EASC 2702 Lecture 18 Basins

basins formed in extensional settings

• continental rift basins and proto-ocean troughs

• passive continental margins

continental rift basins

• forms where the crust and mantle lithosphere is being pulled apart

• the asthenosphere rises below thinned lithosphere

• on the surface forms grabens and half-grabens, filled with prograding sediment and coeval volcanics

• common during periods of supercontinent breakup - now buried beneath deposits of the passive continental margins

evolution of rifts into oceans

continental rifts → proto oceanic troughs → pavssive continental margins and oceanic basins

passive continental margins

• formed when a continent rifted apart and new oceanic crust developed

• but there is now no tectonic interaction occurring

proto-ocean troughs

elongated depressions that form during the earliest stage of continental rifting.

margins connected to rivers…

will form deltas, estuaries, clastic shelf deposits, and submarine channels and fans

margins not connected to rivers…

are starved and may form thick carbonate platforms

basins formed in compressional settings

formed along major collisional plate tectonic boundaries

• arc related basins

• orogenic basins

arc related basins

trench, forearc, back-arc basins

Volcanic arcs form above subduction zones because …

the release of water from subducting slabs reduce the soludus of the mantle, producing melts.

the steeper and faster the rate of subduction…

the more extension there is in the plate above the subducting oceanic crust

trench

A deep, narrow depression on the ocean floor that forms at a subduction zone where old, cold, dense oceanic crust bends and sinks beneath another plate.

• forms at the front of the accretionary prism

accretionary prism

A wedge-shaped mass of sedimentary material that forms when light, deep-marine sediments on the subducting oceanic plate are scraped off and piled onto the overriding plate.

fore-arc basins

• form above subduction zones, between the accretionary wedge and the arc

• main source of sedimentation is the arc (which produce volcaniclastic detritus continuously)

• sediment is mostly mineralogically immature and dominated by volcaniclastic sed rocks

• can also be sediment starved and form carbonate platforms

Fore-Arc Basin Deposits

Steep slopes, deep water, and abundant volcanism cause volcaniclastic turbidites and deep-marine fan facies to dominate, with abundant ash fall beds and subaqueous volcanic flows.

There is a direct relationship between volcanic eruptions and sedimentation events.

back-arc basins

• extensional basins behind the volcanic arc

• may start as rifts and proto-oceanic troughs and can evolve into oceanic crust with their own spreading ridges

• deposits are similar to rift deposits, but with a lot more fresh volcaniclastic material

foreland basins

• form adjacent to mountain ranges (mountains are where the crust piles up creating a orogenic load)

• the load pushes down on the underlying lithosphere, causing it to sink, forming a basin on the edge of mountain belts

• peripheral and retroarc

peripheral foreland basins

occur at continent-continent collision zones (e.g., Himalayas) where the overriding plate, the subducted plate, and sediment accumulation all add to the load

retroarc foreland basins

form on the craton-ward side of continental subduction zones, where compression and terrane accretion lead to loading (e.g., the Western Canadian sedimentary basin)

foreland basin sub-basins: the wedge top (piggyback) basin

the part of the basin that forms above the thrust front of the orogen. It is characterized by syn-deformational sedimentation, including growth sequences and unconformities above blind thrusts.

foreland basin sub-basins: the foredeep

the deepest part of the foreland basin, where most sediment accumulates. The presence of a mountain range adjacent to these basins means that sediment supply is abundant

foreland basin sub-basins: the forebulge

the zone of uplift from the crest of the attenuated flexural wave. It may be covered or form a basement high where additional sediment may be sourced.

foreland basin sub-basins: the back bulge

a shallow basin that may (or may not) form craton-ward of the forebulge

foreland basin sub-basins

basins formed in strike-slip settings

form only local pull-apart basins where there are bends, lateral displacement, terminations, and bifurcations

• releasing bends

• pull-apart basins

• strike-slip terminations

releasing bends

result in transtension, and the opening of narrow, sigmoidal basins.

• basin formed in strike-slip settings

pull-apart basins

occur where there is a lateral displacement of a strike-slip fault.

strike-slip terminations

leave behind remnant stress that is taken up by transtension.

• basin formed in strike-slip settings

intracratonic basins

• broad basins formed on stable continental crust, and over 100s of millions of years episodically experienced subsidence, including widespread flooding during the Paleozoic eustatic rise

• origin is not understood