Mechanical Microdeformation and Kink-band Formation in Mica from the Colonia Impact Crater, Sao Paulo, Brazil

A comprehensive examination using a transmitted light optical microscope was conducted to analyze the morphology and geometric parameters of kink band development in mica from the Colônia impact crater's crystalline basement rocks. Significant differences were observed between kink bands formed perpendicular and parallel to the [001] plane. Kink bands perpendicular to the [001] are narrow and elongated, exhibiting two distinct coplanar orientations with regular spacing and parallel planar fractures, which suggest a slip-dislocation strain mechanism. In contrast, kink bands parallel to the [001] plane exhibit a greater variety of shapes and a more complex internal structure. These kink bands can be grouped into six types- sigmoidal-shape, S-shape, lenticular-shape, Z-shape, Z-shape with internal dislocation, and Z-shape with internal rotation. The deformation patterns in these kink bands indicate two primary processes- flexural and shear strain. The most notable deformations induced by these strain mechanisms include the curvature of cleavage lamellae, delamination cracks, and fractures along the boundaries of kink bands. More severely deformed kink bands exhibit dislocation, rotation, and partial obliteration of internal cleavage lamellae. These characteristics differ from those of typical kink bands in tectonically deformed rocks, thereby reviving the longstanding debate regarding their potential use as alternative evidence in impact cratering investigations.