Classification of plate boundary types (bit of review and expansion):

• Divergent:
  • continental rifting, for example the EAR.
  • seafloor spreading, for example the MAR with Iceland as an anomaly where the rift is exposed.
• Convergent:
  • subduction zones, where oceanic crust is recycled into the mantle.
  • terrane accretion, where a terrane such as an arc is added to the active margin.
  • collision zones, where continental fragments collide, produce a mountain belt and close an ocean basin.
• Transcurrent: where two plates slide past each other. For example, parts of the the San Andreas system.
• Intermediates (with oblique motion components).
  • transpression: oblique motion with dominant boundary parallel motion, but with a convergent component.
  • transtension: oblique motion with dominant boundary parallel motion, but with a divergent component. For example the Dead Sea Transform

Subduction-arc complexes - the return flow:

• review - site of arc volcanism in the overriding plate, and of a deep trench (up to 12 km below sea level) that marks the boundary.
• surface elements: trench, inner and outer trench walls, arc-trench gap, arc, rear arc basins or fold-thrust belts (click on adjacent diagram).
• Benioff zone: This is a zone of earthquakes that in 3-D defines a surface that slants into the mantle to depths of 670 km. It represents the colder top of the subducting oceanic lithosphere. There is debate as to the mechanisms that produce the deep earthquakes. They must be different than those that produce shallow earthquakes.
• accretionary wedges and melanges: melange (meaning mixture) is composed of blocks of basalt, limestone, serpentine, gabbro, blueschists and eclogites (and more) that are embedded in a highly contorted mess of sedimentary rocks, including deep marine oozes and volcanogenic sediments. This collects above the descending plate as an accretionary wedge.
• a chain of thought: large, deep positive gravity anomaly over the subducting plate can be related to phase and density changes in the mantle, which in turn suggests the mechanism of trench pull.

Mountain building - collisional processes and complexes:

• Wilson cycle framework: basic idea is that oceanic basins open, grow and then close along lines of previous closure.
• arc and continental crust is nonsubductable, so impingement must be produced by other processes. It is non-subductable because of lighter density, greater thickness, and weaker character.
• four major processes involved in forming mountain belts:
  • accretion of exotic and suspect terranes: occurs during subduction. you might think of this process as accreting extra large piece of nonsubductable type material.
  • crustal thickening: primarily by folding and faulting (will talk about next lecture). The crust beneath the Himalayas and Tibet is about 70 km thick, double the normal thickness.
  • escape tectonics: fault bounded wedges shoved out of the way to the side. Himalayas as the example.
  • gravitational collapse/spreading: mountain can only grow so high, and when the roots collapse they spread out similar to the way an ice cap spreads out.
• Himalayas, Appalachians, Urals: as some of many examples.

Transcurrent tectonics:

• complexities from locking and releasing bends:
• 
• San Andreas as an example.

Supercontinents:

• Gondwana
• Laurasia
• Pangea
• Rodinia

Hot spots and mantle convection.

classic story of Hawaiian chain and hot spot track.

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