Insights into post-orogenic extension and opening of the Palaeo-Tethys Ocean recorded by an Early Devonian core complex in South China

Abstract

The Palaeo-Tethys Ocean formed during the Devonian as a consequence of blocks separating from northern Gondwana. The record of the Palaeo-Tethys Ocean stretches across Eurasia from the European Alps eastward to the southwestern South China block (SCB). Tectonic domes of the SCB record the contraction, extension, decompression, and partial melting of deep crust that occurred as a result of detachment faulting and tectonic evolution of the Palaeo-Tethys Ocean. This study investigates post-orogenic (Kwangsian) extension and opening of the Palaeo-Tethys Ocean as recorded by the Motianling core complex in the southwestern SCB, based on structural analysis and 40Ar/39Ar geochronology. The Motianling core complex in the southernmost part of the Jiangnan fold belt comprises the Sanfang gneissic granite surrounded by low-grade metasedimentary rocks. The core complex records at least three deformation (D) events: pre-D1, D1, and D2. The pre-D1 event produced large-scale NE-striking folds with axial surface cleavages and was coeval with emplacement of the Sanfang batholith. D1 deformation is represented by a top-to-the-NW ductile–brittle normal faulting system with mylonitic foliation and was coincident with greenschist-facies metamorphism, indicating that the core complex formed during NW–SE extension. D2 deformation is dominated by Cenozoic NW–SE-trending normal faults. The timing of D1 deformation and subsequent cooling was constrained by 40Ar/39Ar geochronology. Syn-kinematic muscovite yields an 40Ar/39Ar age of 417 ± 2.3 Ma (1σ) and suggests that the dome was initially developed in the Early Devonian through ductile shear zones. Magmatic biotite from the granitic mylonite yields a weighted mean 40Ar/39Ar age of 363.0 ± 3.1 Ma. Therefore, the Motianling core complex records at least two major stages of tectonic evolution: (1) Kwangsian (Caledonian) orogenic collapse and subsequent first-stage cooling at ∼417 to 363 Ma; and (2) Cenozoic regional extension and final cooling at ca. 45 Ma, as inferred from published apatite fission track data. The present results, combined with previous studies, indicate that the D1 deformation event resulted in the formation and exhumation of the Motianling core complex in a continental rifted margin during the opening of the Palaeo-Tethys Ocean.