Lecture Seven - Extension tectonics Flashcards
When does crustal extension occur?
Crustal extension occurs when the maximum princlipal stress is roughly vertical.
Note that σ1 (usually z axis) vertical = normal faults.
Force of gravity acting on the rock pile is greater than the tectonic stresses applied in the horisontal plane.
Areas where the horisontal tectonic stresses are small and/or upwelling asthenosphere produces gravitational potential energy.
Areas where slab roll-back removes the horisontal tectonic stress from a plate boundary.
Show how progressive development of a continental rift towards separation causes the formation of new continental crust.
Explain how normal faulting can create space problems.
When there is bookshelf tilting occuring, causing faulting, there are areas where there is apparent free space.
These spaces can be filled by the hanging will folding over the footwall, forming a synform over an antiform.
These spaces can also be filled by tectonic errosion of fault walls, causing the brecica (rock fragments from faulting rocks) to fill the gap.
The gaps can also be filled with magma - this is a way in which volcanism can occur.
What do asymmetrical faults look like, and what are their characteristics?
Half graben - Graben which has been filled in with sediements. But unlike graben, half grabens are surrounded on one side by the fault, and the other side by the tilted fault block. Whereas grabens are usually surrounded on both sides by faults.
Sharp on one side and blunt on the other.
Listric faults - Curved ramps. Dip’s gradient decreases as depth increases.
What can asymmetric extensional settings lead to?
Asymmetric extension can lead to metamorphic core complexes (middle or lower crustal rocks exhumed along low angle normal shear zones).
Hot crustal rocks come up and cool on the surface.
How do extensional settings have significant effects on the thermal regime of the lithosphere?
The heat from the asthenophere heats the lithosphere, driving hydrothermal fluid convection (in faults particularly) and deposits minerals.
This heating of rocks in the lithosphere causes stretching of the crust in increase in rate (as they are made more ductile, soft and less viscous due to heating).
The increase in crustal stretching causes the crust to further heat.
This is a positive feed back loop and ultimatly leads to crustal breakage, and the break up of continents.
Or in Roberto’s words:
Thinning of the lithosphere is accompanied by upwelling of the asthenosphere - which melts and carries heat into the crust with the resulting magmas.
Thinning of crust and lithosphere increases the heat flux and the geothermal gradient.
Upshot - extending terranes tend to be hot
—-> High temperature-low pressure metamorphism
