Low‐Angle Shear Within the Exposed Mânzălești Diapir, Romania: Salt Decapitation in the Eastern Carpathians Fold‐and‐Thrust Belt

Abstract

AbstractIn salt‐detached fold‐and‐thrust belts, contractional modification of salt structures may include decapitation by thrusting, but examples are not well known in the subsurface and undocumented in outcrop. Here we present a surface exposure of an intrasalt, sub‐horizontal shear zone at the boundary between the Tarcău and Subcarpathian nappes in the Romanian Eastern Carpathians. The Mânzălești diapir forms the largest rock salt outcrop in Europe, with unique salt‐karst geomorphology. Numerous wells show that the outcrop is above deep‐seated salt of a precursor salt‐cored anticline or passive diapir whose base is at >3,500 m. Multiscale observations using UAV‐based digital outcrop models, fieldwork, and microstructure analysis show that the outcrop is characterized by sub‐horizontal foliation with isoclinal folds, unlike the subvertical fabric of most Romanian diapirs. The halite is rich in clastic inclusions, with a power‐law size distribution caused by tectonic reworking of originally impure salt. Microstructures show that the halite matrix is strongly deformed by dislocation creep, forming subgrains and dynamically recrystallized grains around large porphyroclasts with piezometry indicating relatively high differential stress of around 4 MPa, at pressures sufficient to suppress dilatancy. The observations are best explained by sub‐horizontal shear generated by an overriding nappe, overprinting an original coarse‐grained salt fabric during decapitation of the salt body.