Post on 11-Dec-2015
transcript
Class 9. Mid-Ocean Ridges
Dax Soule
OCEAN/ESS 410
Mid-Ocean Ridges
Dax Soule
Modern Map
What makes these so different?
From MacDonald 1982
Learning Goals
• Be able to sketch the structure of fast and slow spreading mid-ocean ridges and describe how they differ
• Describe how and why the structure and across-axis bathymetry of slow and fast ridges differ in terms of magmatic, tectonic and hydrothermal processes that form the crust
A favorite mid-term question
Catalyst Questions
• List one example for both a slow and fast spreading rate mid-ocean ridge
• Indicate on which you would expect for find high temperature hydrothermal venting
• Indicate on which you would expect to find the larger earthquakes
Oceanic Crust
Stratigraphy
From Karson 2002
• Deep sea drilling program
• Dredging of fracture zone scarps
• Ophiolites
What’s the difference?
Gabbro Basalt
Upper crust
From Karson 2002
Upper basalt – morphology primarily pillow, but lobate and sheet also evident; variable thickness; high porosity
Lower basalt – lobate and sheet are common; increased fracturing and hydrothermal alteration
Transition zone – fractured sheet flows cut by dikes; gradational
Sheeted dikes – subparallel alignment; ~1m width; dip away from ridge
A/B/C
C/D
Modified from Nedimovic and Carbotte 2008
Upper crust – Seismic layer 2
Multi-channel seismic (MCS) studies are able to identify layer boundaries based on impedance contrasts; tomography identifies changes relative to reference model
Nature of transition from 2A to 2B is focus of community debate:1) Lithologic – boundary between high-
porosity basalt flows to low-porosity sheeted dikes
2) Hydrothermal – alteration front within upper extrusive volcanic layer
Fracturing and hydrothermal alteration contribute to seismic anisotropy
Seismic Layer 3
Compare and contrast
• Examine these schematic drawings of fast and slow spreading mid-ocean ridges
• List the ways in which the structure and across-axis bathymetry of slow and fast ridges differ in terms of magmatic, tectonic and hydrothermal processes that form the crust
Pair up
Compare your answers Share what you have observed
• Magmatic systems:• Tectonics:• Hydrothermal systems:
Fast-spreading < 8 cm/yr
seismic zone
Courtesy of D. Fornari
Slow-spreading 2-5 cm/yr
Temporary magma chamber
Mantle meltadiabatically rising
mantle material
magma
MOR
Mantle
sediments, igneous crust & mantle
island arctrench
earthquakes
ocean crust earthquakescontinental crust
melt
fracture zone
trench
Plate Tectonics
Magmatics
Slow spreading ridges• Mush-filled chamber with
no melt lens• Short lived AMC feeds
localized volcanic structures within the axial valley
• Undifferentiated lavas
Fast spreading ridges• Thin, narrow, sill like body
of melt overlying a thicker, wider crystal mush zone.
• Steady state AMC• Wide range of differentiated
lavas • Large low velocity zone
extending to the base of the crust
Tectonics
Slow spreading ridges• Large rift valleys (10 – 20 km
wide)• Rugged topography with relief
up to 1000 m • Earthquakes can occur to depths
of 8 km or more• Maximum EQ magnitude = 5.5
Fast spreading ridges• No major tectonic faults
bounding the axial valley• Axial summit with trough
~100 wide and 10-20 m deep• Small relief (smooth
topography like a dome) • Seismic activity constrained
to depths shallower than the AMC (max depth 2 km)
• Maximum EQ magnitude = 2
Hydrothermal Circulation Fast spreading ridges
• Circulation depth controlled by the depth of the AMC
• Along axis convection of hydrothermal fluids?
Slow spreading ridges
• Circulation controlled by large regional faults
• Across axis convection of hydrothermal fluids
What about intermediate spreading-rate ridges
4-8 cm/yr
Are they more like slow or fast spreading ridges
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. Gorda
Ridge
Juan de Fuca Ridge
Melt Production at ultraslow spreading ridges
Base of Lithosphere
Melting shuts off because uppermost mantle cool conductively
Average Crustal thickness < 6km
Ultraslow spreading ridges. Not enough melting to generate continuous crustal
coverage
Exit Slip
• Explain how slow and fast spreading centers differ in terms of the cause and intensity of seismic activity
• Sketch your choice of:– A slow spreading center– A fast spreading center – A depth section showing seismic layers 1 – 3