Tectonics of the Oceans

Ocean Basins Part 2

Oceanic Plate Margins

Divergent Plate Margins- topographically high regions usually in the middle of ocean basins called mid-ocean ridges, where oceanic crust is produced by the partial melting of upwelling mantle material to form basaltic magma.. One notable exception is the East Pacific Rise, which is not in the middle of the Pacific Ocean. Length ~40,000 km, rising ~2.5 km above the abyssal floor. Widths between 1,000 and 3,000 km. The relative motion of the plates creates structures in the crust that accomodate stretching- such as normal faults near the surface and ductile thinning at deeper levels.

Images from USGS site. Left, the mid-ocean ridge system of the Atlantic Ocean. Click on the picture of Iceland for a picture of the fissure running through Thingvellir.

Fracture Zones- rectilinear fracture systems within the oceanic crust. Transform fault boundaries mark the seismically active portions of the fracture zones. Characterized by pronounced topographic break and deformed oceanic rocks. Up to 10,000 km long and 100 km wide. Oceanic crust near fracture zones tends to be thinner than average. Structures are typically systems of strike-slip faults or, at deeper levels, vertical zones of ductile deformation that have a subhorizontal direction of displacement. See below for a schematic of a transform margin..

Convergent Plate Margins or Subduction Zones- See Figures Below. These are places where oceanic crust is recycled back into the mantle. Partial melting of mantle above the down-going plate produces characteristic volcanic arcs on the over-riding plate. Boundaries marked by volcanic islands bounded on one side by a parallel trench. (deepest places in the oceans- up to 12 km depth) May extend for 1000's of km's. Systems of active thrust faults characterize the landward side of trenches. Active normal faults near island arcs and regions behind the arcs. Trenches associated with negative Bouger gravity anomalies- indicates a great mass deficiency below the sea-floor. Older island arcs are thicker than normal oceanic crust, ~25km. Younger arcs are thinner than older arcs.
Definition: Bouger Correction/Anomaly- a correction which takes into account the nature (density) of the material between the surface of equal gravity or geoid and the measuring station. Any anomaly remaining after all these corrections have been made is called the Bouger Anomaly and represents the effect of rocks with densities different from the average or structural controlled rock masses not in isostatic equilibrium.

Images taken from USGS site. Both show convergent plate margins which end in subduction.

Next Page, Ocean Basins Part 3 (Oceanic Plate Interiors)