I have some samples of micaceous metapelites which were mylonitized in during normal sense motion. The strand of mylonitic rocks covers a transport parallel thickness of around 5-6 km. Quartz within metapelitic mylonite are only partially dynamically recrystallized (mostly bulging with subordinate SGR process). But we noticed that lower part of the shear zone is older by 1-2 Myrs than the upper part. How to test that strain hardening in the lower part happened and strain was localized in the upper part later?
If you already have differences in the ages (cooling ages isn´t it), that´s one point. Perhaps you could also try to see if there are differences in the deformation mechanisms, conditions, kinematics, etc, between the boths parts. We recently presented one example of strain localization under progressive retrograde conditions, also recording a kinematic change (Oriolo et al. 2015, International Journal of Earth Sciences)
Mr. Oriolo,
1) The obtained difference in age (Ar-Ar) is actually the difference in age of recrystallization (dynamic) of mica.
2) I have measured aspect ratio in quartz in micaceous quartzite rocks affected by this shear movement. For equivalent percentage of weaker minerals (mainly muscovite) there is variability in quartz aspect ratio------can we take this as an evidence of strain-hardening due to changing deformation conditions? How can I measure these?
3) How can I ensure that dislocation density difference in a partially recrystallized rock implies strain hardening?---What would be strategy on this line could you please highlight?
Dear Nirmalya,
you should consider that Ar-Ar ages may reflect not only recrystallization but also could indicate cooling differences (i.e., due to differential exhumation). On the other hand, if you find differences in quartz fabrics between overpinted and non-overprinted areas, they could be considered as an evidence of strain softening/hardining processes.
Thanks a lot again Oriolo!..But I have sampled only in transport parallel section...not along the depth profile!....How could I test the differential exhumation in our sample?...Do you have some papers on that?
Regards,
Nirmalya
Ar-Ar ages have to be carefully exhamined as they can be interpreted as the age of cooling, recrystallization, etc (e.g., Dunlap 1997, Chemical Geology; Mulch and Cosca 2004, Journal of the Geological Society of London). If you have different ages for the same mineral across the shear zone, this coud indicate differential exhumation, e.g., being the sector yielding the younger ages the one which was later exhumated.
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