Flow and strain patterns at the terminations of tapered shear zones Nibir Mandal a , Susanta Kumar Samanta a , Chandan Chakraborty b, * a Department of Geological Sciences, Jadavpur University, Calcutta 700032, India b Geological Studies Unit, Indian Statistical Institute, 203 B.T. Road, Calcutta 700035, India Received 20 December 2000; revised 1 April 2001; accepted 9 May 2001 Abstract With the help of corner ¯ow theory, this paper numerically analyzes the deformation pattern at the terminations of tapered shear zones, the walls of which are rigid and move parallel to each other in opposite directions. The overall ¯ow pattern is characterized by curvilinear particle paths that show convexity towards and opposite to the tapering direction respectively for low ,58) and high .108) inclinations of the wall verging opposite to the sense of wall movement. In tapered shear zones there are two distinct ®elds of instantaneous shortening and extension parallel to the direction of wall movement. Numerical models reveal that the ®nite strain distributions are generally asymmetrical with larger strain concentration occurring near the wall verging opposite to sense of wall movement. The S-foliation trajectories show a curvilinear pattern, convexing against the tapering direction. The analysis of rotationality vorticity) indicates that the sense of vorticity near the synthetically verging wall is reverse to the sense of wall movement; however W k is one everywhere within the shear zone. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Non-parallel walls; Simple shear; Ductile ¯ow; Vorticity; Foliation 1. Introduction Theoretical, experimental and ®eld studies over several decades have led to a comprehensive understanding on the kinematics of parallel-sided, ductile shear zones. In such shear zones the strain pro®les generally remain almost constant in differing transects through the zones. However, shear zones can show walls which converge and diverge, and this type of non-parallelism is commonly observed at the terminations of most natural shear zones Ramsay and Huber, 1987, p. 595). The deformation near the tapering ends of these shear zones hereafter called tapered shear zone) is essentially heterogeneous, and the nature of strain distribution is extremely complicated with complex, laterally variable strain pro®les Freund, 1974; Ramsay and Graham, 1970; Ramsay, 1980; Simpson, 1983; Ingles, 1986; Ramsay and Huber, 1987). The intricacy of the defor- mation pattern within tapered shear zones and its dis- similarity with that of parallel-sided shear zones can also be demonstrated by means of simple physical model experi- ments Fig. 1). An appropriate deformation model that describes the heterogeneous ¯ow within tapered shear zones is, however, still lacking see Ramsay, 1980; Ramsay and Huber, 1987). This paper investigates the heterogeneous ¯ow within tapered shear zones with special reference to particle paths, strain distribution pattern and foliation trajec- tories, using a simple continuum model. The continuum-mechanics approach is a useful way for the study of macro-scale ductile shear zones Cobbold, 1977; Ramsay, 1980). Several workers have applied con- tinuum models to analyze the deformation patterns in large- scale, parallel-sided shear zones involving transpressional movement Sanderson and Marchini, 1984; Fossen and Tikoff, 1993; Tikoff and Teyssier, 1994; Dutton, 1997; Jones et al., 1997). The results of numerical simulations based on these models conform well to the structural features observed in natural, analogous transpression zones e.g. Dutton, 1997; Jones et al., 1997). This paper also uses the continuum approach but applies the corner ¯ow theory to study the ¯ow and strain patterns in ductile, tapered shear zones with rigid walls. Corner ¯ow model Batchelor, 1967) was utilized by several workers to explain exhumation in convergent settings Cowan and Silling, 1978) and emplacement of exotic blocks in me Âlange terranes Cloos, 1982, 1984). To study the ¯ow kinematics of tapered shear zones we have, however, slightly modi®ed the corner ¯ow model of Batchelor 1967) as enumerated in the following section. The modi®ed model has been used for numerical simulations Journal of Structural Geology 24 2002) 297±309 0191-8141/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0191-814101)00065-7 www.elsevier.com/locate/jstrugeo * Corresponding author. Fax: 191-33-577-6680. E-mail addresses: nibir@jugeo.clib0.ernet.in N. Mandal), chandan@isical.ac.in C. Chakraborty).