Active-passive margin transition in the Cathaysia Block : thermochronological and kinematic constraints

Abstract

The Cathaysia Block, located in southeastern China, is characterized by a widespread magmatic belt, prominent NE-striking fault zones and numerous rifted basins filled by Cretaceous-Eocene sediments. The geology denotes a transition from an active to a passive margin, which led to rapid modifications of crustal stress configuration and reactivation of older faults in the Cathaysia Block. However, the timing and kinematics of the active-passive margin transition need to be better constrained. The SW Cathaysia Block, near the coastal area of the South China Sea, is selected for studying the transition. There are two major geological units in this region: the Nanling Range and the Yunkai Terrane. The Nanling Range is a magmatic belt composed of granitic plutons with formation ages ranging from Caledonian to Cretaceous. The Yunkai Terrane is a metamorphic terrane of Caledonian age.

Thirty zircon fission-track (ZFT) data and thirty apatite fission-track (AFT) data were obtained from the granitic plutons in the SW Cathaysia Block. The distribution of ZFT ages shows two episodes of exhumation of the granitic plutons: the first episode is found to occur during 170 Ma – 120 Ma and affect the SW part of the Nanling Range; the second episode, a more regional exhumation event, occurred during 115 Ma- 70 Ma. The AFT dating results show a general cooling sequence from south to north during Late Cretaceous – middle Eocene, contrary to the conventional passive margin model.

Numerical geodynamic modeling of subduction zone indicates that (1) high slab dip angle, high geothermal gradient of lithosphere and low convergence velocity favor the subduction process and the reversal of crustal stress state from compression to extension in the upper plate; (2) the late Mesozoic magmatism in South China was probably caused by a slab roll-back; and (3) crustal extension could have occurred prior to the cessation of plate subduction.

The numerical model results of lithospheric deformation associated with subduction process reveal that granitic crust is easily deformed compared to gneissic crust. The results could be used to explain the observation that the granite-dominant Nanling Range was exhumed earlier than the gneiss-dominant Yunkai Terrane. In addition to the difference in geology between Yunkai and Nanling, the heating from Jurassic- Early Cretaceous magmatism in the Nanling Range may have softened the upper crust, causing the area to exhume more readily. On the other hand, the numerical models of crustal extension with pre-existing faults dipping ocean-ward demonstrate a trend of fault reactivation sequence from south to north. Assuming that granite exhumation in the Cathaysia Block was mainly a product of rifting and fault reactivation, the numerical models support the AFT results.

The integrated FT dating and numerical model results suggest that roll-back of the subducting paleo-Pacific Plate slab during Late Cretaceous is likely to be the driving force of the transition from Mesozoic subduction to Cenozoic extension in the Cathaysia Block. The timing of the transition is suggested to have taken place at ~ 92 Ma, according to a rapid cooling as revealed by the thermal history modeling of AFT length data.