Dating fossil lower-crustal earthquakes by in-situ apatite U-Pb geochronology

Determining the precise age of fossil earthquakes is essential for understanding the tectonic setting in which they occurred. Pseudotachylytes are quenched frictional melts that occur along coseismic fractures and represent evidence of fossil earthquakes in the rock record. Here, we present the first in-situ apatite U-Pb ages for three samples from Lofoten in the northern Norwegian Caledonides, that occur within and adjacent to lower-crustal pseudotachylytes: a pristine pseudotachylyte, a mylonitized pseudotachylyte, and a mylonite that contains pseudotachylyte transposed into the shear plane. The apatite U-Pb ages are combined with cathodoluminescence and electron backscatter diffraction analysis to provide microstructural constraints on the interpretation of ages. Apatite shows evidence of either fragmentation and annealing, or dislocation creep in response to coseismic rupture. The results suggest that the short-lived thermal pulse generated by the earthquake is sufficient to reset the apatite U-Pb system. The age of 426 ± 19 Ma is interpreted to date lower-crustal seismicity during the early Caledonian orogeny. Later fluid-rock interaction along the same structures led to partial resetting of the apatite U-Pb systematics, indicating that seismic brittle failure of the lower continental crust occurs at the incipient stages of continental collision, and that the resulting structures are preferential pathways for later fluid flow in the otherwise impermeable anhydrous lower crust.