16' MidOcean Ridges Morphology William Wilcock

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16. Mid-Ocean Ridges Morphology William Wilcock
OCEAN/ESS 410
Fall 2007
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Mid-Ocean Ridge Crustal Structure

• Evidence from
• Ophiolites
• Drilling
• Geological mapping on fault scarps
• Crustal thickness is 6 km and varies little with
  spreading rate

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Fast-Spreading Ridges
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Intersecting Profiles Sections from the East
Pacific Rise
Axial magma chamber is 1 km wide, 100 m thick
but can extend along axis for 10s of kilometers.
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Fast-Spreading - East Pacific Rise
Magma chamber is probably more continuous along
axis than shown here
Sinton Detrick (1992)
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Hydrothermal Vents and Sulfide Deposits
Image courtesy of John Delaney and Deborah Kelley
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Structure of a Hydrothermal System

• Alt, 1995

Heat Uptake Zone
Alt, 1995
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Phase diagram for seawater
Sub-Critical
Seafloor
Critical point (CP)
Critical Point
Super-Critical
2-phase
1-phase
Moho
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Hydrothermal Circulation Above an Axial Magma
Chamber
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Structure of a fast-spreading ridge
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Depth, km
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Depth of magma chamber controlled by balance of
heat supply (magma) and heat loss (hydrothermal
circulation). The higher the spreading rate, the
higher the magm supply and the shallower the
magma chamber
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Magma chamber depth versus spreading rate
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Slow-Spreading Ridges
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Hydrothermal Circulation when no magma chamber
is present. Cracking front propagates down
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Slow-Spreading - Mid-Atlantic Ridge
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What about intermediate spreading-rate ridges

• Are they more like slow or fast spreading ridges

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Cross-axis relief
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Juan de Fuca and Gorda Ridges - Both have a full
spreading rate of 6 cm/yr.JdFR - looks like a
fast spreading ridge but magma chamber is
deeper.Gorda looks like a slow spreading.
Juan de Fuca Ridge
Gorda Ridge
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Along-axis segmentation
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Fast - East Pacific Rise
Slow - Mid-Atlantic Ridge
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Fast-Spreading
100 km
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Structure can be remarkably two-dimensional
within a segment Between 910 and 950N height
of rise axis of EPR changes by lt50 m
25 km
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Along-axis segmentation-- fast-spreading
ridges Different orders 1st Fracture
zones 2nd Overlapping spreading centers (can
migrate along-axis) 2nd order segments are long
(100 km) and continuous reflect a relatively
constant supply from the mantle or efficient
along-axis transport of melt at shallow levels.
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Juan de Fuca MagneticsEvidence that Overlapping
Spreading Centers Migrate
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Slow Spreading
100 km
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Structure varies substantially along axis within
a segment. The axial valley is more extended
at segment ends mantle may be exposed
(peridotite). It is more elevated in the segment
center Each segment center is associated with a
focus of mantle upwelling and melt injection
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more magmatism at segment centers
large faults, especially at segment ends
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Along-axis segmentation-- slow-spreading
ridges Different orders 1st Fracture
zones 2nd Non-transform offsets (generally do
not migrate along-axis) Segments are generally
shorter (40-80 km) reflect discontinuous supply
from the mantle and inefficient along-axis
transport of melt at shallow levels.
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Sections ALONG fast (left) and slow (right)
segments
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Why are ridges segmented

• Two ideas
• Bottom Up Explanation. Reflects pattern of
  mantle upwelling and melt delivery. Upwelling is
  more intense below segment centers
• Top Down Explanation. It is a natural
  consequence of the way brittle boundaries break.
• This is an area of active research