EESC 101 Lecture 6: Convergent Boundaries

Exam 1

Subduction Zone Structure

• Trench: deep depression where oceanic plate begins to sink

• Subducting Plate: denser oceanic lithosphere descending into the mantle

• Overriding Plate: continental or oceanic plate above the subducting slab

• Volcanic/ Magmatic Arc: chain of volcanoes on overriding plate from magma generated by slab melting

• Accretionary Prism: sediments scraped off subducting plate forming a wedge at the trench (like a snow pile pushed by bull dozer)

• Back Arc Basin: depression or basin behind a volcanic arc at a subduction zone cause by overstretching of the plate

• Forearc Basin: basin between a volcanic arc and the trench at a subduction zone, forms as a result of accumulated sediment from accretionary prism

Slab Rollback

lower plate bends more steeply over time “pulling” the upper plate forward and mantle upward

Where do subducting slabs end up?

• unclear but some continue down into lower mantle

• may even sink all the way to the core-mantle boundary

  • potential cause of mantle plumes

Foreland Basin

a basin that forms next to a mountain range due to the weight of the mountains pressing down on the crust, bc crust bends downward (flexes) under the mountains, creating space for sediment to accumulate

• Appalachian Basin

Thrust Faults and Overthrust Belts

• Formed: convergent plate boundaries where plates are pushed together against gravity

  • Involve large-scale compressional faults called thrust faults

  • series of these faults creates an overthrust belt, stacking slices of crust over one another

Ophiolites

slices of oceanic crust accreted to continents

• Formation: through obduction (oceanic crust pushed onto continental crust)

• Significance: reveal oceanic plate structure

• Notes:

  • not fully representative of mid-ocean ridges

  • usually from back-arc or peripheral basins

Ophiolite Distribution in Eastern Mediterranean

important source of metals like copper

• cyprus ophiolite

  • major source of copper in the ancient world

    • first mined over 5000 years ago

    • finally extinguished major deposits in 1972

  • also contained many sulfur rich minerals

Subduction Related Earthquakes

• occur along the subducting plate down to ~660 km depth

• strongest quakes happen at plate boundaries:

  1. shallow (<~70 km): from faulting in the lithosphere, strongest

  2. deep (~70–300 km): from deformation within the slab

  3. very deep (>300km): from shifts in mineral structure, powerful but less likely to be destructive at surface

Wadati-Benioff Zone

• Definition: zone of earthquakes that traces the downgoing subducting slab

• Significance: maps the path of the subducting plate through the mantle

• Named after: Kiyoo Wadati and Hugo Benioff, who studied deep earthquakes

Volcanism in Subduction Zones

• types of volcanic arcs:

  • continental arc: volcanoes on continental plates

  • island arc: volcanoes on oceanic plates

• Location: volcanoes form at a specific distance from the plate boundary, where the subducting slab is ~150 km deep

• Mechanism:

  • subducting slab carries sediments and water

  • upper mantle has volatile minerals

  • water and volatiles lower melting point, causing melting at ~150 km depth

Ocean-Ocean Collisions

• in ocean ocean collisions, the older plate (more time to cool = more dense) will be the subducting plate

  • creates island arcs

  • EX Aleutian Islands

Ocean-Continent Collisions

• create some large volcanic mountain chambers

• volcanic material tends to be felsic (partial melting of continental crust)

• lower angle of descending slap correlates to wider mountain range

Continent-Continent Collisions

• once all intervening oceanic crust has been subducted, continents will collide

  • continental crust cannot be subducted

• Underplating: when one continental plate is pushed beneath another, slices of crust fuse to the base of the overriding plate thickening continental crust

• creates large, less volcanic mountain ranges

• Orogenesis: process of mountain formation through tectonic forces

India-Eurasia Collision

• largest collisional feature on earth today

  • not volcanically active, but still has many earthquakes