S339 Tutorial 4 April 2011

1. S339Tutorial 4 April 2011 • Folding and kinematics3.3 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 • 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)

2. 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/

3. Morcles fold nappe Helvetic Alps Switzerland (cf. Figs. 3.3 and 5.13)

5. Sheath fold in rheomorphic welded pyroclastics, Pantelleria Island, off Sicily

6. Sheath fold in rheomorphic welded pyroclastics, Pantelleria Island, off Sicily

7. Kinematics – determining sense of movement Choosing a fold limb to determine cleavage vergence (Box 4.2)

8. Lateral ramps

10. 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 tests.’ Key clues to mineral identity are provided in Table 6.2.1 (Activity 4.6.2): PPL Relief Colour and pleochroism Cleavages/fracture Crystal form (e.g. fibrous) Crossed polars Isotropic/anisotropic 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.

11. 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 colours. 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.

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

13. 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-undersat- urated (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

15. 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 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).

16. 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 • 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 2 question

17. 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.

18. Archaean tonalite genesis models 1

19. Archaean tonalite genesis models 1 b Same general pattern for Archaean genisses worldwide From: Igneous rocks and processes – a practical guide. Wiley-Blackwell 2010

20. Why? Remember REE partition coefficients, especially for garnet From: Igneous rocks and processes – a practical guide. Wiley-Blackwell 2010

21. Archaean tonalite genesis models 2 GA = garnet amphibolite GFA = garnet-free amphibolite ECL = eclogite

22. Archaean tonalite genesis models 3

23. Archaean tonalite genesis models 4