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ORIGIN AND MORPHOLOGY OF OCEAN BASINS

Detailed investigations of axial morphology, using side scan mapping, submersibles, high resolution seismics, and deep-sea drilling, have greatly advanced our knowledge about ridge dynamics. Oceanic ridges (depressions) with steep and irregular slopes, and oceanic rises (uplifts) gentle slopes are giant mountainous features of deep-ocean basins. These systems are entirely volcanic and composed of lavas with basaltic composition (characteristic of oceanic crust).

The Mid-Atlantic Ridge is separating the Atlantic Ocean into equal halves. It is a slow spreading ridge with a rate of about 1 centimeter per year. The crest is marked by a central rift valley, 30 to 50 km-wide and 1 km-deep or more. The crest is characterized by very rugged morphology, by shallow earthquakes (centers less than about 60 km deep), by active volcanism, and by high heat flow values. The upwelling of mantle material pulls apart the crust, producing central rift, and generating earthquakes, and bringing up heat from the Earth’s interior. Hot mantle material filling the gap is less dense than old oceanic crust, because of thermal expansion. Away from the ridge, it becomes denser as it cools, sinks, and tends to have smoothed morphology toward the flanks. Generally, the ridge crest has an elevation of averaging from 2500 to 3000 m. However, the ridge is much shallower than average in the North Atlantic, where it has one of the most active hot spots associated with it, namely Iceland. Along its entire length, the ridge is segmented between major fracture zones (or propagated along the transform fault). Each segment has its own morphology.

On slow-spreading ridge areas (the Atlantic Ocean, the Red Sea), mushroom-shaped magma (narrow) chambers (with roofs only 1.5 to 2.5 km below the sea floor) cause uplift and formation of narrow axial graben structures. The upwelling material forms pillow basalts and lava sheets after contact with the cold sea water and fills these narrow axial valleys.

On fast-spreading ridge areas, as on the East Pacific Rise, the supply of lava is such that no big rift valley develops. Instead there is an axial summit with or without a narrow graben. The magma chambers have finite extents in all directions. Transform faults do exist on fast-spreading ridges, however they are fewer, farther apart, and not deep gashes which cut into lithosphere (unlikely slow spreading ridges). What is observed is a double spreading center with overlapping zones of upwelling lava in place of the transform fault. These peculiar “69-like” structures have been observed all up and down the east Pacific Rise. These overlapping spreading centers occur at transform fault intersections and result in complex fracture zone scars in old crust as one center survives and other slowly freezes. Excessive heat in magma chambers causes them to overshoot as they are pinched out at transform faults. Alternatively, small adjustments in ridge strike as poles of rotation slowly move relative to each other cause the overlapping spreading centers.

The pattern of magnetic anomalies or stripes (as lava freezes and takes on the current polarity of the earth’s magnetic field) was found to be controlled by the degree of volcanic cycle. That depends mostly upon spreading rate. If slow spreading is occurring, a wide crustal accretion zone produces a mixed bag of positively an negatively magnetized rock. In other words, as the plates are pulled apart slowly, there is much more overlap than the cleaner eruptive events on the fast-spreading ridge. Therefore, the mid-Atlantic ridge does not produce as clear set of magnetic stripes as the fast spreading East Pacific Rise where a narrow intrusion zone. An even more extreme mode of plate-boundary reorganization is the propagating rift, where a change in pole positions causes a ridge axis to change orientation by propagating the new axis across the old. A characteristic wedge-shaped zone of magnetic anomalies is recognized to accompany ridge propagation. High heat flow and positive gravity anomaly occur over the rise axis due to the rise of warm mantle rock beneath abnormally thin crust. However, negative gravity anomaly occurs over the rift valley of the mid-oceanic ridge axis.

Heat removed from the ridge by its contact with seawater. This cooling of the hot rock causes fissures, cracks and fractures. As seawater gets down through these fissures it gets heated (up to 350°C), the water becomes so buoyant that it shoots directly up to seafloor in the form of hot water-jets called hydrothermal vents . These vents have significant impact on the chemistry of seawater. The result id spectacular hot-spring activity at the ocean floor: white and black smokers , as they are called. As water moves through basalt, it changes chemically, giving up its sodium and magnesium and taking up calcium and potassium. Bordering these vents are found rich deposits of metal sulfides (iron, copper, zinc) and other minerals. These chimneys form ore deposits called massive sulfides.

Fracture Zones:

It is unlikely that single magma chamber can run along the whole length of an ocean ridge. The oceanic ridges (and rises) are offset by rugged fault scars called fracture zones.

Fracture zones are major lines of weakness in the earth’s crust that cross the mid-ocean ridge at approximately right angles. It occurs in more or less straight portions which are offset from each other. The ridge crest is not continuous but segmented. Thus, axial magma chambers do not extend right up to transform faults. They are unlikely to be in contact with a similar chamber beyond the fault. It appears that the axial magma chambers have extra breaks apart from those at transform faults. The breaks in the magma chamber are also responsible for the existence of the overlapping spreading centers on the East Pacific Rise.

There is motion along this fault during active spreading, there are earthquakes on it. These earthquakes are shallower than 7-9 km and define the active part of the fracture zone, that is, the ridge-ridge transform fault . Beyond this active part, the fracture zone is frozen trace of the fault; scarps subside as the seafloor ages on both sides of the zone. These extensive linear zone have an usually irregular topography with large seamounts, steep-sided or asymmetrical ridges, throughs or escarpments. Although fracture zones are difficult to trace under the sediments of abyssal plain and continental rise, some think that they can trace the extensions of fracture zones onto continents.

Fracture zones may have a thermal control on the evolution of the ridge. Along the slowly spreading ridges, transform faults can act as thermal barriers to the ridge axis. That may indicate disconnected magma bodies under the ridge axis. This can be controlled by the depth of magma and distance between transform faults.

The observed topographic anomalies along the fracture zone are probably caused also by vertical motions of crustal blocks. Changing in direction of stress acting on transform-ridge system may produce transform migration and changing orientation of the ridge axis. This event causes vertical tectonic motions that produces compression and uplifting of ultramafic crustal blocks (peridotites, gabbros). During the vertical tectonic motion (without volcanism), uplifted valleys form across the fracture zone. Vema and Romanche Fracture zones have uplifted walls. In the Red Sea, these features rise up above sealevel: Zabargad island.

Trenches:

Some observations: trenches are roughly 100 km wide and from hundreds to thousands of kilometers long. The cross-section is usually V-shaped, and the deepest part may be flat due to ponded sediment. The greatest depths are in the southern Aeagen Sea, Hellenic Trench, in the western Pacific, in sediment-starved trenches off island arcs: Challenger deep of Mariana Trench (11 k m), and Tonga, Philippine, Japan, Kermadec trenches. The “ Ring of fire ” around the Pacific is associated with the trenches: these andesitic volcanoes (about 800 active volcanoes arranged in long parallel to oceanic trenches) sit on top of the dipping earthquake planes (Benioff seismic zones) or the down going lithosphere under continents or island arcs. The ring of trenches of Pacific is the site of most of the deep and intermediate earthquakes on Earth: some earthquakes also occur in Mediterranean, Iran and central Asia at the boundary of Himalayas.

Oceanic trenches are marked by abnormally low heat flow compared to oceanic crust. This implies that the crust in trenches may be colder than normal crust. High heat flow occurs over the volcanic arc. The greatest negative gravity anomalies in the world are found over trenches. These anomalies are interpreted to mean that trenches are actively being held down or the thickening of continents from below causes the crust to be out of isostatic balance (crustal rocks tend to rise or sink gradually until they are balanced by displacing mantle rocks). Isostatic adjustment may occur at subduction zones. A subducting plate may generate molten magma, which sometimes rises all the way to the surface to erupt as lava or in certain cases the magma stops and accumulate within the continent. This accumulation of magma thickens the continent.

Marginal Seas:

These seas are common features of the oceans. These seas, the Mediterranean, Black, and Caribbean are examples. They have a structure between the ocean margin and basin. They have thick sediment accumulations due to their long geologic history, their closeness to continents that source of sediment, and many large rivers enter into marginal seas. They have restricted oceanic circulation that favor the organic matter accumulation. This fact results in an environment favorable for the accumulation of oil and gas (Persian Gulf and Gulf of Mexico). There are marginal seas that result of the rifting continent and seafloor spreading. The examples are Red Sea and Gulf of California.

The Mediterranean Sea can be classified as an ocean in its final stages of its life cycle, which is the only remnant of the once-extensive Tethys Ocean. The Mediterranean is shrinking as the African plate continues to thrust northwards beneath the European plate. The Mediterranean would be floored by oceanic crust dating back to Jurassic times, about 170 Ma. The crust of the eastern Mediterranean has an age of Cretaceous (110 Ma), bordered by the Tertiary aged (25 Ma) crust (south of Italy), as a result of back-arc spreading. There is a collision zone running south of Cyprus, where African plate meets the Eurasian plate. Active volcanism and frequent earthquakes show that it is still evolving. Miocene and younger (10-2 Ma) oceanic crust in the western Mediterranean is believed to have formed in a back-arc setting associated with the subduction. Ocean floor older than about 70 Ma is represented in the Mediterranean region by slivers of oceanic lithosphere and upper mantle which have been tectonically removed from the ocean floor and emplaced over a continental margin, usually during a collision event. A piece of ocean floor preserved in this way is called an ophiolite . Ophiolite complexes are land-based sequences of fragments of oceanic crust and mantle. In a ophiolite sequences, sediments of deep-sea-origin (including chert), overlie, basaltic pillow lavas, then basaltic dykes (magma solidifies inside the fissure to form a vertical sheet rock), then gabbros, and finally peridotites.

Oceanic Plateaus

There are number of local elevated plateaus rising 2 to 2 km above sea floor. The crustal materials beneath these plateaus could be somehow thickened by the gain of the asthenosphere or effected by mid-ocean ridge system (Voring and Yermak Plateaus). Some of these are fragments that have broken off of existing continents. Others are probably the result of large volcanism (Ontong Java and Kerguelen Plateaus) associated with hot spot events.

Seamounts, Abyssal Plains, Guyots, and Aseismic Ridges

The ocean floor is underlain by oceanic crust many volcanic peaks above the ocean floor. Those conical undersea mountains extend more than 1 km above the deep-ocean floor are called seamounts. The Pacific has about 10 000 of such seamounts. If they less than 1 km above the deep-ocean floor are called abyssal hills and areas dominated by abyssal hills are called abyssal hill provinces . They sometimes rise above sea level to form islands occur in linear chains (Hawaiian chain, Emperor seamounts). They are scattered on the sea floor, including abyssal plains (exceptionally flat regions probably due to large sediment deposition by turbidity currents). It is thought that most seamounts are extinct volcanoes. Only active volcanoes are found on the ridge crest. Two active volcanoes of the island of Hawaii is not associated with the ridge. Volcanoes build up on top of the crust over the hot spot, a site of a plume of rising magma. As the plate moves, a trail of extinct volcanoes forms behind the active tip of the line. Hot spot plumes (with diameters of a few hundred kilometers) originate from lower mantle. They constitute an important component of the mantle convection. Basalts are enriched in so called incompatible elements (potassium, rubidium, strontium, thorium, and rare earth elements), compared with ocean ridge basalts. Hotspot tracks are found in the Indian Ocean: plumes may be stationary over periods as long as 100 Ma as seen on the Ninety-east ridge, and the Kerguelen-, Marion- and Reunion-hot spots appear to have begun with massive outpouring of flood basalts (Deccan and Rajmahal)

Guyots (table mounts) are flat-topped seamounts. No rocks older than Cretaceous were ever dredged from the guyots. Flat tops were cut by waves, the guyots then invaded by reef corals and atoll formation and subsided after erosion took place. There are types of reefs: fringing (attached to shore: reefs bordering the Hawaiian islands), barrier (parallel to the shore but separated by lagoon: off northeastern Australia, Florida Keys), atolls (circular reefs that built on volcanic cones: the Bikini atoll in the south Pacific).
Many seamounts are aligned in chains (for example, the Hawaiian-Emperor line). Such volcanic chains are called aseismic ridges; that is, they are submarine ridges, different from mid-oceanic ridge where earthquakes occur along the rift valley, that are not associated with earthquakes.

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Nilgün Okay

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Last modified on Wed Oct. 31 17:35:47 PST 2001