High strain melt transfer: signatures and implications

We present microstructural and outcrop-scale evidence for melt migration pathways that formed during deformation within high-strain zones cutting through both partially molten (supra-solidus) and solid (sub-solidus) rocks. High-strain zones with more than 10% undeformed felsic or leucocratic material are easily recognized as melt pathways, especially in supra-solidus rocks. However, identifying similar pathways in sub-solidus rocks—or those with very low melt content (1–2%)—is more difficult, as they often lack visible felsic components at the outcrop scale and closely resemble typical mylonite zones, leading to frequent misinterpretation.

Even where large volumes of melt once moved through these zones, direct microstructural evidence can be subtle or only detectable under the microscope. When melt-rock interaction is more intense, textural signs may remain faint, but geochemical signatures can be detected at both the micro- and macro-scale. In these settings, melt tends to flow through porous, high-strain networks in a highly reactive manner, producing complex reaction fronts.

We urge geologists to carefully assess whether deformation occurred in the presence of melt, even in zones that appear solid-state. This distinction is critical because: (1) rocks deformed with even small amounts of melt are significantly weaker than those in the solid state; (2) melt-rock interactions can lead to metasomatism; and (3) these zones can channel and sustain prolonged melt movement, contributing to large-scale chemical differentiation of the crust.