Laser ablation 40Ar/39Ar step-heating analyses for encapsulated and unencapsulated pseudotachylytes from a Neoproterozoic normal fault belonging to the St. Lawrence rift system (Canada) preserve the absolute timing of rifting and initial opening of the Iapetus Ocean. The total gas and retention ages for encapsulated pseudotachylytes from the Montmorency fault (Quebec City) are 610.3 ± 4.6 Ma and 619.0 ± 2.5 Ma. Ten unencapsulated analyses from two pseudotachylyte veins with varying matrix/clast ratios yield total gas ages of 634.7 ± 1.6–663.9 ± 1.8 Ma. These ages show an excellent linear relationship with the proportion of clast inclusions, resulting in lower intercept ages (i.e., no host rock) of 613.3 and 614.2 Ma. These statistically indistinguishable ages constrain major seismic faulting along the St. Lawrence rift system and significantly improve prior estimates for late Neoproterozoic rifting of Iapetus. The upper intercepts, reflecting host-rock ages, match cooling ages of Grenville basement in the area. We conclude that the time of major continental rifting along the northern Laurentian margin and initiation of the Iapetus Ocean occurred at 613–614 Ma, coeval with emplacement of the 615 Ma Long Range dikes of Labrador. This study also demonstrates that Ar geochronology of pseudotachylytes using varying clast/matrix ratios is a robust method to date ancient faulting.

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