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

This paper presents a generalized theoretical approach towards two-dimensio nal 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 synkine matic size reduction of the porphyroclast with concomitant mantle developme nt in response to dynamic recrystallization. Numerical simulations reveal t hat the principal factors governing the geometry of mantle structures are: (1) the initial aspect ratio of the porphyroclast (a/b), (2) the rate of cl ast-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-co axial deformation. In general, porphyroclasts develop delta-, phi- and fina lly, sigma-type mantle structures, as the rate of clast-size reduction is p rogressively increased. The tails of equant porphyroclasts tend to be chara cterized by wings with increase in bulk sheer during progressive deformatio n. In contrast, inequant objects (a/b > 1) develop composite tails with mul tiple wings, even at low finite shear strains. However, with increase in as pect ratio delta geometry tends to dominate the overall mantle structure. P orphyroclasts with a large aspect ratio (a/b = 3) form tails with overturne d delta-wings, as described in Passchier, C.W., Simpson, C., 1986. "Porphyr oclast system as kinematic indicators", Journal of Structural Geology 8, 83 1-844. In general the type of non-coaxial deformation, with decrease in kin ematic vorticity (or increase in S-r), porphyroclasts irrespective of their initial shapes, lend to form atypical delta-like tails that do not cross t he central reference plane. (C) 2000 Elsevier Science Ltd All rights reserv ed.