Extension and Doming In (Ultra-)High-Pressure Terranes Driven by Syn-Convergent Exhumation of (U)HP Nappes

Many ultra-high pressure (UHP) terranes are characterised by structural domes cored by nappe stacks comprising UHP eclogites in tectonic contact with overlying high-pressure (HP) and lower-grade rocks. Contacts between UHP and HP nappes are generally interpreted as thrusts, whereas contacts with overlying lower-grade rocks are typically normal-sense shear zones. Numerical models for subduction and exhumation of continental crust in a subduction channel during the onset of collision provide an internally consistent explanation for these observations, including syn-convergent extension. The models involve subduction of a weak continental margin and stronger continental interior, and include strain weakening and reversible density changes accompanying metamorphic phase transformations. Results are interpreted in terms of the exhumation number, E, which expresses the relative contributions of Couette (down-channel) and Poiseuille (up-channel) flow components. During subduction, margin material progressively weakens, leading to diachronous detachment and exhumation of UHP rocks. Decoupling initiates at depth along a shear zone that propagates upwards. The buoyant UHP plume tunnels up the shear zone until it encounters HP material in the upper part of the subduction channel; the HP nappe is then folded over and exhumed by the rising UHP plume. The resulting structural dome is cored by the (U)HP nappe stack, with attenuated remanants of the suture zone overlying the UHP plume and its HP carapace. During the later stages of exhumation, a prominent low-angle extensional shear zone forms above the rising nappe stack, creating foreland-directed normal faults that separate the (U)HP nappes from lower-grade cover. Other nappe geometries can be produced by models significantly stronger or weaker than the reference model, but all include doming and accompanying extension that are driven by the buoyant rise of a substantial volume of UHP material. Model geometries and PTt paths are consistent with observations from Dora Maira, Tso Morari, and other natural examples.