TUTORIAL 3 CRIB SHEET QUESTION 1 NORTH PACIFIC

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Practical 1

Tutorial 3 crib sheet

Question 1.

North Pacific

2500km of new crust in 65Myr
average rate ~38.5km/Myr

Note Spreading rate is this amount in each direction

Question 2

2a) #2 t=0 & t₀ =10Ma

#2 linear velocity of plate B = 100km/Myr, but since plate A is fixed, ½ of this is due to addition of crust to plate A west of ridge and ½ to addition of new crust to plate B west of ridge. Spreading rate of each plate is thus 50km/Myr.

ridge has moved 500km E due to new plate A
but 1000km of plate B subducted
however, also 500km of new plate B so net loss of B is 500km
anchor subducted and ammonite moves E 1000km from start position of 500 to 1500km
E of ridge active transforms, W inactive

#3 ridge E 20My x 50km/Myr --- 1000km (new plate A)

plate B also has 1000km new crust, plus has moved east 1000km due to new plate A. Thus over 20Myr 2000km of plate B subducted, anchor and ammonite have gone; net loss of B is 1000km.

2b) linear velocity is 50km/Myr; spreading rate = 25km/Myr Note A is subducting beneath B

At t₀ = 10Ma  ridge is 10x25km 250km E of start

subduction zone is 10x(25+25km)=500km E

At t₀ = 20Ma  ridge is 20x25km 500km E of start

subduction zone is 20x(25+25km)=1000km E

2c) B subducted under A and A is fixed therefore subduction is static

linear velocity of B is 40km/Myr; there is no ridge

B therefore moves 400km NW by t₀ = 10Ma and 800 km NW by t₀ = 20Ma

2d) A is subducting under B and therefore trenches will migrate NE; again linear velocity is NW 40km/Myr.

TUTORIAL 3 CRIB SHEET QUESTION 1 NORTH PACIFIC

x = 40 cos 45° (or sin 45°)

= 28

Therefore horizontal velocity component i.e. westwards  28km/Myr

Therefore in 10Myr subduction zones migrate 280 km W, and in 20Myr moves 560 km W. Relative distance between them is maintained.

 the eastern spreading ridge propagates northwards as the transform is gradually subducted, however transform itself propagates NW maintaining same relative length of transform

2e) Remember A is fixed so N subduction zone remains static whilst southern subduction zone moves west at 28km/Myr. Transform is consumed.

After t₀ = 20Ma transform has reverse sense of motion.

2f) Linear velocity 80km/Myr, spreading rate 40km/Myr (in 4 Myr - 10km)

After 10 Myr spreading ridge is 400km to west

400km of new oceanic crust on plate A

400km of new oceanic crust on plate B

800 km of plate B subducted

After 20 Myr x2

2g) B linear velocity NW 40 km/Myr

W velocity component 28km/Myr

A is fixed so lower subduction zone doesn’t move

Upper zone migrates NW. Transform lengthens