Metamorphism of Upper Crust

Question 1

What is typical conditions for crustal extension?

Answer 1

Low P, regional metamorphism, high geothermal gradient

Question 2

Name 5 characteristics of extensional metamorphism

Answer 2

1. Regional with large scale heat source
2. Low/med P assemblages
3. Flat lying high grade extensional fabrics
4. Typically associated with basic magmatism, may cause low grade contact
5. P-T-t paths - uplift followed by isobaric (cooling at constant pressure) cooling

Question 3

Name five features of contact metamorphism

N.b. also remember diagram

Answer 3

1. Small short lived heat source, local high geothermal gradient, overlapping aureoles may create large scale
2. Igneous intrusions exploit crustal weakness, fabrics localised in contact zone or heating causes ductile deformation
3. Polymetamorphism and post-tectonic, typically overgrow earlier low grade regional fabric, granite melts formed at depths migrate up causing contact
4. Magmatic loading - increased pressure in crust due to magma weight
5. Retrogression common from fluids released from crystallising plutons, variable fluid composition, catalyse reactions

Question 4

State six features of crustal thickening

Answer 4

1. Complex tectonics with polymetamorphism, lots of syntectonic growth
2. Regional, units separated by faults, evidence of lateral heat flow
3. Barrovian metamorphism series - (Chl, Bi, Gt, Staur, Ky, Sill)
4. Typically clockwise P-T-t path
5. Associated with granite magmation
6. Uplift driven by isostasy gives erosion giving clastic sediments and extension giving shallow angle detachments, mylonites

Question 5

Describe a metamorphic core complex

Answer 5

Thickened crust, thinned by extensional detachements. Major mylonite zone separates high grade core from low grade overlying rocks

Question 6

What does crustal unloading result in?

Answer 6

Uplift of metamorphic core

Question 7

What lubricates the mylonite zones?

Answer 7

Granitic melts

Question 8

What can weaken detachment zone?

Answer 8

Melts moving into weak zones

Question 9

Describe transtension dominant strike slip

Answer 9

Oblique extension, basin opening (periodic compression, giving small scale short lived orogenesis)

Question 10

Describe transpressional strike slip

Answer 10

Orogenic setting, in thickened crust, major strike slip movement associated with collision. Periodic transtension can trigger erosion, uplift and unroofing

Question 11

What happens during strike slip in an arc setting?

Answer 11

Small continental terranes migrate along pacific-type margins collecting/colliding embayment terranes