000m34

1

35 36 37 38

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134000m

35 36 37 38

139000m

000m957

96

97

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00

01

000m028

795000m

96

97

98

99

00

01

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

B

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F

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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500HEIG

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HORIZONTAL DISTANCE (M)

1:10000

LON

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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

4746

494048

54

50

28

4236

30

4042

484838

24

18

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52

40

58

3856

33

31

4042

3844

40

48

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48

12

12

22

11

18

1830

19

40

38

30

34

25 28

40

30

36

3028

49

48

37

2428

16

42

2026

1828

24

38

44

28

44

44

48

6222

34

2024

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1824

52

62

74

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31

35 36 37

35 36 37

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00

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97

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

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HT (

M)

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35000 9

9920

NG

36380 9

9920

stri

ke-s

lip f

ault

HEIG

HT (

M)

500

400

300

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100

0

-100

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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.