Tectonic evolution of the Cretaceous Gyeongsang Back-arc Basin,SE Korea: Transition from sinistral transtension to strike-slip kinematics

The Gyeongsang Basin, the largest Cretaceous nonmarine sedimentary basin in Korea, formed as a continental back-arc basin related to NNW-directed subduction of the (proto-) Pacific Plate underneath the Eurasia Plate. The basin can be divided into an earlier-formed western back-arc depression and a later-formed eastern volcanic arc platform. We investigated the basin evolution and the associated tectonic settings, largely based on an analysis of structures individuated in the field in the back-arc region. From 127 Ma, the basin initiated as a NNE-trending narrow depression bordered by NNE-striking sinistral faults, and then progressively expanded under a transtensional kinematics induced by progressive trench roll-back. Sinistral shearing of inherited NNE-striking structures played an important role in basin subsidence, and secondary WNW- to NW-striking transverse faults acted as normal faults. The NNE-striking principal displacement zone in the west of the basin runs along the western marginal area of the Jinju and Daegu domains and passes through the Uiseong domain from south to north, but most of this zone is now preserved as deep structures. Volcanic activity starting at ca. 115 Ma was characterized mainly by episodic basaltic eruptions occurring contemporaneously with back-arc deposition of a sedimentary sequence. After ca. 90 Ma, a transtensional kinematics changed to a strike-slip one, and the basin expansion and sedimentation in the back-arc region terminated. During the strike-slip event, rhyolitic-dacitic volcanism increased in intensity as a large NE-trending volcanic arc developed close to subduction zone and its loading caused the stratal flexure in the back-arc region, and the orientations of the shortening and stretching axes remained NW–SE and NE–SW, respectively. Additionally, continuing sinistral shear generated local depressions along the faults located in the west of the back-arc region and within the volcanic arc.