Origin of subhorizontal shear zones: Examples from a syntectonic laccolith within a metamorphic core complex

The origins of subhorizontal shear zones in the tensile middle crust are unclear. Structures in the syntectonic Congjia laccolith (128–126 Ma) in the Linglong metamorphic core complex (MCC; 137–108 Ma) of eastern China provide an opportunity to investigate the formation of such subhorizontal shear zones. Heterogeneous uplift of the MCC triggered SE-directed subhorizontal magmatic flow to solid-state deformation within the syntectonic laccolith, leading to the development of top-to-the-SE subhorizontal high-to low-temperature shear zones at 128–125 Ma. The magmatic flow and solid-state deformation were driven by gravity and facilitated by intrusion-induced thermal weakening. As the laccolith was exhumed, magmatic fluid overpressure and corresponding cyclical changes in the principal stress axis led to the development of alternating vertical and subhorizontal hydrofracturing within the brittle-ductile regime. Some of the subhorizontal hydrofractures were overprinted by top-to-the-SE brittle-ductile shear zones during the period from 125 to 123 Ma, and subsequent top-to-the-SE brittle faults formed between 123 and 108 Ma, indicating persistent subhorizontal shearing in the ductile, brittle-ductile, and brittle regimes. The structural evolution of the Congjia laccolith demonstrates that gravity-driven subhorizontal magmatic flow can trigger the development of subhorizontal shear zones in middle crust under a regional tensile setting.