STRUCTURE AND EMPLACEMENT OF DEEPLY SUBDUCTED ULTRAHIGH-PRESSURE ROCKS: AN EXAMPLE FROM THE SEVE NAPPE COMPLEX OF THE SWEDISH CALEDONIDES

Ultrahigh-pressure, microdiamond-bearing paragneisses of the Seve Nappe Complex (SNC) in western Jämtland, Sweden are suited for studying exhumation of deeply subducted terranes. Geologic mapping shows that (1) the upper and lower SNC are highly sheared with top-E/SE vergence, (2) the migmatitic middle SNC contains garnet-amphibolite lenses and rare paragneiss blocks (± kaynite) with discordant foliations, and (3) top-E/SE shearing occurred widely in the middle SNC except for a rheologically similar, strike-parallel zone (100‒500 meters wide) underlying the upper SNC showing top-W/NW shearing. ⁴⁰Ar/³⁹Ar dating of synkinematic muscovites (formed along the mylonitic foliation) and dynamically recrystallized biotite from the boundary between the SNC and Köli Nappe yields plateau ages of 418.1±1.7 Ma and 415.7±1.4 Ma, respectively. Muscovite and potassium-feldspar from a leucosome of the middle SNC are dated to be 416.7±1.4 Ma (plateau age) and ca. 400‒340 Ma (laser step-heating ages), respectively. The mica ages indicate that the timing of shearing of the upper SNC and the cooling and/or shearing of the middle SNC were broadly coeval. In this chronologic context, structural data suggest the following: (1) the Köli Nappe extruded toward foreland relatively faster than the SNC that, in turn, extruded relatively faster than the Särv Nappe; (2) the ultrahigh-pressure blocks of the SNC exhumed incoherently; and (3) the middle SNC had various extruding rates at different structural levels indicating heterogenous flow. Combined with other reported data, the Jämtland SNC likely experienced rapid cooling (>20°C/m.y.) from 860−850°C to 400−350°C between 440 Ma and 416 Ma followed by slower cooling (~3°C/m.y.) to 180‒150°C from 416 Ma to 340 Ma. The rapid cooling is considered to reflect exhumation of the SNC from the depth of migmatization to mid-crustal levels until the shearing of the SNC ceased. The slower cooling is interpreted to record the passive piggyback translation of the SNC toward the foreland during the subduction/exhumation of the Western Gneiss Region at 415−380 Ma and the subsequent unroofing. This study supports the piggyback model for the long-distance translation of the SNC and provides structural context for the exhumation of the SNC with implications for exhumation mechanism(s).