MOR Discontinuities

Question 1

MOR discontinuities: 1st - 4th order

Answer 1

1. Transform faults
2. Large overlapping spreading enters (OSCs)
3. Small OSCs
4. Deviations from axial linearity (DEVALs)

Question 2

Fracture zones

Answer 2

• Long linear bathymetric depression
• Perpendicular to ridge
• Active transforms and fossil traces

Question 3

Fracture zones at slow spreading ridges

Answer 3

• Narrow fracture zones
• Deformation <1km wide
• Form valley <15km wide, 1-5km deep
• Deformation not wide compared to valley
• Ex. Vema, Romanche on Mid Atlantic

Question 4

Fracture zones at fast spreading ridges

Answer 4

• Wide shear zones
• 10’s of km wide
• Ex. Clipperton on East Pacific rise

Question 5

Examples of 3 fracture zones w/ offset and age contrast

Answer 5

• Clipperton, 85km, 2Ma
• Vema, 310km, 20Ma
• Romanche, 950km, 55Ma

Question 6

Why do slow ridges have narrow fracture zones?

Answer 6

• Large difference in age, seafloor depth and lithospheric thickness
• Prevents fault migration, keeps fault zone narrow

Question 7

Why do fast ridges have wide shear zones?

Answer 7

• Small contrast in age and lithospheric thickness, more young material
• Little constraint on exact location of transform fault

Question 8

Transverse ridges

Answer 8

• Form at transform w/ mostly boundary-parallel relative plate motion
• But small component of compression/extension
• Small changes in spreading direction (tectonic or flexural uplift)

Question 9

Leaky Transforms

Answer 9

• Mostly boundary-parallel relative plate motion
• Small component of extension
• Change in rotation pole?
• Ex. Gibraltar, Gulf of California, Cayman Islands

Question 10

MOR discontinuities: Transform faults

Answer 10

• Ridge offset >50km, occur every 300-500km
• Axial depth increase 100’s of m
• Offset ridge segments evolve separately, different basalt chemistry

Question 11

Clipperton Transform Fault

Answer 11

• East Pacific Rise, approx. 10 degrees N
• South ridge segment is shallow, broad, has a swollen magma chamber, axial graben in places
• North ridge is deep, narrow ridge, no axial graben, no AMC, magnetically starved

Question 12

Slow spreading ridge along-axis profile

Answer 12

• FZ crust typically thin, fractured, altered basalt underlain by serpentinized ultramafic rocks
• Reduced magma supply at ridge offsets
• Magma supply typically highest beneath central part of segment
• Magmatic centers separated by transform faults

Question 13

Spreading center overlap = ?

Answer 13

• Approximately 3 x the offset distance

Question 14

When transforms fail to develop is?

Answer 14

• Deviation on fast ridges with < 15km lateral offsets

- Lithosphere is too weak/thin

Question 15

OSC

Answer 15

Overlapping Spreading Center

Question 16

OSC on EPR

Answer 16

• 8km of offset

- 600m deep overlap basin

Question 17

How are ridge tips abandoned?

Answer 17

• Spreading initialized, then propagation of spreading centers along strike
• Spreading centers overlap, curve towards each other, encircle a zone of shear and rotational deformation, OSC established
• Progressive shear and rotational deformation continues until one OSC links with the other
• Continuous spreading is established and OSC abandoned, overlap zone rafted away

Question 18

V-shaped wake

Answer 18

• Migration of OSC can reveal a v-shaped pattern of abandoned ridge segments

Question 19

Large OSC’s vs Small

Answer 19

• Large: 3-20km offset every 50-300km, axial depth increase >100m, Create off-axis scars, or Abandoned Ridges and overlap basin due to OSC migration, Long-lived 0.5-3Ma
• Small: 0.3-3km offset every 30-100km, axial depth increase 10’s of m, Minimal off-axis scars, short-lived <10Ka
• Both have magma supply max at segment centre, minimum at ends and different magma sources for segments

Question 20

DEVALs

Answer 20

Deviations in Axial Linearity

• Occur every 10-50km
• Bend in axis but no depth change
• Generally detected by geochem variation only
• Incomplete mixing in a single magma chamger

Question 21

Ridge segmentation is related to what?

Answer 21

• Mantle upwelling
• 30-60km beneath ridge decompression partial melting in upwelling asthenosphere
• Melt partitioned at different levels to feed ridge segments