Tectono-magmatic reactivation in cratonic settings: a case study from the Superior province, Canada

Phanerozoic intraplate magmatism has frequently been observed in association with ancient sutures, palaeorifts and strike-slip fault zones across multiple ancient cratons, including Laurentia, Baltica, Australia, Siberia. However, it is still unclear whether these lithospheric discontinuities were passive conduits for the melts generated in the asthenosphere, or if their tectonic reactivations acted as a primary control on melt production and distribution. In the Superior province of the Canadian shield, we explore the relationship between intraplate tectonic and magmatic activity along two segments of the Proterozoic St. Lawrence failed rift system, which hosts two Jurassic kimberlite fields (Kirkland Lake, Timiskaming) and a Cretaceous alkaline province (Monteregian Hills). Our goals are 1) to examine the structural settings of these provinces and 2) investigate the potential role of these lithospheric structures in melt production and channelling under the Mesozoic stress regime.

Basement fault structures associated with kimberlite pipes and alkaline intrusions were identified using available aeromagnetic data from Timiskaming and Montérégie. Magnetic data were employed  to construct a constrained 3-D inversion of the magnetic susceptibility distribution using Oasis montaj VOXI software package. Additionally, the regional stress field in the Superior province in the Mesozoic was reconstructed based on 542 measurements of joints, shear fractures, veins and dykes taken at 36 sites across the Palaeozoic cover of St. Lawrence lowlands. The Right Dihedron and Rotational Optimisation methods implemented in WinTensor 5.9.2 were used to compute stress tensors for structural associations of different relative ages.

The results demonstrate that Kimberlite pipes of the Timiskaming and Kirkland Lake fields tend to cluster around the intersections of two fault families: 1) thrust faults of Neoarchean Destor-Porcupine and Esker – Larder Lake sutures (trending W–E), and 2) normal faults of the Proterozoic Timiskaming graben (trending NNW – SSE). Intrusions of the Monteregian Hills alkaline province are also emplaced at the intersection of two fault families: 1) normal faults of the Proterozoic Ottawa – Bonnechere graben (trending W–E), and 2) a N–S trending set of faults of unclear kinematics or age. Reconstructed stress tensors for the Mesozoic are indicative of an extensional regime and a progressive counter-clockwise rotation of the stress-field throughout the Mesozoic (subhorizontal σ₃  trend shifts from 86 to 306).

The spatial distribution of intrusions within the Timiskaming and Ottawa-Bonnechere grabens, indicates that intraplate magmatism was strongly controlled by St. Lawrence paleorift structures. However, intrusions are preferentially localized in areas where the paleorift is intersected by other fault systems. We speculate that these local fault systems are transfer faults oriented perpendicular to the normal faults of St. Lawrence system, creating pull-apart-like structures that accommodated intraplate magmatism. This emplacement model aligns with geochronological data, which indicate Jurassic intrusions of the Timiskaming and Kirkland Lake fields were emplaced along NNW–SSE-trending graben under a SW–NE  trending σ₃ , while the Cretaceous Monteregian Hills were emplaced along the W–E-trending Ottawa–Bonnechere graben under a N–S trending σ_3.