Fracture-driven intrusion and upwelling of a mid-crustal pluton fed from a transpressive shear zone--The Rieserferner Pluton (Eastern Alps)

摘要

The Rieserferner Pluton was emplaced at a depth of 12–15 km into steeply dipping, greenschist-facies mylonitic rocks of the Austroalpine basement, just south of the Tauern Window (Eastern Alps). Intrusion occurred during north-south–directed shortening and east-west horizontal extension in front of the rigid Southern Alpine Indenter. The regional strain field is transpressive, with a strong coaxial component, and is characterized by east-west stretching. Tonalitic melt ascended through a feeder channel preserved in the steep southern part of the Rieserferner Pluton, within and adjacent to the steep mylonitic foliation of a major shear zone, the Defereggen-Antholz-Vals (DAV) Line. Melt ascent was perpendicular to the north-south shortening direction in the country rocks. Magma emplacement involved melt-induced hydro-fracturing to form a subhorizontal, tabular pluton that protruded northward from the DAV Line into the previously folded country rocks. During the late stage of emplacement, buoyant upwelling of the partly recrystallized magma induced doming of the pluton roof as well as vertical ductile shortening of the directly overlying country rocks. Magma pressure therefore locally exceeded the lithostatic pressure. Thermal modeling constrains the maximum time for doming and solidification of the pluton to have been 32,000 yr. The wavelength of the domes in the pluton roof indicates the viscosity contrast between country rock and partly crystallized tonalite in the pluton to have been at least 100:1.