Tectonophysics, 158 (1989) 163-171 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 163 Deformation partitioning, shear zone development and the role of undeformable objects T.H. BELL I, A.C. DUNCAN ’ and J.V. SIMMONS 2 ’ Department of Geology, James Cook University, TownsviNe, Qld. 481 I (Australia) ’ Department of Civil and Systems Engineering, James Cook University, Townsville, Qld, 4811 (Australia) (Received February 16,1987; accepted July 27.1987) Abstract Bell, T.H., Duncan, A.C. and Simmons, J.V., 1989. Deformation partitioning, shear zone development and the role of undeformable objects. In: A. Ord (Editor), Deformation of Crustal Rocks. Tectonophysics, 158: 163-171. The formation of a mylonite zone by homogeneous progressive simple shear is highly unlikely because heteroge- neous strain of grains with different compositions causes deformation partitioning and results in progressive inhomoge- neous simple shear or non-coaxial progressive bulk inhomogeneous shortening. However the presence of rigid bodies in a zone of rock undergoing bulk simple shear spreads and homogenizes the stress field such that the instantaneous displacement field is essentially homogeneous at a scale up to at least four times the diameter of the rigid object. That is, the portion of rock surrounding the rigid body deforms by homogeneous progressive simple shear and the rigid object is forced to rotate. This has considerable implications for the rotation or lack thereof of unstrained porphyroclasts and porphyroblasts, and hence the information such grains preserve about the sense of shear on their margins or through inclusion trails. Geometric relationships between porphyroclasts or porphyroblasts and the surrounding matrix that arc superficially similar can indicate exactly the opposite sense of shear depending on whether the porphyroclasts or porphyroblasts have or have not rotated leading to considerable problems in interpretation of the bulk movement in a shear zone. Resolution of these relationships enables ready distinction of whether the deformation history involved progressive bulk inhomogeneous shortening or progressive simple shear. In particular these results have considerable implications for the origin of blueschist minerals preserved in garnet porphyroblasts via the roles of strain softening and hardening during mylonitization due to the destruction of feldspar porphyroclasts and growth of garnet porphyroblasts respectively. Introduction Recent work on the role of deformation parti- tioning during foliation development has consider- ably advanced our understanding of the micro- structural processes associated with deformation. Of particular significance is the concept that porphyroblasts do not rotate if deformation parti- tioning occurs and, indeed, can only rotate if the deformation involves homogeneous progressive simple shear alone (Bell, 1985). The latter type of 0040-1951/ 89/ $03.50 0 1989 Elsevier Science Publishers B.V. deformation history is very unusual because most rocks deform heterogeneously due to the role of phyllosilicates and graphite versus other minerals (Bell et al., 1986). This is because phyllosilicates and graphite, unlike most other minerals, localize progressive shearing causing deformation parti- tioning and hence progressive inhomogeneous simple shear or even progressive bulk inhomoge- neous shortening. Hence we decided to investigate deformation partitioning in a bulk shear environment in order