Microstructures

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Shear Sense Indicators:
Crenulation Cleavage

Click image to enlarge

100. Crenulation cleavage - Metasedimentary rock from the Brooks Range, Alaska, shows a well-developed crenulation cleavage defined by white mica grains. An original metamorphic fabric, dipping here gently to the left, is defined by white mica and fine-grained, quartzo-feldsapathic lenses within which all grains have their boundaries pinned by the mica flakes. Within the steeply-dipping cleavage zones (really the limbs of asymmetric microfolds), there is a significant reduction in the amount of quartz and feldspar present, indicating that there has been pressure solution or diffusional mass transfer of material out of the cleavage domains. Thus, quantification of the amount of shortening accomplished by the crenulations would require a comparative chemical volume change study in addition to geometric analysis.

FOV 1.5 mm, Nicols Crossed.

Shear Sense Indicators: Crenulation Cleavage	Click image to enlarge
100. Crenulation cleavage - Metasedimentary rock from the Brooks Range, Alaska, shows a well-developed crenulation cleavage defined by white mica grains. An original metamorphic fabric, dipping here gently to the left, is defined by white mica and fine-grained, quartzo-feldsapathic lenses within which all grains have their boundaries pinned by the mica flakes. Within the steeply-dipping cleavage zones (really the limbs of asymmetric microfolds), there is a significant reduction in the amount of quartz and feldspar present, indicating that there has been pressure solution or diffusional mass transfer of material out of the cleavage domains. Thus, quantification of the amount of shortening accomplished by the crenulations would require a comparative chemical volume change study in addition to geometric analysis. FOV 1.5 mm, Nicols Crossed.