Petrological evidence for co-seismic slip in extending middle-lower continental crust: Heier's zone of pseudotachylyte, north Norway

Multiple generations of pseudotachylyte occur in the normal, shallowly dipping (30°NW) Eidsfjord and Fiskfjord shear zones, northern Norway, which developed during lateto- post-Caledonian crustal extension of Precambrian crystalline basement. Garnet-bearing amphibolite- facies mineral assemblages occur in metamorphosed pseudotachylyte and in sheared metaanorthosite. Similar mineral assemblages (+clinopyroxene) occur in mylonitized pseudotachylyte cut by normal shear bands at Fiskfjord. These assemblages permit calculation via geothermobarometry of pressure and temperature for the crustal level at which pseudotachylyte was repeatedly generated by co-seismic slip during crustal extension. Equilibration conditions at Eidsfjord of 700±50 MPa and 650±25°C correspond to z = 22-29 km; conditions for Fiskfjord of 950±200 MPa and 640±50°C correspond to z = 30-40 km. Both ranges are significantly greater than the standard depth of the seismogenic zone. Creep at depth in the normal fault accumulated elastic strain in the shallower levels of the fault, which upon release initiated rupture downwards into the ductile regime at high strain rates resulting in frictional sliding and the generation of pseudotachylyte. Garnet-bearing ultramylonitic pseudotachylytes attest to a return to steady-state creep following co-seismic slip at depths well below the standard seismogenic zone.