To prof. Eltherige and others
a further consideration is deserved for upper crustal rocks where dating the
shear process is difficult unless suturing sediments can provide the 'not
younger than' timing.
In the Canadian Rockies and in the Southern Apennines of Italy it has been
found that faulting can induce the stransformation of smectite to illite
with important contributions to the faulting process (i.e. production of H2O
from within the fault zone, see Casciello et al.). Uptake of K during
illitization can be used for K/Ar dating of the shearing. As Vrolijk states
'no other clay minerals have built-in radioetric clocks' ......
Emilio Casciello
Vrolijk, P. & van der Pluijm, B. A. 1999. Clay gouge. Journal of Structural
Geology, 21, 1039-1048.
Casciello, E., Cesarano, M., Cosgrow, J.W., 2004. Shear deformation of
pelitic rocks in a large scale natural fault. Geol. Soc. London, Special
Publications, 224, 113-125
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