UPLIFT AND EXTENSION ALONG THE PROTEROZOIC MARGIN OF EASTERN LAURENTIA

Combined structural and geochronologic research in the northeastern Grenville Province (Ontario and New York State) shows protracted periods of extension after the last episode of contraction. The Grenville Province in this area is characterized by syn-orogenic extension at ca. 1040 Ma,supported by U/Pb data on sphenes and Ar-Ar data on hornblendes, followed by regional extension occurring along crustal-scale shear zones between 945 and 780 Ma, shown by Ar-Ar analysis of hornblende, biotite, and K-feldspar. At ca. 780 Ma the eastern portion of the Grenville Province, from Ontario to the Adirondack Highlands, is uplifting as a uniform block. Tectonic hypotheses have invoked various driving mechanisms to explain the transition from compression to extension; however, such explanations are geodynamically unconstrained. Our numerical models indicate that mechanisms such as gravitational collapse and mantle delamination may act over timescales that cannot explain a protracted, 300 myr extensional history that is contemporaneous with ongoing uplift of the Grenville Province. The presence of a plume upwelling underneath the Laurentian margin may contribute to uplift and extension in the Grenville Province during this time. The uplift history, while on slightly different timescales, is similar to that seen in numerical models of uplift and extension caused by the interaction of a plume with the base of the lithosphere. Alternatively, some of the protracted extension could be a far-field effect, possibly caused by tectonic activity in the southwestern U.S. in which deformation is transmitted through the rigid Laurentian craton to its margin, effectively changing the stress distributions in the Grenville Province of northeastern North America.