Morphology of Ocean Basins

Transcription

1 Morphology of Ocean Basins Elements Continental margins Shelf Shelf break Continental slope Continental rise Submarine canyons Abysal plains Mid-ocean Ridges Guyots and seamounts

2 Two types of continental margins: a) Passive (Atlantic type) b) Active margins (Pacific type) Elements of continental margins Shelf Shelf break Continental slope Continental rise Submarine canyons

3 Earth s interior: Crust, lithospere, upper mantle active and passive margins

4 Physical and geochemical characteristics of continental crust, oceanic crust and upper mantle: Isostasy

5 Elevations of Earth crust on land and in the oceans

6 Surface Percent of Earth s Total Surface Area Area Square Kilometers Land area 29.2% 148,940,000 Area covered by Water Pacific Ocean Atlantic Ocean Indian Ocean Southern Ocean 70.8% 361,132, % 155,557, % 76,762, % 68,556, % 20,327,000 Arctic Ocean 2.8% 14,056,000

7 Continental Margins The oceans cover 70.8% of the earth surface Total area of the earth: 560 x 10 6 km 2 Area of oceans: 361 x 10 6 km 2 (71%) Area of land 149 x 10 6 km 2 (29%) Continental Margins Shelf 27 x 10 6 km 2 Continental slope 28 x 10 6 km 2 Continental Rise 19 x 10 6 km 2 (present only at passive margins) Total 74 x 10 6 km 2 (20.5% of ocean area)

8 Continental Shelf Shelves are large parts of the low-lying areas of continents covered by the sea. They are submerged during interglacial periods and most parts of it exposed during Glacial periods The edge of the shelf lies at different water depths, usually between -100 and -200 m Taking the shelf edge at 200m depth, gives an area of 28 x 10 6 km 2, wich is about 5% of the earths surface, 7-8% of the ocean floor Terms assoicated with shelf area Littoral: intertidal Supralittoral: above the high tide, spray zone Sublittoral: offshore from the tidal area

9 Base level Sea level is the level which defines a base level for erosion and deposition of sediments. Sea level is the baseline for continental erosion. It is also the baseline for marine deposition. The sediments deposited offshore can only built up to the sealevel. This is why coastal areas and offshore areas close to the sea level are flat.

10 SEDIMENTATION ON CONTINENTAL MARGINS Continental margins are depositional sites. Especially the shelves are the depositional sites for the detrital sediments, where thick (10 km or more) wedge of sediment accumulate. Margins are also the most fertile parts of oceans with high organic carbon burial: organic rich sediments, which convert into HCs. These areas are therefore favourable sites for petroleum exploration. Especially the Atlantic-type margins are important for petroleum, because of high sedimentation rate, gradual subsidence and lack of tectonic disturbance.

11 Passive (Atlantic-type) and active (Pacific-type) continetal margins Atlantic margins are steadily sinking regions, accumulating great thicknesses of sediments. Pacific-Type margins are rising associated with volcanism, earthquakes, folding, faulting and mountain-building processes. Atlantic type margins In the Atlantic, the margins formed initally by rifting apart of continents, then evolved through subsidence and loading of sediment. Best example are the Red Sea-East African Rift System The sediment consists of coarse continental clastic sedimentary rocks interbedded with alkaline volcanic rocks. Evaporites are common and deposited in the early phase of rifting. Reefs may develop on the faulted and submerged blocks and form reef Carbonates.

12 Evolution of passive margins a) Uplift of cotinental crust above a hot spot in mantle & erosion a, b) Extension and graben formation b,c) Coarse clastic sediment deposition and alkaline volcanism c) Evaporite deposition d) Rifting and drifting: seafloor spreading, new ocean crust formation by melting of mantle beneath

13 Block medel for contient-ocean transition (Seibold and Berger, 1996)

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15 Development of triple junction and a new ocean with passive margins: Red Sea example

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17 Basins due to thermal subsidence after heating following rifting passive margins Passive continental margins are very important for petroleum potential Sedimentation due to primary rifting. Lake and river deposits of shale and clastics During drifting evaporites and black shales and marls will deposit (organic rich) Post-rift stage offshore deep marine deposits

18 Typical form of a passive margin

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20 Active (Pacific-type) continental margins Pacific-Type margins are related to subduction (convergent margins) which can be classified according to the collosion type: Continent-continent (Himalayas) Continent-ocean (Peru-Chile margin) Island arcs (Marianas and Japan in Pacific) These margins are characterized by: Trenches on the offshore side with steep slopes Accretion of sedimentary and magmatic material from down-going plate; all piled up against the margin as mélange.the sediments include pelagic deposits and magmatic rocks of ocean-lithosphere origin Gravitaty controlled sedimentation (turbidites) in the trenches Deformation of sediments (folding and shearing) High fluid pressure in acretionary prism: fluids such water, CO 2, methane released by metamorphic reactions Magmatic and hydrothermal activity on the overlying plate Association with back-arc spreading and back-arc basins

21 Three types of active continental margins: Ocean-island type Ocean-continent type Continent-Continent type

22 Divergent boundaey Mid-Ocean ridge Convergent boundary Active or Pacific type margin

23 Convergent plate boundary: Acitve (Pacific-Type) margins

24 Active margins Peru-type continental margin Aubouin (1984) Island arc (ocean-ocean) With back-arc basin spreading Curray et al. (1974)

25 Trenches Associated with active margins 100 km wide and several hundred to thousands of km long 7-11 km deep Slopes 8-15 degrees Flat floor filled with sediment Turbidite sedimentation common and accretionary prisms of highly deformed sediments Eastern Pacific ones bordered by land: high sedimentation rate Oceanic ones more deep and with less sediment Examples in Pacific Ocean: Kermadec, Japan, Marianas, Tonga Peru-Chile

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27 Mariana Trench The trench is about 2,550 kilometres long but has a mean width of only 69 kilometres. It reachess a maximum-known depth of about km at he Challenger Deep, a small slot-shaped valley in its floor, at its southern end, although some unrepeated measurements place the deepest portion at km. [

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29 Sediments and fluids at active (Pacific-type) margins Nankai subduction zone (Japan) with accretionary prism of Miocene sediments over a decollement surface Cold seeps

30 Continental Slopes The continetal slopes join the shelf with the deep basin The slope angle commonly varies between 4 and 6º. In some cases it can be more than 15º For example, in the Marmara Sea, the northern slope has up to 30º slope angle. In the Bahama Plateau it is more than 40º It is generally the case that in a specific continental slope the slope angle decreases with depth. Width: km; rarely km wide Depth: usually from m to m; up to 5000 m Pacific continental slopes are deeper than Atlantic ones Covers an area of 28 Million km 2 Covered with finer grained sediments than thosee on the shelf 60% mud, 25% sand, 10% gravel, 5% shells

31 Submarine canyons Submarine canyons are a common feature of continental slopes. The smooth suface of the slopes are interrupted by canyons. Examples: Monterey Canyon, off Central California; Hudson Canyon; Nazare Canyon in Portugal. They are uncommon on gentle slopes and where sediment supply is low or absent. They have tributary systems in shallow parts (shelf area); some can run several 100s km to 1000s km beyond the continental rise Commonly connect to a river mouth on the shore. Erosion and mass-wasting processes are common in canyons. They are important in transporting sediment to deep basins by turbidity currents. Origin: Sealevel lowering and subareal erosion (e.g., Mediterranean during Messinian, or during sealevel glacial lowstands) Erosion by turbidity currents that are triggered by storms, earthquakes, volcanic explosions (current velocities of cm/s, or more)

32 { Submarine canyons and deep sea fans

33 Submarine canyons and turbidites

34 Deep sea fans Proximal Proximal Distal Distal

35 Canyons in Sea of Marmara

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