In-situ apatite U-Pb geochronology coupled with microstructural analysis reveals the age of lower-crustal seismicity

Pseudotachylytes (quenched frictional melts produced during coseismic slip) represent unambiguous evidence of fossil earthquakes. They are routinely studied to investigate the processes operating before, during, and immediately after seismicity. To place this knowledge into the relevant tectonic context, requires the capability of determining the age of seismicity. However, pseudotachylytes are thin (typically below 1 cm thick) and extremely fine grained, rendering the application of geochronological tools challenging. In the upper crust, the ⁴⁰Ar/³⁹Ar method has been successfully used to date pseudotachylytes, however, no successful attempts of dating lower-crustal pseudotachylytes have been reported until now.

We present results from in-situ apatite U-Pb geochronology applied to lower-crustal pseudotachylytes exposed in Lofoten, northern Norway. The sample suite includes a pristine pseudotachylyte, a mylonitized pseudotachylyte, and a mylonite with pseudotachylyte veins transposed into the foliation. The results are scrutinized by detailed microstructural investigations using electron backscatter diffraction and cathodoluminescence (CL) imaging. By doing so, we are able to identify apatite that deformed by crystal plasticity in response to the seconds-to-minutes-long thermal pulse generated by the earthquake. The corresponding apatite U-Pb data define a tight regression with a lower intercept at ~426 Ma; the age of lower-crustal seismicity in the Lofoten exposures.

Our analyses also yield an age population significantly younger than the age of seismicity. The single spot dates of this population are microstructurally controlled, i.e., spots within the same microstructural framework yield similar ages, and characterized by significant variations in CL intensity within single grains. The U-Pb data of this age population do not define a clear regression, but rather indicate several Pb-loss events. Patchy CL zoning in some of these apatites indicates modification by dissolution-precipitation, and thus the involvement of a fluid in partial resetting of the apatite U-Pb system. We interpret this age group to reflect protracted fluid percolation along the pathways provided by the pseudotachylytes and shear zones. In-situ apatite U-Pb dating coupled with microstructural investigations thus provides (1) the first robust age of a fossil lower-crustal earthquake, which indicates that seismicity occurred during the early stages of continental collision, as well as (2) evidence that such structures serve as long-lived fluid pathways long after seismicity occurred.