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Low-angle shear within the exposed Manzalesti salt diapir, Romania: incipient decapitation in the Eastern Carpathians fold-and-thrust belt
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  • Dan Mircea Tamas,
  • Alexandra Tamas,
  • Jessica Barabasch,
  • Mark Rowan,
  • Zsolt Schleder,
  • Csaba Krézsek,
  • Janos Urai
Dan Mircea Tamas
Babes-Bolyai University, Babes-Bolyai University

Corresponding Author:danmircea.tamas@ubbcluj.ro

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Alexandra Tamas
Durham University, Durham University
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Jessica Barabasch
RWTH Aachen University, RWTH Aachen University
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Mark Rowan
Rowan Consulting, Inc., Rowan Consulting, Inc.
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Zsolt Schleder
OMV Exploration & Production GmbH, OMV Exploration & Production GmbH
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Csaba Krézsek
OMV Petrom, OMV Petrom
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Janos Urai
RWTH Aachen University, RWTH Aachen University
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In salt-detached fold-and-thrust belts, contractional modification of diapirs may include decapitation by thrusting, but examples are not well known in the subsurface and unreported 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 an original salt-cored anticline whose base is at >3500 m. Multiscale observations using UAV-based digital outcrop models, fieldwork, and microstructure analysis show that the outcrop is characterised 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 dirty 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 a relatively high differential stress around 4 MPa. The observations are best explained by sub-horizontal shear generated by an overriding nappe, overprinting the original coarse-grained salt fabric during incipient decapitation of the salt diapir at a depth sufficient to supress dilatancy.