RAPID EXTRUSION AND PERVASIVE HORIZONTAL FABRIC: A CASE FOR LATERAL OVERTURNING OF DEEP CRUSTAL MIGMATITES, NE BOHEMIAN MASSIF

In the Sudeten Variscides of Bohemia, structural and geochronometric evidence from a widespread migmatitic massif uncover an apparent conundrum of rapid sub-vertical extrusion as indicated by the telescoped chronologies with the existence of regional sub-horizontal foliations and lineations which trend parallel to narrow sub-vertical shear zones that contain the (U)HP assemblages. Metamorphic U-Th-Pb ages from the migmatites of the Orlica-Snieznik Complex constrain the supra-Barrovian conditions during mid-crustal residence to 345-330 Ma coeval with rapid cooling as indicated by regional Ar-Ar white mica and biotite ages of 342-334 Ma. Mesoscopic petrofabrics combined with anisotropy of magnetic susceptibility (AMS) results from migmatitic metasedimentary and metaigneous units reveal remarkably consistent shallow (10°) N-S lineation attitudes on both steep and sub-horizontal planar fabrics. Strain analysis conducted using the AMS data reveals a dominant oblate (flattened) fabric with only local prolate fabric. Structural evidence for exhumation by crustal extension is absent. We propose that during closure of a Devonian seaway, the leading edge of Bohemia was subducted to >70 km depths culminating in (U)HP metamorphism of primarily continental material. Following failure of the subducted slab and terminal arc-continent collision, the low-density / low-viscosity orogenic lower crust traveled an initial buoyancy-driven (isothermal) ascent path, which was sub-vertical in nature– due in part to the rigid continental backstop of the over-riding plate. Migmatization attributed to isothermal decompression, breakdown of hydrous minerals, and the heat advection with the rise of lower crustal material weakened the overthickened crust, allowing continued rapid sub-vertical ascent of a low-viscosity matrix carrying small, resistant pods of eclogite, granulite and garnet peridotite. Vertical extrusion of the orogenic root was arrested in the mid-crust, where the lower ductile crust was laterally overturned at the base of rigid upper crustal blocks. Upon reaching the crustal high-strength lid and cessation of the rapid vertical exhumation, the ductile mass of continental material horizontally spread, underwent subsequent supra-Barrovian metamorphism, and quickly cooled.