Faculty of Science Geology Department Sedimentary Basins Lecture

Faculty of Science Geology Department Sedimentary Basins Lecture 5

Active vs. passive margins • Active margins are found on the edge of a continent where subduction occurs. These are often marked by uplift and volcanic mountain belts on the continental plate. Less often there is a strike-slip fault, as defines the southern coastline of W. Africa. Most of the eastern Indian Ocean and nearly all of the Pacific Ocean margin are examples of active margins. • While a weld between oceanic and continental lithosphere is called a passive margin, it is not an inactive margin. • Active subsidence, sedimentation, growth faulting, pore fluid formation and migration are all active processes on passive margins. Passive margins are only passive in that they are not active plate boundaries.

Passive margins are found at every ocean and continent boundary that is not marked by a strike-slip fault or a subduction zone. Passive margins define the region around the Atlantic Ocean, Arctic Ocean, and western Indian Ocean, and define the entire coasts of Africa, Greenland, India and Australia. They are also found on the east coast of North America and South America, in western Europe and most of Antarctica. East Asia also contains some passive margins

Passive margin basin • A passive margin is the transition between oceanic and continental lithosphere which is not an active plate margin. • It is constructed by sedimentation above an ancient rift, now marked by transitional lithosphere. • The transition between the continental and oceanic lithosphere that was originally created by rifting is known as a passive margin.

Passive margin basin • 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

Passive margin basin

Passive margin basin Mechanism: 1 -In the first stage a continental rift is established due to stretching and thinning of the crust and lithosphere by plate movement. 2 -The second stage leads to the formation of an oceanic basin, similar to the modern Red Sea. The subsiding continental crust undergoes normal faulting as transitional marine conditions are established. Areas with restricted sea water circulation coupled with arid climate create evaporates deposits. Crust and lithosphere stretching and thinning are still taking place in this stage. Volcanic passive margins also have igneous intrusions and dykes during this stage

Passive margin basin 3 - The last stage in formation happens only when crustal stretching ceases and the transitional crust and lithosphere subsides as a result of cooling and thickening. • Economic significance Passive margins are important exploration targets for petroleum. continental passive margins account for 31% of the world's giant oil fields. Continental rifts (which are likely to evolve into passive margins with time) contain another 30% of the world's giant oil fields.

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. They commonly accumulate deep sea fan or basin plain sediments. Due to the advanced cooling of the aging oceanic crust, subsidence is usually low, unless it is activated by thick sedimentary loading near the continental margin. Fault-bounded basins of limited extent are common in conjunction with the growth of midoceanic ridges.

Oceanic sag basins