Oceanic Lithosphere Deformation

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Oceanic Lithosphere Deformation
GEOL466 - Lect. 11
1. Oceanic Core Complexes
2. Asymmetric Ocean Rifting
3. Seafloor Hydrothermal Activity
4. Rheological implications
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1. Oceanic Core Complexes

• Transform Zones cut Ocean Ridges at a right angle

• Extensional forces acting on oceanic plates near
  a spreading center produce a variety of seafloor
  morphologies.

• One of the yet unexplained seafloor morphologies
  consists in large, domal massifs that appear to
  form during periods when much of the extensional
  strain is localized along a detachment fault
  Oceanic Core Complexes

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1. Oceanic Core Complexes

• Sustained relative motion along the fault
  results in the footwall being unroofed to expose
  lower crustal gabbro and upper mantle peridotite
  at the seafloor.
• Adjacent upper crustal blocks (hanging wall)
  show typical volcanic seafloor morphology.
• Such large domal massifs are often found at the
  inside corner of ridge-transform intersections
  (RTI)

inside corner
outside corner
outside corner
inside corner
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1. Oceanic Core Complexes
Cross-section of an oceanic core complex

• detachment fault
• footwall
• hanging wall
• asymmetry

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1. Oceanic Core Complexes
An outstanding example of an oceanic core complex
has been mapped at the eastern intersection of
the Mid-Atlantic Ridge and the Atlantis transform
fault.
MAR 30N
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Northern Mid-Atlantic Ridge
MAR 30N
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1. Oceanic Core Complexes
oceanic dome
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1. Oceanic Core Complexes
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Perspective view of MAR 30N core
complex vertical exageration 2x
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1. Oceanic Core Complexes
Map view of MAR 30N core complex
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1. Oceanic Core Complexes - The Atlantis Massif
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2. Asymmetric Ocean Rifting
Is ocean rifting symmetric or asymmetric with
respect to ridge axis ?
fast
intermediate
slow
it depends on the spreading rate ...
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2. Asymmetric Ocean Rifting
Multibeam bathymetry
Interpretation
Ze, Zw neovolcanics Be, Bw Brunhes M Matuyama F fa
ult
Magnetic profile
Magmatic accretion is extremely asymmetric at the
ends of slow-spreading ridges (Allerton et al.,
2000)
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2. Asymmetric Ocean Rifting
Magmatic accretion history - past 0.7
Myr (Allerton et al., 2000)
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2. Asymmetric Ocean Rifting
Symmetric accretion model - segment
center (Allerton et al., 2000)
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2. Asymmetric Ocean Rifting
Asymmetric accretion model - segment
ends (Allerton et al., 2000)
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3. Seafloor Hydrothermal Activity
Hydrothermal activity off ridge axis (Kelley et
al., 2001)
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3. Seafloor Hydrothermal Activity
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3. Seafloor Hydrothermal Activity

• A huge new type of hydrothermal vent field has
  been discovered at 30 North near the
  Mid-Atlantic Ridge.
• These structures stand up to 60 metres above the
  seafloor, making them the largest hydrothermal
  chimneys of their kind ever observed.
• Most known hydrothermal fields are found on
  young crust, driven by hot basaltic material, but
  the newly discovered 'Lost City' field is located
  on 1.5-million-year-old crust and fuelled by vent
  fluids much cooler than the norm for 'black
  smokers'.
• This discovery implies that the area of oceanic
  crust supporting hydrothermal activity and
  abundant microbial life is much greater than was
  previously thought.

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4. Rheological implications
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4. Rheological implications
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4. Rheological implications
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4. Rheological implications
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4. Rheological implications