Granitic magma emplacement and deformation during early-orogenicsyn-convergent transtension: The Staré Sedlo complex, Bohemian Massif
Abstract
tThe Late Devonian Staré Sedlo complex, Bohemian Massif, was emplaced as a subhorizontal sheeted sillpluton into a transtension zone. The transtensional setting is documented by strong constrictional fabric,corroborated by the anisotropy of magnetic susceptibility (AMS), with variably developed subhorizontalmagmatic to solid-state foliation suggesting vertical shortening.
Intrusive contacts of the granitoids withmetapelitic screens and tapered sill tips indicate that magma wedging was the dominant process ofsill propagation. The sills exhibit two intrusive styles, ranging from thin lit-par-lit injections to widelyspaced meter-thick sills.
These two styles are interpreted as reflecting variable viscosities of intrudingmagma where low-viscosity magma percolated along foliation planes whereas high-viscosity magmaproduced more localized thicker sills. We propose that the magma/host rock system in transtensionmust have evolved from initial crack tip propagation and vertical expansion due to new magma additionsthrough conduit flow to ductile thinning after the magma input had ceased.
The sill emplacement andtheir subsequent deformation are then interpreted as recording early-orogenic syn-convergent sinistraltranstension along the rear side of an upper-crustal wedge, which was extruded both upward and laterallyin response to subduction and continental underthrusting.