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