Comment and Reply on "Model for the Precambrian evolution of the Avalon terrane, southern New Brunswick, Canada" COMMENT Albert Leger, Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218 Paul F. Williams, Department of Geology, University of New Brunswick, Fredericton, New Brunswick E3B 5A 3, Canada Nance (1987) presented a model for the Precambrian evolution of the Avalon terrane of southern New Brunswick, Canada, and interpreted major shear zones as dextral and transtensional. We agree with inter- pretation of the faults as dextral (Leger, 1986; Leger and Williams, 1986) and believe that Nance's model sheds light on a geologically very complex area. However, two critical problems remain: the date of mylonit- ization in the Pocologan and Belleisle zones, and the evidence for transten- sional forces in southern New Brunswick during the Precambrian. Nance (1987) stated that the dike complex between the Belleisle and Pocologan zones is "spatially and temporally associated with the Pocolo- gan mylonite zone, which . . . formed in response to intense dextral shear that affects earlier but not later dikes" (p. 753-754), Because the dikes are believed to be Precambrian, he concluded that the mylonitization also occurred in the Precambrian. He presented no new evidence for the age of the dikes but referred to "dikes and coeval mylonitization that formed prior to the deposition of the (unconformably) overlying Eocambrian and early Paleozoic successions" (p. 753). We have mapped the mylonites along the same shear zones in rocks of uncertain age (possibly Paleozoic, according to McCutcheon and Ruitenberg, 1985), but found no clear evidence of an unconformity between the mylonites and the overlying Eocambrian sequence (Leger, 1986; Leger and Williams, 1986). Rast and Dickson (1982) presented reasonable arguments for interpretation of the dikes as Precambrian, but they also were unable to present any cogent evidence for the age. Currie (1987a) has pointed out that some of the dikes show signs of deformation at their contacts, whereas others are apparently undeformed. Nance (1987) interpreted this situation as indicating that early dikes in- truded the faults. We agree with Currie's observation (Leger, 1986) but note that shear-zone deformation is heterogeneous and that more detailed structural work, in this area of poor outcrop, is necessary before Nance's interpretation can be substantiated. Contrary to Nance (1987), we have interpreted the mylonites as "Acadian" in age on the basis of correlation with other northeast-striking northern Appalachian faults of known Silurian through Carboniferous age. These faults are all dextral during their early movement history and have strikingly similar fabrics, including juxtaposed steeply and shallowly plunging stretching lineations. They vary in orientation from a little east of north to east-west and commonly curve gradually from one orientation to the other. They include the Cobequid/Chedabucto fault (Nova Scotia), the Dover fault, and the Indian Islands fault (Newfoundland), all of which are being worked on currently by one of us (Williams) and/or students at the University of New Brunswick. Along with other faults with which we are less familiar, they form an anastomosing network that we believe was active at least between Silurian and Carboniferous time. Should members of this network prove to have been active in Precambrian time, we are faced with the interesting problem of explaining how they survived the opening and closing of Iapetus. The date of mylonitization in the Pocologan and Belleisle zones is thus crucial to a correct reconstruction of the Avalon terrane and the Appalachians in general. A detailed structural study of the dikes, with radiometric ages on both deformed and undeformed examples is necessary before asserting that they are Precambrian. Finally, we consider Nance's interpretation of the dike and mylonite geometry as the coeval products of transtension to be incompatible with the field evidence (Fig. la). With transtensional dextral shear of both the Belleisle and Pocologan faults, the extensional stress axes (ct3) would have been oriented north-northeast-south-southwest, and we would expect the dikes to have been initially oriented west-northwest-east-southeast (Fig. lb). Dikes of this orientation are not encountered in the area in question. Figure 1. a: Sketch of southern New Bruns- wick showing distribution of dikes within dike complex (D.C.). Dike complex is bounded by two dextral shear zones: Belleisle fault zone (BFZ) to north and Pocologan mylonite zone (PMZ) to south. Heavy short lines represent dikes; heavy long lines represent faults. Map after Rast and Currie (1976) and Chandra (1982). b: Diagram showing approximately ideal distribution of dikes injecting within dex- tral transtensional tectonic regime; a1 repre- sents maximum compressive stress;CT3repre- sents minimum compressive stress. "OLOGY, May 1988 475