faults with relatively low D±L-values plot in the
central to upper parts of the data envelope of two previous
studies (Cartwright et al., 1995.; Nicol et al., 1996.;
Fig. 11.). Hence in the Central basin faults have built
much higher displacements for their length than has
generally been identi®ed in previous studies.
The anomalously high D±L values in the Central basin
are largely associated with en echelon faults in multifault
synthetic transfer zones. The faults display asymmetric
smooth pro®les or multiple peaked pro®les where a larger
displacement fault has linked with smaller displacement
faults. Previous outcrop studies (Peacock & Sanderson,
1996.; Cartwright & Mans®eld, 1998.) have determined tip
gradients ranging up to 0.25; some faults in the Mae Moh
mine exhibit very high tip gradients (®rst maximum tip
gradients up to 0.6). Faults with simple symmetric and
asymmetric displacement pro®les and relatively high D±L
values appear to be associated with faults that propagated
early in their development into overlapping synthetic
transfer zone geometries (e.g. Fig. 16b.).
The map geometries of the normal faults indicate a
number of synthetic transfer zones are present, characterized by relay ramps. However, displacement transfer
between overlapping neighbouring faults is highly
variable (ranging from high to low) and appears to re¯ect
whether faults propagated relatively early or late into
overlapping con®gurations (with respect to building of
signi®cant displacement). The relatively rapid development of overlapping con®gurations creates a greater
potential for high conservation of displacement between
adjacent faults (e.g. faults 1-2-3 and 11-12-13-17). The
strains not accommodated by displacement transfer
325
C. K. Morley and N. Wonganan
between two or more principal faults must be accommodated instead by (1) zones of relatively high-strain
ductile deformation and/or (2) a relatively broad region
where strain is transferred between faults of similar and
opposite dip. Fault groups 6-7-8-9 shows low conservation of displacement, with displacement dropping up to
100 m within the relay ramp system between faults 6 and
9. However, summed displacements across all mapped
faults in the study area consistently add up to < 150 m
across fault group 6-7-8-9 (Fig. 16., pro®le N80uE). These
data indicate that fault group 6-7-8-9 did not act as a selfcontained displacement system. Instead, strain was
conserved across a relatively broad region between
mappable faults. Signi®cant increases in ductile strain
to compensate for the drop in displacement did not
apparently develop.
ACKNOWLEDGMENTS
We would like to thank the EGAT staff at the Mae
Moh mine, particularly the Geotechnical Engineering
Department, Geology Department and Administrative
Division for their helpfulness in enabling the project to be
conducted; they provided accommodation, transport, other
logistical help, excellent geological data and an enjoyable
working atmosphere. The Universiti of Brunei Darussalam
is gratefully acknowledged for providing funding for the
project. D. Peacock and C. Childs are thanked for detailed
and constructive reviews that helped improve the manuscript.
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Received 19 January 2000; revision accepted 27 September 2000
327