Development of different types of pull-apart microstructures in mylonites: an experimental investigation

Abstract

Intragranular fracturing of feldspar porphyroclasts in granite mylonites of a ductile shear zone has led to development of diverse types of pull-apart and bookshelf structures. Porphyroclasts with centrally located fractures show pull-aparts with parallel walls (Type 1), whereas those with off-centered fractures display pull-aparts with both parallel (Type 1) and non-parallel (Type 2) geometry. Analog model experiments, performed by embedding segmented rigid elliptical objects within a viscous matrix, indicate that the orientation and location of the fracture and the aspect ratio of the porphyroclast are the principal parameters in controlling the development of bookshelf structures and pull-aparts. In the case of centrally located fractures, porphyroclasts of moderate aspect ratios develop either Type 1 pull-apart or bookshelf depending upon the fracture orientations, whereas those of large aspect ratios (>3) form only Type 1 pull-aparts irrespective of the fracture orientation. Off-centered fracturing of porphyroclasts gives rise to fragments of unequal size, which rotate independently at equal or contrasting velocities, forming Type 1 or Type 2 ('V'-) pull-aparts, respectively. In the latter case, depending upon the orientation of fracture, the smaller fragment rotates faster or slower than the larger fragment, showing relative tilt synthetic (Type 2a) or antithetic (Type 2b) to the bulk shear sense, respectively. Type 2b geometry generally develops when the fracture angle with respect to the short axis of porphyroclasts is low, antithetic to the shear direction, and the long axis of the porphyroclast is at a high angle to the shear plane.