SYNTHESIS OF REGIONAL OTTAWAN SHEAR ZONES IN THE ADIRONDACK AND MORIN TERRANES: TOWARDS A UNIFIED TECTONIC MODEL

The Morin (MT) and Adirondack (Highlands and Lowlands) terranes share many similarities (e.g., ca 1155 Ma AMCG suites; ca 1180 Ma Shawinigan high-grade metamorphism; and late Ottawan collapse), making it likely that they share a common tectonic evolution. The Adirondack Highlands underwent ca 1090-1050 Ma granulite facies metamorphism, and – although similar metamorphic zircon ages have not yet been reported for the central MT - at the southeast extremity of MT, zircon ages indicate high-grade Ottawan metamorphism at ca. 1087 Ma. MT is bounded on the west by the steep, sinistral Labelle Shear Zone (LSZ) and on the east by the dextral Lake Taureau Shear Zone (LTSZ), each with shallow lineations. Both zones bottom out to form a trough shaped decollement along which MT was thrust NNW at ca 1090-1070 Ma. To the southeast both MT and LTSZ are truncated by the extensional, east dipping ca. 1070-1040 Ma Tawachiche Shear Zone juxtaposing amphibolite and granulite facies gneiss (Nadeau, 2006).

Projected south, LSZ aligns with the ~10km wide, mylonitic NNE Black Creek Shear Zone (BCSZ) separating the marble-rich Adirondack Lowlands from the quartzite-rich Frontenac terrane of Ontario. BCSZ dips steeply east with shallow lineations, apparently truncates the ca 1160 Ma Kingston dikes, and forms a discontinuity in ä18O(zircon) of AMCG granitoids. We interpret BCSZ as the southern extension of LSZ, each accommodating Ottawan thrusting of both AT and MT. Some 50 km east, the extensional Carthage-Colton Shear Zone (CCSZ) dips gently northwest with down dip lineations and juxtaposes the hanging wall amphibolite grade Lowlands against the granulite facies Highlands. CCSZ and LSZ appear to be unrelated. Dating of synkinematic Lyon Mt. Granite in CCSZ fixes faulting at ca 1045 Ma suggesting it represents the southwestern analogue of the Tawachiche Shear Zone. Together these two large, gently dipping detachments accommodated late Ottawan exhumation of both AT and MT. Note that AT was proximal to the orogen core and may have experienced more intense Ottawan metamorphism and fabric formation, as well as somewhat later exhumation, than MT. Notwithstanding, this synthesis offers a unified tectonic model that clarifies the history of the Monocyclic Allochthonous Belt and suggests that the Adirondack and Morin terranes form a coherent entity.