Rolling structures at large shear strain

Abstract

At large shear strain, shear criteria are often obliterated or become ambiguous. From examination of both natural examples and experimental models we describe a new criterion called ‘rolling structure’, widely represented in sheared rocks. A typical rolling structure is composed of a rigid or competent object (e.g. porphyroclast, boudin, fossil, etc.) with two tails asymmetrically disposed around it. In most cases tails are of the same material as that of the rotating object, and result from strain softening and grain-size reduction at the object periphery, forming a mantle. Z and S asymmetries of rolling structures represent dextral and sinistral senses of shearing, respectively. Tails must not be confused with pressure shadows which usually present an opposite asymmetry for a given shear sense. Besides the determination of the sense of shear, the occurrence of these structures allows a minimum estimate of the strain intensity in strongly sheared rocks, since rolling structure length is proportional to shear strain.