S339 Tutorial 4 April 2011
• Folding and kinematics
Sheath folds
Fold exercises
Revision of structural terms and concepts
• Metamorphism during mountain building (Section 6)
A6.1 Mineral ID in the Virtual Microscope
• Earthquake fault-plane solutions (Figs. 7.12, 7.45)
• Dehydration melting (Figs. 7.35, 7.36)
• Exam questions to work on in pairs
(answers at
Out of your depth in structural geology?
Block 4 not helping much?
If so, try visiting Rob Butler’s on-line introduction to the subject at:
Morcles fold nappe
Helvetic Alps
Switzerland (cf. Figs. 3.3
and 5.13)
Sheath fold in rheomorphic welded pyroclastics, Pantelleria Island, off Sicily
Sheath fold in rheomorphic
welded pyroclastics,
Pantelleria Island, off Sicily
Kinematics – determining sense of movement
Choosing a fold limb to determine cleavage vergence (Box 4.2)
Lateral ramps
Mineral ID in the Virtual Microscope
‘Mineral identification is like a medical diagnosis: no one symptom
may be sufficient on its own; initial suspicions are followed by further
Key clues to mineral identity are provided in Table 6.2.1 (Activity 4.6.2):
Colour and pleochroism
Crystal form (e.g. fibrous)
Crossed polars
Interference colour and birefringence
Straight or oblique extinction
Twinning (plagioclase)
NB: Many optical properties depend on crystal orientation (i.e. you
need to rotate the stage) and which cross-section through the mineral
the thin section offers you.
Identify the following from Table 6.2.1
1. Colourless low relief equant crystals with white interference colour and patchy
or undulating extinction.
2. Elongate high-relief crystals with pale blue to colourless pleochrism. Some crystals
exhibit 2 non-perpendicular cleavages.
3. High-relief colourless fibres with straight extinction and grey to blue interference
4. Colourless low relief equant crystals and fine opaque inclusions, some with
yellowish pleochroic haloes. Grey to white interference colour.
5. Colourless high-relief crystals with two cleavages. Grey to yellow interference
colour and straight extinction.
6. Colourless low relief elongate crystals with marked single cleavage parallel to
elongation. Bright intense interference colours and straight speckly extinction.
Fault-plane solutions explained (Figs. 7.12, 7.45)
Dehydration melting
Dehydration melting is the name given to crustal
melting associated with the breakdown (= dehydration)
of a hydrous mineral such as muscovite at high T.
In dehydrating, it reacts with other minerals present
to form a hydrous melt and a new, wholly anhydrous
mineral assemblage (see equation).
Granite formation by dehydration melting does not
occur under water-saturated conditions (dashed
solidus right), because any melt formed would be
unable to ascend without solidifying. The curved path
shows how dehydration melting under water-undersaturated (vapour-free) conditions yields a melt that can
ascend through dykes and form higher-level sills
before the solidus is crossed. This fits the emplacement
mechanism for the Himalayan leucogranites.
After Inger and Harris 1993
TMA04 hints 2011K
Q1(a) (i) & (ii): the transition between the 2 episodes of movement involves some
anomalous bumps and jerks. Ignore these and concentrate on the 2 separate lines.
Q1(a)(iii) and (iv): be sure to give the units.
Q1(b)(i): how has the tectonic mechanism changed? How is the movement
Q1(b)(ii): Note the 20 marks allocated to this section. It needs to embrace all the
relevant possibilities covered in Block 4, though not necessarily at great length.
Q2(a): The one rotatable view of the porphyroblast is not ideally oriented to show
body colour, pleochroism or extinction angle. Most of the marks are allocated to
accurate description of the mineral properties. The mark for identifying this mineral
will be awarded flexibly on the basis of consistency with your description, so don’t
get in a lather about it!
The same applies to the matrix minerals in (b).
Exam questions to plan
Part 1 question
Question 1
For each of the components A-D of a typical mountain belt, choose two of the features I-VII that are characteristic of them.
A Deep crustal shear zone
I Pressure solution
B Foreland thrust belt
II An ophiolite complex
C Internal zone basement massif
III Ductile behaviour
D Orogenic suture zone
IV Partial melting textures
V An imbricate fan
VI Sheath folds
VII A blueschist mélange
Part 2 question
Question 2
(a)Discuss the relationship between plate convergence and uplift of narrow mountain belts by considering:
(i) how crustal shortening is achieved in the Himalayas.
(ii) how this varies along the Himalayan chain.
(iii) what is the cause of Himalayan uplift?
(iv) the relationship between uplift rate and local structures in the western Himalayas.
(4 marks)
(4 marks)
(3 marks)
(4 marks)
(b) Discuss the relationship between plate convergence and uplift of wide plateaux by considering possible causes for rapid
uplift of the Tibetan plateau millions of years after plate collision.
(8 marks)
Outline answers at
3600 Ma-old Isua ‘grey’ gneisses, West Greenland
(‘grey’ because, being tonalitic, they lack pink K-feldspar.)
Note podded mafic dykes and later leucocratic sheets.
Archaean tonalite genesis models 1
Archaean tonalite genesis models 1 b
Same general pattern for Archaean genisses worldwide
From: Igneous rocks and processes – a practical guide. Wiley-Blackwell 2010
Why? Remember REE partition coefficients, especially for garnet
From: Igneous rocks and processes – a practical guide. Wiley-Blackwell 2010
Archaean tonalite genesis models 2
GA = garnet amphibolite
GFA = garnet-free amphibolite
ECL = eclogite
Archaean tonalite genesis models 3
Archaean tonalite genesis models 4
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