Plate boundaries Divergent plates moving apart Once a

Plate boundaries Divergent (plates moving apart) ØOnce a divergent boundary has started, new oceanic crust is extruded at the mid-ocean ridge as the two plates move apart. As the crust cools, it subsides. ØInitially, doming occurs, followed by rifting. Continued extension leads to a narrow sea and eventually an ocean basin. ØOceanic crust is initially hot and buoyant, and forms a rise. As it ages and cools, it gets denser and eventually becomes denser than the underlying mantle.

Breakup of Continent Ø Rifting does not start with a single graben that eventually becomes the ocean. There a series of parallel grabens, only one of which eventually becomes the sea. Rifting can also stall, resulting in failed rifts. Ø Rifts can accumulate huge, thick sequences of sediments. Old rifts are common sites of oil deposits (anoxic conditions, heating by basalts, thick sedimentary sequences often with well sorted sandstones). Ø Examples: East African Rift Zone, Dead Sea, Red Sea, Atlantic

Plate boundaries Convergent (plates moving towards each other) o Oceanic lithosphere can be subducted. Oceanic plate bends down at the trench. o Partial melting of oceanic lithosphere and maybe some mantle gives rise to an arcuate chain of volcanoes on the overlying plate where the oceanic plate gets to 100 km, which depends on age/density of downgoing slab. With older oceanic crust, the plate descends at a steeper angle and the volcanic chain is closer to the trench. o Continental crust does not get subducted, because it is less dense than mantle material and oceanic crust.

Plate boundaries Convergent

Opening and closing phases

Plate boundaries Transform (plates moving past each other, strike-slip features) Unless the boundary is a perfect great circle (straight line-equivalent), there will be tiny areas of convergence and of divergence, resulting in small compressive and tensional regions.

Plate Interiors • In plate tectonics theory, most deformation occurs at the boundaries. Plate interiors are more passive and only respond to loading and unloading, as thin, elastic plates. • Hot spots are trails of volcanic activity that appear unrelated to plate boundaries, but do trace plate motion. These seem to be fixed with the mantle, and the tracks record plate motion. Typically, there is a chain of volcanic features of progressive age. The youngest feature marks the current position of the hotspot.

Wilson cycles: based on birth and closure of oceans (Atlantic Ocean), 1966

Intracratonic Basins location and tectonic setting: in anorogenic areas on cratons Rift basin • Geological Origin: The down-dropped basin formed during rifting because of stretching and thinning of the continental crust

Passive margin basin Passive Margin Basin are those basins that form on the margins of continents that are not active tectonic or fault-bounded margins. They are typically very large and may have a substantial thickness. • Geological Origin: Subsidence along a passive margin, mostly due to long-term accumulation of sediments on the continental shelf • Example: East coast of North America

Oceanic sag basins or nascent ocean basins Occupy the area between a mid-oceanic ridge, including its rise, and the outer edge of the transitional crust along a passive continental margin

Subduction-related basins Trench (accretionary wedge) • Geological Origin: Downward flexure of the subducting and non-subducting plates (sites of accretionary wedges) • Example: Western edge of Vancouver Island

Subduction-related basins Forearc basin • Geological Origin: The area between the accretionary wedge and the magmatic arc, largely caused by the negative buoyancy of the subducting plate pulling down on the overlying continental crust • Example: Georgia Strait

Backarc or interarc basins form by rifling and ocean spreading either landward of an island arc, or between two island arcs which originate from the splitting apart of an older arc system