Magnetotelluric Evidence for Asymmetric Simple Shear Extension and
Lithospheric Thinning in South China
Abstract
Extension and rifting of the lithosphere is fundamental to the evolution
of the continents, but the mechanism by which the lithosphere thins
remains enigmatic. Using new dense magnetotelluric array data collected
within the rifted margin and adjacent areas of Southeast China, we
resolve the three-dimensional electrical structure of the lithosphere to
constrain the process of rifting and thinning. Our measurements discover
a brittle-ductile transition zone featuring low electrical resistivity
and low seismic velocity in the Cathaysia Block. A southeast-directed
dip is resolved for the Jiangshan-Shaoxing Fault that documents the
Neoproterozoic suturing of the Yangtze and Cathaysia Blocks, and has
been reactivated by the Early Paleozoic and Early Mesozoic
intracontinental orogenies. It acted as a low-angle detachment fault
during the Mesozoic-Cenozoic extension and rifting. Given the
asymmetries of topography, electrical resistivity, Bouguer gravity
anomaly and Mesozoic volcanism across the Gan-Hang Rift, an asymmetric
simple shear extension model is proposed for the South China
Mesozoic-Cenozoic rift system. Water content of up to 0.1 wt% and melt
fraction of up to 1% are estimated at 70 km depth beneath the central
Wuyi Mountains, suggesting hydration of the mantle lithosphere. The
hydration weakening of the mantle lithosphere promoted both the
gravitational instability and convective removal of the lowermost
lithosphere in South China.