Feather features: A low-shock-pressure indicator in quartz

Abstract

Feather features are a recently discovered type of microstructure in shocked quartz. They occur as short, parallel to subparallel lamellae with spacing similar to that of planar deformation features (PDFs). These lamellae are always found in combination with a planar fracture (PF). Feather features are crystallographically controlled; (0001), {1011}, and {1122} orientations are common, while {1013} and {1012} orientations are lacking. Samples from the Nrdlinger Ries and the Matt Wilson impact structures are compared with plane wave shock recovery experiments <16 GPa. Microscopic analysis of quartz grains with feather features show that their formation is linked to shearing along the associated PFs during shock deformation. The generation of shear fractures with feather features in plane wave shock recovery experiments suggests a pressure range of ∼7-10 GPa, although further constraints are needed for an upper and lower pressure limit. The orientation of feather features in rock samples is suggested to be controlled by the direction of the principal axis of stress, with sheared PFs at ∼45° angles to the axis and most feather feature lamellae aligned parallel to the axis ±20°. The majority of feather feature lamellae emanate from the PFs in the same direction and in experiments point in the direction from which the shock wave came. On the basis of their appearance in shocked quartz grains from 26 impact craters and the current lack of reports of these features in endogenically deformed crustal rocks, their uniqueness as a new type of impact-induced planar microstructure alongside PDFs and PFs is proposed. Copyright © 2011 by the American Geophysical Union.