Strain and strain rate in a synkinematic trondhjemitic dike: evidence for melt-induced strain softening during shearing (Bohemian Massif, Czech Republic)

Emplacement-related deformation of a Cambrian trondhjemitic dike, located at the western border of the Teplá-Barrandian unit (central European Variscides), is studied in detail. The trondhjemitic melt was emplaced at T = 750 °C into an active low-temperature (ca 350 °C) east-northeast trending dextral transcurrent shear zone. Thermal modelling indicates that the rheologically critical melt fraction and the solidus of the dike were achieved after 1 and 3 days of cooling, respectively. The major part of the shearing occurred within 8 days after the melt emplaced. nation of the dike, in strain magnitude, mineralogy and geochemistry, can be explained by the laterally varying rheology (from margin to centre) during cooling and shearing. The deformation was volume-constant, and the strain data plot in the apparent constrictional field. The unusually high value of the calculated longitudinal strain rate (> 2.8 × 10−6s−1; equivalent to a displacement rate of > 2 cm h−1) is probably related to the intruding melt that ‘lubricated’ the shear zone and thus enhanced its displacement velocity. Consequently, melts that intrude as dikes into active transcurrent shear zones may significantly weaken the strength of the middle and upper continental crust.