How does orogenic crust deform? Evidence of crustal-scale competent behaviour within the partially molten middle crust during orogenic compression

Granitic partial melts are generally thought to significantly weaken the orogenic crust and, ultimately, lead to the collapse of an orogen. Studies from different orogens have shown that the syn-melt deformation behaviour at the orogenic scale is, however, more complex. In addition, once fully crystalline, granitic material strengthens the crust. Linking the evolution from melt-present to melt-absent deformation at the scale of the orogen is a challenging but necessary task if one wishes to investigate the overall behaviour of the middle crust. In this paper, we make observations of orogen-scale deformation, developed over a period of 30–40 Ma during crustal partial melting of the middle and lower crust of a Palaeoproterozoic orogen. The crust shows a globally common pattern where coeval partial melting, compressional deformation, and transtensional structures co-exist. We demonstrate this complex interaction through an integrated approach using multiple datasets. The key observation is that of widespread, regular, orogen-scale shortening by folding of sub-horizontal anatectic granitic sheets and highly migmatized lithologies; this shortening interacted with modest orogen-parallel lateral stretch expressed as local extensional/transtensional structures. The pervasive and dominant deformation style is that of folding of the syn-kinematic granite sheets and voluminous migmatites: this demonstrates that the principal deformation style of deformation within the partially molten mid-crust is competent rather than weak. The observed evidence of weak behaviour such as strike-slip or transtensional shear zones accommodating lateral escape are localized in comparison and not necessarily associated directly with the granite sheets or the highly migmatized volumes of the crust. Moreover, the strain intensity (fold wavelengths and amplitudes) seems independent of the overall melt fraction. The implications are that, while widespread volumes of partial melt will weaken the crust overall (compared to the brittle upper crust) and while individual melt bodies can possibly persist for up to some millions of years in the middle crust, i) the relative strength between (partially) molten volumes is not primarily controlled by melt fraction; and ii) individual melt volumes may be too short-lived at the time-scale of orogenic deformation (i.e. orogenic strain rates) to significantly influence the overall deformation style.