Thermobarometry of metamorphosed pseudotachylyte and associated mylonite: Constraints on dynamic Co-seismic rupture depth attending Caledonian extension, North Norway

• Garnet occurs in metamorphosed and mylonitic pseudotachylyte.
• Mineral assemblages are conducive to mineralogic thermobarometry.
• Calculated P corresponds to depths of ca. 20 and 40 km, well below the seismogenic zone.
• Results are consistent with downward dynamic rupture on a seismogenic normal fault.

The exhumed post-Caledonian Eidsfjord and Fiskfjord extensional shear zones of northern Norway exhibit evidence of coseismic rupture propagating into the ductile crust as evidenced by the presence of mylonitic and metamorphosed pseudotachylyte. Geothermobarometric calculations on garnet-bearing mineral assemblages in mylonitic gneisses associated with mylonitic pseudotachylyte and in metamorphosed pseudotachylyte permit determination of the depth and ambient temperature of seismogenic low-angle ductile extension. Average pressures from Eidsfjord (570 ± 115 MPa at ca. 650 °C) and Fiskfjord (1120 ± 220 MPa at ca. 650 °C) correspond to faulting depths of 21 ± 4 km and 41 ± 9 km, respectively. The Fiskfjord results agree with previous thermobarometric calculations on mylonitic Cpx + Grt-bearing pseudotachylyte at Fiskfjord. The calculated depths are 5–25 km below the depth of the standard seismogenic zone. These results demonstrate that low angle normal faults may cut through a large portion of continental crust. This occurrence of mylonitic pseudotachylyte in an extensional crustal setting is most easily explained by dynamic downward rupture into the ductile regime and/or unusually high shear stresses to account for coseismic rupture at such depths, implying a direct connection with a seismogenic normal fault in the upper crust.