Page 212 - Physical Principles of Sedimentary Basin Analysis

7

Subsidence

This chapter deals with the mechanisms responsible for subsidence and basin formation.
Before we look at these mechanisms we will recapture a little bit of the plate tectonic
setting of sedimentary basins.
The surface of the Earth is made of two types of rigid plates – those beneath the oceans
and those beneath the continents. The oceanic plates are produced at mid-ocean ridges
and are consumed by subduction beneath other plates at ocean trenches. The continents
are placed on stable plates that are much older than the oceanic plates. Plate tectonics is
the theory of these plates and their movements. The rigid part of the Earth’s interior is
called the lithosphere and the lower boundary for the lithosphere is given by an isotherm
◦
(∼1300 C) in the mantle. The mantle below this isotherm is called the asthenosphere,
and it is sufficiently hot to behave like a fluid on a geological time scale. The lithosphere
is viewed as a body that floats on a fluid-like mantle asthenosphere, and it behaves like
a rigid body that can transmit stress over large distances. The forces that push a plate
away from the ridge and pull the plate down into the Earth at an ocean trench are carried
through the plate. The lithosphere is divided into the crust and the lithospheric mantle by
the boundary named the Moho after the Croatian seismologist Mohoroviˇ ci´ c who discov-
ered the discontinuity in 1909. The Moho is a material contrast between a crust that is less
dense than the lithospheric mantle below. The crust of the continental lithosphere is older,
less dense, more heterogeneous and thicker than the oceanic crust (see Figure 7.1). We will
see that the reason why large parts of the continental plates are above sea level is because
the continental crust is thicker than the oceanic crust.
In this chapter we will look at basin subsidence related to isostatic equilibrium,
crustal stretching and thermal subsidence from decay of temperature transients caused by
stretching.

7.1 Isostatic subsidence
The lithosphere is a rigid plate floating on the ductile asthenosphere like a block of wood
floating on water, and when it is loaded it floats deeper. The lithosphere is, for instance,
loaded when sediments are deposited in a water-filled basin. The pressure remains the

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