Geotherms, maps and timing

Question 1

Describe three variations of geotherm

Answer 1

1. Convection of fluids near surface initial drop, settling then fall
2. Straight geotherm, all lower crust is molten
3. Textbook geotherm - inefficient heat transport near surface

Question 2

Name and describe 3 age constraints on metamorphic rocks

Answer 2

Timing of mineral growth
relative to deformation, gives age of deformation not rock as deformation is a catalyst for reactions. Need to correlate with fabrics
Absolute age determinations on metamorphic minerals
difficult as only retain radioactive daughter at low temp so show time of cooling. Needs garnet/monazite to see peak T metamorphisms
Thermal modelling
Thrust thickens during orogeny - fast relative to geothermal changes, thrust distorts and resets geotherm

Question 3

Describe the thermal evolution of thermal crust

Answer 3

Inverted gradient rapidly lost, initial cooling in upper sheet, major heating in lower sheet, overall heating due to double heat production, thick crust thins = rock uplift, rock follows P-T loop

Question 4

What does effect does local uplift have on rocks next to it?

Answer 4

Hot rocks next to cold rocks = metamorphism through lateral flow

Question 5

Describe the Barrovian zone

Answer 5

Medium pressure, crustal thickening

Chl, Bi, Gt, Staur, Ky, Sill

Question 6

Describe the Buchan zone

Answer 6

Low pressure, extension

Bi, Cord, And, Sill

Question 7

Detail three problems with polymetamorphic rocks

Answer 7

High grade with later low grade, partial regression if fluids can get in
Low grade with later high grade, early assemblages destroyed
Two events of similar grade, may show two events

Question 8

Where are problems in polymetamorphic rocks typically found?

Answer 8

Orogenic belts