Page 123 - Physical Principles of Sedimentary Basin Analysis

6

Heat flow

The Earth is losing heat through its surface, partly from cooling of the Earth and partly
from heat generation by decay of radioactive isotopes. The heat loss from the Earth’s sur-
face is typically 0.05 W m −2 . This is much less than the influx from the Sun, which is
typically 500 W m −2 on a sunny day. However, almost all the energy received from the
Sun is returned back into space as infrared radiation. The energy from the Sun powers
the processes in the biosphere and in particular the water cycle (evaporation and rainfall),
and it is therefore the energy source for erosion. On the other hand, the energy from the
interior of the Earth drives large-scale geological phenomena like mantle convection, plate
tectonics, volcanos, earthquakes and mountain building, see Figure 6.1.
In models of the heat flow and temperature in the subsurface it is important to distinguish
between the crust and the mantle and also the lithosphere and the asthenosphere. The crust
is the upper part of the Earth with a thickness in the range from 10 km to 70 km. It is
made of more silica-rich and less-dense rocks than the mantle below. The crust has typical
densities in the range from 2700 kg m −3 to 2900 kg m −3 , and the mantle has a typical
density 3300 kg m −3 . The crust is therefore both chemically and mechanically different
from the mantle.
The lithosphere is the outermost part of the Earth which is considered rigid, and where
heat transfer is by conduction. This part extends down to a mantle depth of 100 km to
250 km in continental areas. The mantle below the lithosphere is the asthenosphere, where
the temperature is dominated by convective heat transfer. The transition from lithosphere
to asthenosphere is a thermal boundary layer which is not sharp, although we assume that
in the models.
Sedimentary basins are thin covers on top of the crust, and the temperature in a basin is
to a large extent controlled by processes underneath the basin – in the crust and the mantle.
Models for the heat flow through a sedimentary basin therefore involve both the crust and
the lithospheric mantle.
This chapter presents the temperature equation and some solutions that are relevant
for sedimentary basins. For most applications a simple temperature equation is sufficient,
which is derived from a simple energy balance. We will therefore start with the derivation
of a basic 1D temperature equation, and look at some applications, before a more complete
temperature equation is derived.

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