Continental Collision Zones

Question 1

Strike slip fault

Answer 1

No vertical movement, plates move horizontally w.r.t. each other
Can be sinistral/dextral

Question 2

Reverse/thrust fault

Answer 2

Hanging wall moves upwards

Compression

Question 3

Normal fault

Answer 3

Hanging wall moves downwards

Extension

Question 4

Himalayas and the Tibetan Plateau

Answer 4

Indian plate colliding with the Eurasian plate
Very large - 2000km profile and 1800-2800km convergence

Major north-dipping thrusts which get younger towards the south

Flat-topped tibetan plateau less than 5000m high

Made up of terranes, accreted onto the south margin of Eurasia before collision, separated by steep sutures

Main Central Thrust (MCT) - reverse fault
South Tibetan Detachment (STD) - normal fault
- active at the same time
- now largely inactive
- bound the Greater Himalayan sequence

Question 5

Terranes =

Answer 5

Series of former volcanic arcs

Question 6

Why does the Tibetan Plateau have the structure it does?

Answer 6

HOMOGENOUS DEFORMATION; CRUST DEFORMS AS A CONTINUUM BY INCREASING THICKNESS
Terranes also make the crust very thick
= high % of radioactive i.e. incompatible elements
= lots of partial melting

This makes the lithosphere weak
Evidence = low resistivity anomaly (liquids are good conductors and can carry charge)

= does not support differential topography
= plateau
= strong lithosphere around acts like a “buttress”

GPS velocity fields also show clockwise rotation of vectors to the NE/SE suggesting “escaping” material
Also gradual decrease in velocity rather than steps
Focal mechanisms show extension on the plateau and compression in the Himalayas

= Suggests plateau is extending outwards to maintain isostatic equilibrium (as it is very high)

N.B. Also triggers formation of leucogranites intruded into metamorphosed sediments

Question 7

Leucogranite =

Answer 7

Granites formed by in situ melting

Question 8

Western Alps

Answer 8

Europian and Apulian (Italian) plates colliding

190km long profile (much smaller)

250-500m shortening

“Wedge profile” like Critical Taper Theory
- due to thrusting and folding

“CONVERGENCE ACCOMODATED BY THRUSTING AND FOLDING”

No evidence of partial melting = stronger crust
Crustal thickness increases to more than 50km in a narrow band
Results in a rugged topography and wedge shape