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? GabbroBasalt

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 melt adiabatically rising mantle material magma MOR Mantle sediments, igneous crust & mantle island arc trench earthquakes ocean crust earthquakes continental 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 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