TRANSPRESSIVE TECTONICS DURING THE OTTAWAN OROGENY : EVIDENCE FROM THE DANA HILL METAGABBRO BODY, NW ADIRONDACK MTNS., NEW YORK STATE

The Dana Hill Metagabbro Body (DHMG) is a small (5 X 1.5 km) gabbroic intrusion located along the boundary between the Adirondack highlands (Central Granulite Terrain CGT) and lowlands (Central Metasedimentary Belt CMB). The DHMG is located within the Carthage-Colton Shear Zone (CCSZ). Petrologic, structural, and geochronologic evidence indicates that the DHMG was multiply deformed and that deformation took place under amphibolite to granulite conditions syn- to post peak Ottawan Orogeny (1020Mya). The shallow plunge on stretching lineations for granulite facies shear zones in the DHMG indicate that the early deformation/recrystallization events record predominately strike-slip motions along the CCSZ and that the structural level did not radically change during these shearing events. In short, early movements along the CCSZ were not caused by unroofing of the orogen, but rather by potentially large-scale, right-lateral strike-slip movements. Later events (shearing, folding and brecciation) span a wide range of conditions terminating with brecciation at greenschist facies conditions. These temporally later events may record dip-slip movements occurring as late as 945 Ma that eventually bring the CMB and CGT to a common structural level. In this model, the CMB and CGT terranes must have been laterally and vertically separated during the Ottawan Orogeny. The lateral (along strike) separation of the CGT and CMB may have been of great enough magnitude to allow these two terranes to undergo very different P-T-t paths during the Ottawan Orogeny without the need to invoke an intervening ocean basin. The tectonic scenario for the CGT during the Ottawan Orogeny is similar to the transpressive environment envisioned for the emplacement of the widely distributed Wrangellia and Cache Creek Terranes in western North America. The recognition of large scale strike-slip movements along shear zones during the Grenville (Ottawan) may shed light on the relative motions between CGT rocks of the Adirondack Highlands and Morin Terrane with other Grenville-aged CGT slices like the Oaxacan Complex in Mexico and Garzon Massif in Columbia.