REASSESSING THE REGIONAL BASAL DECOLLEMENT OF THE UNGAVA OROGEN, NORTHERN QUEBEC, CANADA

The Ungava orogen (UO) of Nunavik, Quebec, Canada, is part of the ca. 1.83–1.79 Ga Trans-Hudson orogen, formed by the collision of the Churchill collage upper plate with the Archean Superior Craton lower plate. While the UO is often cited as an early example of a complete Wilson Cycle of the Manikewan Ocean, its stratigraphy and tectono-metamorphic evolution remain loosely constrained. It is typically divided into four lithotectonic domains (Southern, Northern, Kovik and Narsajuaq) separated by major south-verging thrust-dominant faults. However, inconsistencies in tectonic models persist, as evidenced by recent work in the Northern Domain. This is exacerbated by a lack of detailed information on the timing and kinematics of movement along major bounding structures, leaving key questions about the amalgamation and architecture of the UO unresolved.

To address this knowledge gap, the current study examines the basal décollement of the UO exposed in a klippe of Southern Domain rocks within the Kovik Domain through detailed structural mapping, microstructural characterization and in situ geochronology. Field and microtectonic shear sense indicators suggest normal-sense movement along the décollement, contradicting recent models that propose thrust faults bound the klippe. All in situ U-Pb titanite and apatite dates, as well as in situ Rb-Sr mica dates, fall within the interval ca. 1765–1730 Ma, significantly younger than previous estimates of ca. 1814–1789 Ma based on whole-grain dissolution techniques. The narrow range of dates across different chronometers with varying closure temperatures may indicate either 1) a period of rapid cooling following the complete resetting of all geochronometers during a major thermal event, or 2) deformation-induced resetting of the geochronometers, perhaps enhanced by fluid infiltration during deformation. These results challenge recent interpretations and, therefore, require a revised model of basal décollement and klippe formation, which can be integrated into the broader tectonic and kinematic framework of the Ungava and Trans-Hudson orogens.