MOR Discontinuities Flashcards
1
Q
MOR discontinuities: 1st - 4th order
A
- Transform faults
- Large overlapping spreading enters (OSCs)
- Small OSCs
- Deviations from axial linearity (DEVALs)
2
Q
Fracture zones
A
- Long linear bathymetric depression
- Perpendicular to ridge
- Active transforms and fossil traces
3
Q
Fracture zones at slow spreading ridges
A
- 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
4
Q
Fracture zones at fast spreading ridges
A
- Wide shear zones
- 10’s of km wide
- Ex. Clipperton on East Pacific rise
5
Q
Examples of 3 fracture zones w/ offset and age contrast
A
- Clipperton, 85km, 2Ma
- Vema, 310km, 20Ma
- Romanche, 950km, 55Ma
6
Q
Why do slow ridges have narrow fracture zones?
A
- Large difference in age, seafloor depth and lithospheric thickness
- Prevents fault migration, keeps fault zone narrow
7
Q
Why do fast ridges have wide shear zones?
A
- Small contrast in age and lithospheric thickness, more young material
- Little constraint on exact location of transform fault
8
Q
Transverse ridges
A
- 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)
9
Q
Leaky Transforms
A
- Mostly boundary-parallel relative plate motion
- Small component of extension
- Change in rotation pole?
- Ex. Gibraltar, Gulf of California, Cayman Islands
10
Q
MOR discontinuities: Transform faults
A
- Ridge offset >50km, occur every 300-500km
- Axial depth increase 100’s of m
- Offset ridge segments evolve separately, different basalt chemistry
11
Q
Clipperton Transform Fault
A
- 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
12
Q
Slow spreading ridge along-axis profile
A
- 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
13
Q
Spreading center overlap = ?
A
- Approximately 3 x the offset distance
14
Q
When transforms fail to develop is?
A
- Deviation on fast ridges with < 15km lateral offsets
- Lithosphere is too weak/thin
15
Q
OSC
A
Overlapping Spreading Center
16
Q
OSC on EPR
A
- 8km of offset
- 600m deep overlap basin
17
Q
How are ridge tips abandoned?
A
- 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
18
Q
V-shaped wake
A
- Migration of OSC can reveal a v-shaped pattern of abandoned ridge segments
19
Q
Large OSC’s vs Small
A
- 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
20
Q
DEVALs
A
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
21
Q
Ridge segmentation is related to what?
A
- Mantle upwelling
- 30-60km beneath ridge decompression partial melting in upwelling asthenosphere
- Melt partitioned at different levels to feed ridge segments
