1. off-shoot
cuts foliation
Shallow
dipping sill
(NW Dip)
Shallow
dipping sill
(SSE Dip)
Location 3-A
Facing NNE abandoned tip
Melted lenses of
host rock
fractures cut
sill and host
Sub-vertical
foliation
(a)
Sill contact discordant
to foliation
Bedding (12)
0
90
Dip angle
0
90
180
270
Strike direction
HR-Fractures (53)
0
90
Sill-HR contact (77)
0
90
180
270
Sill-Host Rock Contacts
Bedding & Fractures in Host Rock N
Sill-Host Rock Contact (Poles)
Key
Fractures in Host rock (Poles)
Bedding (Poles)
Sill cuts vertical host rock bedding
Magma pressure and σ1
cause opening of
favourably orientated fractures
fracture opening
inflation
progressive inflation and opening
σ1
σ1
σ2
σ3
Pre-existing fracture, or
fracture formed during
sill emplacement
Exploitation
Model
0
90
Stepped Fractures
0
90
180
270
Dip angleStrike direction
Exploited Fractures
Exploited Fracture (Poles)
Stepped Fracture (Poles)
Key
Fractures in Host rock (Poles)
Foliation (Poles)
Flow Lineations (T,P)
N
20 cm
no sill exploiting
fractures
Arrows indicate amount of
rotation/ displacement due
to sill inflation in fracture
Sill
Sill HR
Exploitation of
shallow fractures
B) Sill stepped against steep fractures
BA Sill
Sub-vertical foliated
Moine host rock
Location 2-A
NNE SSW
Facing E
Step
F
Fracture
F
Fracture
Host Rock
F
4 cm
Facing ENE
Sill exploiting
shallow fracture
F
F
A) Sill exploiting shallow fractures
A B
Fractures
σ1
σ2
σ1
σ2
1 m
Sill
(Segment 2)
Sill
(Segment 1)
Facing S
Down-Step
(to West)
Exploitation of shallow
fracture
No intrusion
in fracture
Fracture
roof‘uplift’
Sub-vertical
foliation
Segment linkage
via step
Sill parallels
foliation
Sill parallels
foliation
Sill
Sill obliquely
cuts foliation
Location 2-B
Facing NW
Sill
Facing N
Down-Step
(to West)
Sill parallel
to foliation
20 cm
no instrusion
in foliation
A
B
B
232.07
bottom contact
top contact
Facing WNW
Sill
A 20 cm
1 m
Sub-vertical
foliation
Sill
C
C
B) Sill stepped against sub-vertical beddingA) Sill cutting through sub-vertical bedding
C) Sill segments linked via westward down-step. upper segment exploits a shallow fracture.
(Sill highlighted red for clarity)
(Sill highlighted red for clarity) (host rock highlighted blue for clarity)
Exploitation v.s Stepping
Observed Sill Geometries, Isle of Mull, UK:
Sill segment linkage
1.5 m
abandoned tip
(exploitation)
Segment 1
abandoned tipLocation 3-B
Facing E
HR
(a)
Host rock
Segment 2
Sills are discordant to host rock bedding and fabric:
Segment Linkage
σ3
σ1
σ1
Fractures and bedding perpendicular
to σ1
are kept closed, preventing
exploitation and promoting stepping.
Vertical bedding
plane/ fracture
Sub-vertical
Moine host rock
B
B
A
Inflation
Increased shear stress at
sill tips on discontinuity
plane
σ1σ1
σ3
σ2
Steping
direction
Step
trend
A
Stepping
Model
Segment Linkage Model
(a’’)
breached
relay
abandoned tip
(b) (b’’)
flow
breached
relay
inflation
flow
(b’)
relay zone
(a) (a’)
relay zone
σ1
σ1
σ3
σ1
σ1
σ3
1
Department of Geology, University of Leicester, Leicester, UK.
2
School of Geosciences, King’s College, University of Aberdeen, Aberdeen, AB24 3UE, UK
3
Department of Earth Science, Durham University, Durham, UK.TLS15@le.ac.uk
Sill emplacement controlled by regional stress state rather than host layering
Tara Stephens*1
; Richard Walker1
; Richard England1
; David Healy2
; Ken McCaffrey3
N
Bedding
Sill - Host rock
contact
References
GUDMUNDSSON, A. 2011. Deflection of dykes into sills at discontinuities and magma-chamber formation. Tectonophysics, 500, 50-64.
SCHOFIELD, N. J., BROWN, D. J., MAGEE, C. & STEVENSON, C. T. 2012. Sill morphology and comparison of brittle and non-brittle emplacement mechanisms. Journal of the Geological Society, 169, 127-141.
WALKER, R. J. 2016. Growth of a transgressive sill in mechanically layered media. Geology.
(a) (c)
Thrust Fault (Hybrid Mode I - III)
Propagation in σ2
orientation
B B’
σ1
σ1
σ3
σ2
σ3
σ1
B B’
Influence of fluid pressureIncipient shear zone, prior to
through-going thrust fault
σ2
σ3
σ1
A A’
A A’
σ1
σ1
σ3
τ
(MPa)
σn
(MPa)
reactivation possible
θ ≈ 10-50°
2θ
Pf
reactivation
envelope
intact
rock
σ3
σ1
σ1
σ3
σ1
σ1
Pf
(b)
σ1
σ2
σ3
σ1σ2σ3
σT (Intrusion tip)
σT (Discontinuity)
σT (I)
> σT (D)
(1a) (1b)
1) Cook-Gordon Mechanism
(a) Tensile stress ahead of the dyke tip
causes opening of a discontinuity above the
dyke.
(b) If the dyke reaches the crack it will
propagate as a sill.
Dyke - Sill Transition Models
(Gudmundsson, 2011)
2) Stress Barriers
(a) Magma chamber pressure causes local rota-
tions in the regional stress field at mechanical
layer boundaries(dashes indicate σ1
trajectory).
(b) Dykes follow σ1
. If the trajectory changes
abruptly the dyke will terminate.
~10 m
3) Material Toughness
(a) Dyke propagates through a‘soft’material
towards a‘stiff’material.
(b) Dyke terminates or rotates to exploit the
contact. (E = Young’s Modulus, v = Poisson’s Ratio)
• All require mechanical layering, which causes
rotation of the σ1
-σ2
plane from vertical (dykes)
to horizontal (sills).
σ1
Pf
(b)
(a)
σ1
σ1
(2)
E & v
E & v
Tuff
Lava
(a)
(b)
(3)
Introduction
Igneous sill complexes represent a significant volumetric
contribution to upper crustal magma systems, and can play
an important role in petroleum system maturation and gas
generation in sedimentary basins. Despite their signifi-
cance, the causes of sill formation, particularly in terms of
the transition from dikes to sills, remains ambiguous. Host
rock mechanical layering is typically accepted as controling
this transition and promoting sill emplacement.
The purpose of this poster is to highlight the exem-
plary Loch Scridain Sill Complex (Isle of Mull, Scotland,
UK) where sills are emplaced into sub-vertical inter-
bedded metasedimentary Moine host rock. Sills show
a consistent ~26° dip to the NNW and SSE, despite cut-
ting horizontal strata, sub-vertical strata, and sub-
vertical pre-existing host rock fractures. Here, host rock
mechanical layering is not the primary control on sill
emplacement.
Conclusions & Further Work
• Sills in Mull show a thrust-type geometry within vari-
ably layered host rocks. Exploitation of shallow fractures
suggests a sub-horizontal σ1
-σ2
plane.
• NW-SE dipping sills suggests an acute angle about the
σ1
axis; they are not parallel to the σ1
-σ2
plane. Can sills
be treated as magma-filled faults, hence as a record of
the stress state?
• Future work will continue to constrain the timing and
stress state during sill emplacement elsewhere, and aims
to consider the distribution of sills as a record of
horizontal shortening.
• Future work will include:
- Fieldwork in Mull, Skye & Northumberland
- Mechanical modelling
- Analogue modelling
Case Study Sites: Isle of Mull, UK
6°0'0"W
6°0'0"W
6°4'0"W
6°4'0"W
6°8'0"W
6°8'0"W
6°12'0"W
6°12'0"W
6°16'0"W
6°16'0"W
6°20'0"W
6°20'0"W
6°24'0"W
6°24'0"W6°26'0"W6°28'0"W
56°26'0"N
56°24'0"N
56°24'0"N
56°22'0"N
56°22'0"N
56°20'0"N
56°20'0"N
56°18'0"N
56°18'0"N
56°16'0"N
Ross of Mull
Pluton
L4
L1-3
L5
Iona
Bunessan
Loch Scridain
Carsaig
Mull Lavas
Loch Scridain Sill Complex
Moine Supergroup
Assapol Fault
Thrust Fault
50 km
N
Scotland
Isle of
Mull
76
Moine
foliation
21
Lava
units
Bedding/ Foliation
average strike.dip
76
Horizontal Fracture Modes
• Sills and dykes are generally considered as forming in
the σ1-σ2 plane, as hydrofractures (Mode I).
• Fractures and faults represent a record of the stress
state: σ1
≥ σ2
≥ σ3
.
• Dykes are used to infer regional tectonic extension,
whereas, sills are thought to represent a local 'layer-
controlled' stress state.
Mode I
(opening)
σ3
σ3
σ1
σ2
Mode II
(in-plane shear)
Mode III
(out-of-plane
shear)
σ1
σ3
σ1
σ2
σ2
σ3
σ1
σ3
σ1
σ3
σ2
Dyke
Sill
Mode I
(opening)
Sill
DykeSub-horizontal
Palaeogene lavas
Sill
Location 5
Facing ENE
Sills climb at a shallow angle through sub-horizontal lavas
Sill
(Sills & dyke highlighted for clarity)
Sills climb at a shallow angle through sub-vertical Moine host rock - arrows indicate sill top & base
Location 3
Facing E
3 m
Facing ENE
Sill (highlighted red for clarity) SSE dip
NW dip
Sill
Moine host rock
3B
3A
(Sill highlighted red for clarity)
Sill - Host Rock Relationships