Progressive development of mantle structures around elongate porphyroclasts: insights from numerical models

Abstract

This paper presents a generalized theoretical approach towards two-dimensional numerical modeling of the mantle geometry of inequant porphyroclasts of varying shapes within a Newtonian matrix during progressive, general type of bulk deformation. The analysis takes into account the effects of synkinematic size reduction of the porphyroclast with concomitant mantle development in response to dynamic recrystallization. Numerical simulations reveal that the principal factors governing the geometry of mantle structures are: (1) the initial aspect ratio of the porphyroclast (a/b), (2) the rate of clast-size reduction, and (3) the ratio of the rates of pure shear and simple shear (S_r) or the kinematic vorticity (W_k) in the general type of non-coaxial deformation. In general, porphyroclasts develop δ-, φ- and finally, σ-type mantle structures, as the rate of clast-size reduction is progressively increased. The tails of equant porphyroclasts tend to be characterized by wings with increase in bulk shear during progressive deformation. In contrast, inequant objects (a/b>1) develop composite tails with multiple wings, even at low finite shear strains. However, with increase in aspect ratio d geometry tends to dominate the overall mantle structure. Porphyroclasts with a large aspect ratio (a/b=3) form tails with overturned δ-wings, as described in Passchier, C.W., Simpson, C., 1986. "Porphyroclast system as kinematic indicators", Journal of Structural Geology 8, 831-844. In general the type of non-coaxial deformation, with decrease in kinematic vorticity (or increase in S_r), porphyroclasts irrespective of their initial shapes, tend to form atypical δ-like tails that do not cross the central reference plane.