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000m34
1
35 36 37 38
000m39
1
134000m
35 36 37 38
139000m
000m957
96
97
98
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000m028
795000m
96
97
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802000m
Scale 1:10000ALISON SANDISONAberdeen University
Apr 01, 2015 09:17
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 km
© Crown copyright and database rights 2015 Ordnance Survey (Digimap Licence). FOR EDUCATIONAL USE ONLY.
A
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G
HI
500 1000HEIG
HT (
M)
HORIZONTAL DISTANCE (M)
A B C
NM
3590 9
752
CROSS SECTION 'ABC' OF THE CENTRAL AND WESTERN SUITE OF THE ISLE OF RUM
NM
3500 9
843
LO
NG
LO
CH
FAU
LTN
M3650 9
744
1:10000
NW SE ESE
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100
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500HEIG
HT (
M)
HORIZONTAL DISTANCE (M)
1:10000
LON
G LO
CH
FAU
LT
CROSS SECTION 'CDE' OF THE CENTRAL SUITE OF THE ISLE OF RUM
DC ESW NE/NW SE
NM
3500 9
843
NM
3662
9754
NM
3684 9
733
DONE BY: ANDREW MOYLE
51228528
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35 36 37
35 36 37
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LOCATION TOPOGRAPHY GEOLOGICAL ZONES OF RUM
SOLID GEOLOGY OF THE WESTERN AND CENTRAL IGNEOUS SUITE OF ISLE OF RUM , INNER HEBRIDES, SCOTLAND
STEREONET SHOWING DISTRIBUTION OF POLES TO FRACTURE PLANES ALONG THE N-S LONG LOCH FAULT [NM36372 00209] TO [NM36189 96940]
STEREONET SHOWING THE 3 MOST COMMON FRACTURE PLANES ALONG THE N-S LONG LOCH FAULT [NM36372 00209] TO [NM36189 96940].
N N
n=382 n=382
1
2
31- Most common fracture plane3- least common fracture plane
R
P
R'
Fig 1.) Reidel Model for strike-slip faults (Fossen 2010)
The 3 major fracture planes show a very similar pattern to the Reidel Model of a Dextal Strike Slip Fault.
In the Reidel Model (fig 1.), the first sets of fractures to form in an overall strike-slip zone are shear fractures. One set, known as Reidel shear fractures or R-fractures, make a low angle with the overallshear zone and show the same sense of slip. P-Shear Fractures (P) usually develops after the establishment of R-fractures, and development is probably related to temporal variations in the local stress field along the shear zone as offset accumulates. A third set of shear fractures (anitithetic fractures) make a high angle to the zone. These are called R’-shear fractures, and are generally less well developed than R-fractures. (Fossen 2010)
ϕ
ϕ = angle of internal friction
HORIZONTAL DISTANCE (M)1:10000
CROSS SECTION 'FG' OF THE CENTRAL AND WESTERN SUITE OF THE ISLE OF RUM
NW SEF G
NM
35000 9
8599
NM
35480 9
6920
HEIG
HT (
M)
500
400
300
200
100
0
-100
-200
-300
500
400
300
200
100
0
-100
-200
-300
1000500 1500
NG
35000 9
9920
NG
36380 9
9920
stri
ke-s
lip f
ault
HEIG
HT (
M)
500
400
300
200
100
0
-100
-200
-300
500
400
300
200
100
0
-100
-200
-300
500 1000
CROSS SECTION 'H-I' OF THE CENTRAL AND WESTERN SUITE OF THE ISLE OF RUM
HORIZONTAL DISTANCE (M)1:10000
H IW E
KEY
LITHOLOGIES
Basic Dyke (Basalt/Dolerite)
Basic Plug (Dolerite)
Olivine Gabbro
Ultrabasic Plug (Peridotite(P) and Feldspathic Peridotite (FP))
Rock Y (Troctolite)
Ultramaifc Breccia (Feldspathic Peridotite and Troctolite Breccia in Feldspathic Peridotite Matrix)
Feldspathic Peridotite
Intrusive Grandodiorite-Ash
Microgranite
Intrusive Breccia (Red Sandstone and Felsic Breccia in Ash Matrix)
Red Sandstone
Gneiss
BOUNDARIES
Inferred Contact
Observed Contact
Fault
Dyke
GEOLOGICAL SYMBOLS
50
70
19 Dip of layering or flowstructures in intrusive igneous rocks
Strike and dip of Sandstone
Direction of fault block movement
Strike and vertical dip of gneissic banding
Strike and dip of gneissic banding
A B
Long Loch Fault reactivates as dextral strike-slip fault, further displacing sandstone, microgranite, the ring fault and the layered igneous suite. Minor faults along the Long Loch Fault form
Unconformity
Gneiss: gneissic banding is disguished by light bands (quartz and plagioclase rich, lacking hornblende) and dark bands (quartz and plagioclase rich, hornblende rich). Structures observed in the gneissic banding include folding,stretching, brittle fracturing, normal and reverse faults, minigrabens, an echleon structures. These structure are possibly formed dring the uplift of the gneiss
Red Sandstone member is composed of <60% Quartz, <30% K-feldspar, <10% Micas. Crystalline around the aurioles of doleritic plugs, indicating contact metamorphism. Grain description vary from: very fine to medium grain size, low to high sphericity, well to very well sorted, sub rounded to rounded grain shape, and well to very well sorted, but all have a general texture of being very gritty. Ocassional detrital quartz and k-feldspar can be seen, with size up to 2cm and 4cm in length respectively. Sedimentary structures such as grading, wavy and planar lamination, and occasion soft sediment deformation can be observed. The grain desciption along with structures could indicate a large fluvial environment.
Unconformity
A microgranite intrusion occured. This felsic intrusion has a very glassy appearance and is observed in the central intrusion and west of the Western mafic intrusion boundary.
Emplacement of ring fault due to microgranite uplifting sandstone, and then caldera collapse.
Intrusive Breccia in an ash matrix. Formed as a result of caldera collapse. Breccia is made up of microgranite, gneiss, and sandstone. Breccias have a melted appearance and are angular in shape.
Long Loch Fault - N-S Dextral strike-slip fault commences with intial displacement of microgranite, sandstone and main ring fault.
Feldspathic peridotie enters the collapsed caldera through the Long Long Loch Fault. The mineral composition is <60% Olivine, <30% Pyroxene and <5-10% Plagioclase Feldspar. Generally equigranular with medium to coarse size minerals. Harrisite texture observed, indicating floor of basement. Harrisite crystals are arranged crudely to chaotic, inequigranular and medium to coarse crystal size (up to 10cm in length, 1cm thick). Mineral
compositon of Harrisites are <60% pyroxene, <30% olivine and <5-10% Plagioclase Feldspar.
Granodiorite- Ash intrusion occurs as a result of melting of the chamber walls composed of microgranite by feldspathic peridotite. The matrix has a chalky/glassy texture, white/grey in colour, and very dense. Mineralogy of the matrix: <30% Plagioclase, <20% K-Feldspar, <30% Quartz, <10% Hornblende, <2% Biotite
Long Loch Fault movement pauses, formation submagmatic central graben.
Ultramafic brecciation occurs. Breccia contained within a medium grained feldspathic peridotite matrix. Breccia is composed of feldspathic peridotite and troctolite. Length of breccia ranges from 0.5cm to 100cm, with an average length of 5cm. Smaller clasts are generally rounded, with larger clasts being angular.
Rock Y (troctolite) forms as successive layers with feldspathic peridotite. Mineralogy: <60% plagioclase, <20-30% olivine, <2-10% Pyroxene. Crystal grains are equigranular and medium to coarse in size. Large olive clusters appear on some troctolite layers. Structures observed have the appearance of flow structure observed in sedimentary rocks, including wavy structures, deformed layering, grading, interlayering, flame structures, slumping and soft sediment deformation.
Plugs of peridotite. Mineralogy: <80% olivine, <10% plagioclase feldpsar. The mineral grains are equigranular and medium sized. Wavy flow structures observed as well a vein of chromite.
Basalt/Dolerite Dykes and Plugs intrude. Mineralogy: <60%: plagioclase, <30% pyroxene, <10% Olivibne. Minerals in dykes are equigranular and medium sized and have a glassy texture. (basalts darker than dolerite). Dolerite plugs are inequigranular with medium sized mineral grains with phenocrysts of plagioclase feldspar.
GENERALISED VERTICAL SECTION OF THE WESTERN AND CENTRAL IGNEOUS SUITE OF THE ISLE OF RUM
TERTIA
RY (
PALA
EO
CEN
E)
PRE-C
AM
BRIA
NARCH
AEAN
OTHER SYMBOLS
Line of cross section
FP
FP
P
Olivine gabbro observed as plugs and sheets. Mineralogy: <60% plagioclase, <30% pyroxene, <5-20% olivine. The mineral grains are equigranularwhere medium sized observed in plugs, and coarse to very coarse observed in sheets. Flow structures observed in sheets including éye structures´and grading. Plugs show no flow structures. Olivine clusters observed in plugs.