Tracking low-temperature tectonism of the St. Lawrence Platform and Humber Zone, southern Quebec Appalachians, through apatite and zircon (U–Th)/He thermochronology

Abstract

The St. Lawrence Platform (SLP) and Humber Zone (HZ) of the southern Quebec Appalachians have been the subject of extensive studies to resolve the degree of thermal maturation, yet the timing of the thermal maximum is not well understood. We have employed apatite (AHe) and zircon (ZHe) (U–Th)/He thermochronometry across a network of Late Cambrian to Late Ordovician siliciclastic and Grenvillian basement outcrops to resolve the thermal history below 200 °C. Single crystal dates from individual samples exhibit age dispersion by as much as 300 million years, with a strong positive to negative correlation with increasing effective uranium (eU) concentration. The data in the southwestern portion of the basin allow for thermal maxima of up to 200 °C during the Late Ordovician or Early Devonian. Regional burial trends combined with local estimates of paleogeothermal gradients indicate that, if sedimentation continued after the Late Ordovician, there was no significant increase in burial in southwestern portions of the SLP as previously suggested. Heating was followed by protracted cooling through 180–100 °C during the Late Jurassic and Early Cretaceous, and the cooling rate increased by an order of magnitude through the uppermost crust (80–60 °C) until ca. 100 Ma. Both the external and internal HZ sections experienced rapid cooling through the Silurian after a Taconic thermal maximum. The HZ witnessed protracted cooling through the Late Jurassic, followed by accelerated cooling until the present. Increased recognition of these low-temperature events can augment our understanding of the evolution of accretionary orogens, consequently increasing the efficiency of petroleum exploration.