Do diapirs ever lose their heads? Insights from the Romanian Eastern Carpathians

Numerous orogenic fold-and-thrust belts contain salt. It serves as an excellent décollement for folds and thrusts, and in some, diapirs had a profound influence on the structural styles. In salt-detached fold-and-thrust belts, decapitated diapirs can form due to thrusting but are poorly documented in the subsurface and not reported in outcrop. Here we present a surface exposure of a sub-horizontal intra-salt shear zone, which is interpreted to have formed as a result of partial decapitation of a deep-rooted salt-cored anticline. The Mânzălești diapir in the Romanian Eastern Carpathians forms the largest rock salt outcrop in Europe, with unique salt-karst geomorphology between the Tarcău and Subcarpathian nappes. Numerous wells show that the outcrop lies over a deep-seated salt diapir, the base of which is at >3500 m. Multi-scale observations using UAV-based digital outcrop models, fieldwork, and microstructure analysis show that the outcrop is characterised by sub-horizontal foliation with isoclinal folds. The halite is rich in clastic inclusions, with a power-law size distribution caused by tectonic reworking of originally dirty salt. Microstructures show that the halite matrix is strongly deformed by dislocation creep, forming subgrains with a dynamically recrystallised grain size of about 1.5 mm. This is indicative of relatively high differential stress, of around 4 MPa. After combining observations on all scales (cross-sectional, outcrop, and microstructural analyses), our preferred explanation is that the Mânzălești diapir has evolved from a salt-cored anticline to a thrusted diapir in front of the Tarcău nappe. Intense shear originating from a thrust partially decapitated the diapir, shifting its upper portion away from its base.