The Onset Timing of Deep Mantle Upwelling Beneath the Northwestern South
China Sea Margin
Abstract
Massive eruptions of OIB-type volcanism in the Leizhou-Hainan area,
Indochina and South China Sea oceanic basin around the northwestern
South China Sea margin indicate occurrence of strong deep hot mantle
upwelling. However, when it started and how it influenced the
northwestern South China margin is still unclear. The mantle upwelling
not only caused rising of lithospheric temperature, but also produced
basement dynamic uplift. Thus, we conduct detailed analysis of the
Cenozoic time-varying residual subsidence by subtracting the predicted
subsidence from the backstripped subsidence along a new seismic
reflection line in the Qiongdongnan Basin in the northwestern South
China Sea margin to study the dynamic uplift and deep mantle upwelling.
For the first time, we give a method to calculate the
subsidence-independent, time-varying strain rates constrained by the
varying faults growth rates. Then we forward predict the basement
subsidence with a basin- and lithosphere-scale coupled finite extension
model, and accurately recover the backstripped subsidence with a
modified technique of backstripping to eliminate the effects of later
episodes of rifting on earlier sediment thickness. Results show no
residual subsidence in 45-28.4 Ma. But after 28.4 Ma, negative residual
subsidence occurred, reached and remained ca. -1000 m during 23-11.6 Ma,
and reduced dramatically after 11.6 Ma. In the syn-rift period (45-23
Ma), the residual subsidence is ca. -1000 m indicating a significant
subsidence deficit, however in the post-rift period (23-0 Ma), it is
positive of ca. 300 to 1300 m increasing southeastwards denoting
considerable subsidence excess. These results suggest that the syn-rift
subsidence deficit commenced at 28.4 Ma, while the post-rift excess
subsidence occurred after 11.6 Ma. Combined with previous studies, it is
shown that the subsidence anomalies cannot be explained only by
lithospheric deformation, such as depth-dependent lithospheric
stretching, post-rifting crustal thinning, lower crust flow and magmatic
intrusion. We infer that the opposite residual subsidence in the syn-
and post-rift periods with similar large wavelengths (>102
km) and km-scale amplitudes are results of transient dynamic topography
induced by deep mantle upwelling beneath the central Qiongdongnan Basin,
which started to influence the margin at ca. 28.4 Ma, continued into the
Middle Miocene, and decayed at ca. 11.6 Ma. The initial mantle upwelling
had precipitated considerable continental extension and faulting in the
Late Oligocene (28.4-23 Ma), however, prohibited the syn-rift basement
subsidence. After ca. 11.6 Ma, vanish of mantle upwelling in the
Qiongdongnan Basin yielded rapid post-rift subsidence, and meanwhile,
the strong mantle upwelling probably migrated beneath the Leizhou-Hainan
area to form huge basaltic lava flow.