SIGNIFICANCE OF SYN- TO POST-OROGENIC EXTENSION ALONG THE PROTEROZOIC MARGIN OF EASTERN LAURENTIA (ONTARIO AND NEW YORK)

Structural and geochronologic studies in the Grenville Province of southern Ontario and northwestern New York State show periods of extension both at the end of and after the last episode of contraction. Following an episode of syn-orogenic extension that is preserved along the Bancroft shear zone (BSZ), the Robertson Lake shear zone (RLSZ) and the Carthage-Colton shear zone (CCSZ) were (re)activated as large-scale, post-orogenic extensional structures. These major late extensional ductile-to-brittle normal faults dip shallowly towards each other and are locally oblique to earlier ductile structures. The timing of syn-orogenic extension at ~1040 Ma is documented by U/Pb isotope data on sphenes and Ar isotope data on hornblendes, whereas regional extension along the RLSZ and CCSZ occurred after 945 Ma, shown by 40Ar-39Ar analysis of hornblende, biotite, and K-feldspar. Based on Ar K-feldspar ages, the eastern portion of the Grenville Province from Ontario to the Adirondack Highlands was uplifting as a uniform block by 780 Ma. This protracted history of extensional events requires multiple mechanisms. Late syn-orogenic motion along the BSZ is suitably explained by orogenic collapse of an overthickened crust. However, post-945Ma extension requires other scenarios. Gravitational collapse and slab breakoff act over timescales that cannot explain a long history of extension that is contemporaneous with continued uplift of the entire region. This differs significantly from the Himalayan-Tibetan system, which has often been proposed as an analogue for the Grenville Province. Post-orogenic extension in the Grenville Province is best explained by a combination of a heat source at the base of the crust (upwelling) that produced the observed regional uplift, and the presence of far-field stresses that drive extension. The latter may reflect local (pre-850 Ma) breakup of the Rodinia supercontinent, possibly in response to plate reorganization driven by mantle upwelling.