Heat - The Solid Earth - page 318

Figure 7.26. The overthrust model after 30 Ma of erosion. Stipple indicates
remnants of overthrust block. T m is the maximum temperature attained by rock
finally at the surface; D t is the depth at which T m was reached. This figure also
shows the times of closure (Ma before the present) of various radiometric systems:
ap, apatite fission track; zr, zircon fission track (maximum age based on closure at
∼175 ◦ C); bi, biotite K/Ar; and hb, hornblende K/Ar. (From Fowler and Nisbet
(1982).)
Figure 7.26 shows the effect of using these various dating techniques across
the overthrust model's 30-Ma erosion surface; the dates shown are the dates that
would be measured by a geologist working on this surface. It can be seen that
these dates are useful for studying the cooling and erosional history of the pile.
In many natural examples, there can be profound differences in closure ages of
minerals that were initially produced by the same tectonic event. An example of
the use of such methods for the Southern Alps of New Zealand is provided in
Fig. 7.27. The plate boundary between the Pacific and Indo-Australian plates runs
the length of the South Island (Fig. 2.2). Convergence between the plates of over
1cmyr − 1 causes comparable uplift in the Southern Alps. Plotting the apparent
ages against the closure temperature provides a cooling history for rocks along
the plate boundary. This cooling history can be understood in the context of
the tectonic setting of the Southern Alps - initially the rocks move horizontally
eastwards; only within about 25 km of the Alpine Fault do they start to rise
(Fig. 7.27(a)). On the cooling-history graph therefore this appears as almost no
cooling until about 2 Ma ago, followed by rapid exhumation.
Implications
To interpret a metamorphic terrain fully, it is not sufficient to know the pressure-
temperature conditions undergone by the rocks. A full interpretation of the ther-
mal history of the rocks also involves studying the erosional and radiometric