Geology of the Adirondack Mountains (cont.)
3 Carthage-Colton Shear Zone (CCSZ)
The Carthage-Colton shear zone (CCSZ) or mylonite zone (CCMZ) separates the Lowlands from the Highlands. It is more than 100 km long and from a few meters so several kilometers wide. It is mostly composed of the 1164 Ma (Hamilton, 2004) Diana syenite deformed into mylonite.
The major question is the nature of this zone between the Lowlands which underwent no metamorphism during the Ottawan orogeny and the Highlands which were exposed to granulite facies metamorphism at the same time. Mezger et al. (1992) noted that when the Highlands reached a temperature of at least 650° C, the Lowlands had already cooled to 400° C. Since a 250° C difference is too large to have been accommodated by the rather thin mylonite zone (the CCSZ), some explanation must be reached. They suggested that at about 1098 Ma, the Lowlands and the Highlands rifted apart and were separated by a basin or a small ocean. Later they reunited, meaning that, in that case, the CCSZ would be a suture. However, Dahl et al. (2004) note that the Lowlands did experience deformation during the Ottawan orogeny, suggesting that the Lowlands and the Highlands had some kind of common history during the Ottawan orogeny. Streepey et al. (2001, and references therein) also mention 1050 Ma metamorphism farther to the West in the Elzevir terrane, Ontario. These researchers, along with others (e.g., McLelland et al., 2013) propose instead that the Lowlands were thrusted over the Highlands (as early as the Shawinigan orogeny) and so were not exposed to the same high temperatures, lying at a shallower level. Thus the CCSZ would have acted as a fault with a vertical displacement of ~7 km. Later the Lowlands downdropped along the CCSZ during late- and post-Ottawan collapse and extension.
Figure 3-1. Deformed anorthosite in the Carthage-Colton shear zone.
Foliation is well visible in this coarse anorthositic gneiss. Blue crystals are plagioclase (andesine) and pink crystals are also plagioclase, colored by hematite intrusions. Penny for scale.
Figure 3-2. CCSZ mylonite, southwest of Harrisville.
Block of mylonite tilted to the northwest (left).
Figure 3-3. CCSZ mylonite close-up in the same area.
Diana syenite (gray) with pink K-feldspar and white quartz layers. Here the northwest is to the right.