of sedimentary basins
Subsidence is a cooling of the surface of the earth. Causes include sediment loading, tectonic activity, and thermal contraction during cooling of the crust. Sedimentary basins are not produced by sedimentation but by tectonic and thermal processes. When a basin is formed, high angle faults force sedimentation in the basins, but that is not enough to form the actual basin. Deep lithospheric cooling, which results in thermal contraction in order to regain isostatic equilibrium, specifically causes thermal subsidence. (Subsidence curve) Thermal Subsidence usually takes over as the main basin forming component only after tectonic subsidence has ceased.
We must first understand how isostasy works in order to better understand why thermal subsidence occurs. Where the thickness of the crust and lithosphere varies equilibrium needs to be maintained. At the compensation depth the weigh of the vertical columns, with the same cross sectional area from the compensation depth to the surface of the crust, (Isostasy diagram) must have equal gravitational forces even though the density varies laterally. Even though isostatic equilibrium is reached, the plates are, however, not in mechanical equilibrium. In other words even though the density is different at different points in the crust and lithosphere, across hypothetical columns of equal depth, the weight will be the same for all the columns.
In the formation of thermally subsided basins a region of crust or lithosphere is heated. This causes expansion, which results in changes in density. Uplifting then occurs followed by erosion. The erosion causes the thickness of the crust to be reduced. As the lithosphere cools and contracts a basin is formed. Basically, it is the horizontal extension and thinning of the lithosphere that forms the basin. This is consistent with passive margin formation and the properties of passive margins. It is a seaward thinning of the crust between the ridge and ocean-continent boundary. The thinning of the lithosphere occurs during rifting, at which point subsidence can occur on the continents. After rifting the region continues to subside due to thermal contraction of the lithosphere. When rifting begins, we have a lot of heat accumulate which rapidly drops off as we move away from the rift. As this is now an ocean basin, sediment accumulation and water weight speed up the basin subsidence.
Thermal subsidence occurs as cooling crust becomes denser and trends towards a deeper equilibrium level. Different sets of tectonic and thermal conditions in different plate tectonic settings can lead to different types of subsidence and subsidence forming processes even though the primary causes are the same.
At extensional zones, rifting is the main cause of basin formation. As rifting occurs mid-ocean ridges begin to form ocean basins. These rift basins eventually lead to formation of a passive margin. This is known as rift and drift. Thermal subsidence is a result of thermal contraction in order to regain isostatic equilibrium. Passive margins are expected to show thermal negative, exponential cooling after rifting. A passive margin is formed as a result of rifting. The original zone of rift drifts away from the newly formed ridge over time. A passive margin is a place where rifting occurred in the past but is no longer tectonically active. Passive margin formation is important to understanding thermal subsidence, because the thermally subsided basin in extensional zones occurs between the ridge and the passive margin. The basin gets larger as it moves away from the ridge zone. As it gets wider it also gets older and colder, as it gets denser. This can eventually lead to the formation of a subduction zone along the passive margin. (Passive margin formation)
In considering passive margins, we generally see more subsidence than can be explained simply by the thinning of the crust and lithosphere. Usually subsidence begins at the start of mid-ocean ridge formation. This is considered to be tectonic subsidence. This subsidence, however, may be caused by low-density asthenosphere that lies from underneath the rifted lithosphere to lithosphere under even thinner growing ocean basins. A plume may bring the hot mantle material to the shallow depths, but it may be of a high enough viscosity so that it does not cool convectively for a very long period of time. Therefore, the hot asthenosphere could remain under the rift for millions of years before it has the ability to flow out. The thinning caused by the low-density asthenosphere could cause water to subside the basin down to over a kilometre. The rate of subsidence would, however, depend on the initial thickness width of the asthenosphere that had pooled at shallow depths, as well as on the rate of plate divergence. For wide rifts and slow seafloor spreading rates, the rate of subsidence is similar to thermal subsidence. We see here that the formation of a passive margin and thermal subsidence are very much related. In fact the formation of a passive margin is as much related to the formation of a basin at extensional zones, as is thermal subsidence.
An interesting feature in considering basins is basin inversion. Basins that are formed by rifting and thermal subsidence don’t necessarily stay basins. They may suffer from uplift and erosion. This is possibly due to tectonic compression before thermal subsidence. Basically, excess sediments are removed by erosion. However this does not provide significant evidence for basin inversion. It has been suggested that basin inversion results from magmatic underplating by basalt. 5km of basalt underplated into the lower crust above the Moho would cause 600m of uplift. Erosion would increase this figure up to about 2.5km.
|Structures of Sedimentary Basins|