S339 Tutorial 4 April 2011

• 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

www.robingill.f2s.com/S339/Answers_folder/Trial-exam-question-answers)

• Folding and kinematics 3.3 Sheath folds 4.2 Vergence

Fold exercises Revision of structural terms and concepts

• Metamorphism during mountain building (Section 6) A6.1 Mineral ID in the Virtual Microscope

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:http://www.see.leeds.ac.uk/structure/learnstructure/

Morcles fold nappeHelvetic AlpsSwitzerland (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 movementChoosing 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 symptommay be sufficient on its own; initial suspicions are followed by furthertests.’

Key clues to mineral identity are provided in Table 6.2.1 (Activity 4.6.2):

PPLReliefColour and pleochroismCleavages/fractureCrystal form (e.g. fibrous)

Crossed polarsIsotropic/anisotropicInterference colour and birefringenceStraight or oblique extinctionTwinning (plagioclase)

NB: Many optical properties depend on crystal orientation (i.e. youneed to rotate the stage) and which cross-section through the mineralthe thin section offers you.

Identify the following from Table 6.2.1

1. Colourless low relief equant crystals with white interference colour and patchyor undulating extinction.

2. Elongate high-relief crystals with pale blue to colourless pleochrism. Some crystalsexhibit 2 non-perpendicular cleavages.

3. High-relief colourless fibres with straight extinction and grey to blue interferencecolours.

4. Colourless low relief equant crystals and fine opaque inclusions, some withyellowish pleochroic haloes. Grey to white interference colour.

5. Colourless high-relief crystals with two cleavages. Grey to yellow interferencecolour and straight extinction.

6. Colourless low relief elongate crystals with marked single cleavage parallel toelongation. Bright intense interference colours and straight speckly extinction.

Fault-plane solutions explained (Figs. 7.12, 7.45)

Dehydration melting

After Inger and Harris 1993

Dehydration melting is the name given to crustalmelting associated with the breakdown (= dehydration)of a hydrous mineral such as muscovite at high T.

In dehydrating, it reacts with other minerals presentto form a hydrous melt and a new, wholly anhydrousmineral assemblage (see equation).

Granite formation by dehydration melting does not occur under water-saturated conditions (dashedsolidus right), because any melt formed would beunable to ascend without solidifying. The curved pathshows how dehydration melting under water-undersat-urated (vapour-free) conditions yields a melt that canascend through dykes and form higher-level sillsbefore the solidus is crossed. This fits the emplacementmechanism for the Himalayan leucogranites.

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 accommodated?

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 showbody colour, pleochroism or extinction angle. Most of the marks are allocated toaccurate description of the mineral properties. The mark for identifying this mineralwill 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

Question 1For 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 solutionB Foreland thrust belt II An ophiolite complexC Internal zone basement massif III Ductile behaviourD Orogenic suture zone IV Partial melting textures

V An imbricate fanVI Sheath foldsVII A blueschist mélange

Question 2•Discuss the relationship between plate convergence and uplift of narrow mountain belts by considering:(i) how crustal shortening is achieved in the Himalayas. (4 marks)(ii) how this varies along the Himalayan chain. (4 marks)(iii) what is the cause of Himalayan uplift? (3 marks)(iv) the relationship between uplift rate and local structures in the western Himalayas. (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 www.robingill.f2s.com/S339/Answers_folder/Trial-exam-question-answers

Part 1 question

Part 2 question

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

From: Igneous rocks and processes – a practical guide. Wiley-Blackwell 2010

Same general pattern for Archaean genisses worldwide

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 amphiboliteGFA = garnet-free amphiboliteECL = eclogite

Archaean tonalite genesis models 3

Archaean tonalite genesis models 4