Microstructures

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Semi-brittle Deformation, with Brittle Deformation of Feldspar

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13. Offset feldspars - Example of brittle offset of plagioclase grains from mylonitic granodiorite of the Borrego Springs mylonite zone, southern California. Two feldspar grains have had their upper halves offset to the left with respect to their lower halves (for confirmation of this see image #14). Very-fine-grained recrystallized quartz, white mica and feldspar line the microfaults between the grain halves. Quartz, a significant component of this rock, has deformed entirely by crystal-plastic mechanisms, but all of the feldspar grains deformed by brittle failure, thus the overall deformation mode for the rock is said to be semi-brittle.

FOV 1.5 mm, Nicols Crossed.

14. Offset feldspars - Same view as image #13 but with a gypsum plate inserted, to help identify which fragments of feldspar originally belonged together.

FOV 1.5 mm, Nicols Crossed + Gypsum Plate.

15. Antithetic shear of feldspar fragments - This large feldspar grain has fractured in a regular pattern along its cleavage planes and each segment has shifted down to the left in response to an overall right lateral shear across the top of the grain. The phenomenon is the same as the antithetic shear of individual books or dominos in a stack when the whole stack slumps over to one side. In this example, from the Borrego Springs mylonite zone, the quartz grains above the feldspar have been plastically deformed into smoothly undulating ribbon-like grains. Brown biotite (extreme top left) also forms elongate ribbon grains parallel to foliation. Note that the fractures within the feldspar grain do not extend into the quartz.

FOV 3.2 mm, Nicols Crossed.

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16. Offset feldspars - A fractured plagioclase grain from mylonitic tonalite of the Borrego Springs mylonite zone, shows antithetic and synthetic offset of the fractured pieces. The antithetic fracture is the main, 'high-angle normal fault' (down to right) and the synthetic fractures are 'low-angle normal faults' (down to left), indicating an overall top-to-left shear for this deformation zone. The large quartz grain at lower right shows slight undulatory extinction and grain boundary migration recrystallized new grains along it's uppermost margin.

FOV 3.2 mm, Nicols Crossed.

17. Offset feldspars - Same image as #16 but with gypsum plate.

FOV 3.2 mm, Nicols Crossed + Gypsum Plate.

Semi-brittle Deformation, with Brittle Deformation of Feldspar	Click image to enlarge
13. Offset feldspars - Example of brittle offset of plagioclase grains from mylonitic granodiorite of the Borrego Springs mylonite zone, southern California. Two feldspar grains have had their upper halves offset to the left with respect to their lower halves (for confirmation of this see image #14). Very-fine-grained recrystallized quartz, white mica and feldspar line the microfaults between the grain halves. Quartz, a significant component of this rock, has deformed entirely by crystal-plastic mechanisms, but all of the feldspar grains deformed by brittle failure, thus the overall deformation mode for the rock is said to be semi-brittle. FOV 1.5 mm, Nicols Crossed.
14. Offset feldspars - Same view as image #13 but with a gypsum plate inserted, to help identify which fragments of feldspar originally belonged together. FOV 1.5 mm, Nicols Crossed + Gypsum Plate.
15. Antithetic shear of feldspar fragments - This large feldspar grain has fractured in a regular pattern along its cleavage planes and each segment has shifted down to the left in response to an overall right lateral shear across the top of the grain. The phenomenon is the same as the antithetic shear of individual books or dominos in a stack when the whole stack slumps over to one side. In this example, from the Borrego Springs mylonite zone, the quartz grains above the feldspar have been plastically deformed into smoothly undulating ribbon-like grains. Brown biotite (extreme top left) also forms elongate ribbon grains parallel to foliation. Note that the fractures within the feldspar grain do not extend into the quartz. FOV 3.2 mm, Nicols Crossed. Click here to view Flash animation in a new window.
16. Offset feldspars - A fractured plagioclase grain from mylonitic tonalite of the Borrego Springs mylonite zone, shows antithetic and synthetic offset of the fractured pieces. The antithetic fracture is the main, 'high-angle normal fault' (down to right) and the synthetic fractures are 'low-angle normal faults' (down to left), indicating an overall top-to-left shear for this deformation zone. The large quartz grain at lower right shows slight undulatory extinction and grain boundary migration recrystallized new grains along it's uppermost margin. FOV 3.2 mm, Nicols Crossed.
17. Offset feldspars - Same image as #16 but with gypsum plate. FOV 3.2 mm, Nicols Crossed + Gypsum Plate.