The Nature And Tectonic Transition Of The Multiphase Rifting In The Northern Margin Of The South China Sea

Multiphase rifting is the typical characteristic in the process of evolution of rifts all over the world.The South China Sea(SCS),one of the biggest marginal sea in the southeast edge of the Eurasia Plate,experience the whole process from intra-continental rifting to breakup.The Pearl River Mouth Basin is a typical rifted basin in the northern South China Sea,experiencing two-phase rifting in the early-middle Eocence and Late Eocene-Oligoecne respectively.More precisely,the first rifting phase can be devided into two stages of rifting: phase Ia and Ib.Our research aims at dealing with some important problems such as the difference between rifting structure,tectonic evolution and as the significance of tectonic transition.This paper can provide reference for global rifted basin evolution by revealing the structure,tectonic characteristics and their difference during multi-phase rifting and uncovering two kinds of tectonic transition.By means of making fine structure characterization of 30 sub-sags,we summarized 4 kinds of structural evolution styles.Our research shows that low-angle boundary faults exhibit a first-order control on those sub-sags striking NE-NEE during the rift phase Ia,however,sub-sags striking EW-NWW are mainly controlled by high-angle boundary faults.At that time,the angle of fault rotation is middle-high and the structure of sags is wedge-shaped half-graben primarily.During the rift phase Ib,the angle of the boundary faults still shows the polarization character.The fault blocks are characterized by high spin intensity,which are weaker than that in the rift phase Ia.Meanwhile,wedge-shaped sub-graben is still the main structural style of the sub-sags.During the whole rift phase I,there are four kinds of structural evolution styles: inheritance,evolution,eruption and neonatal style.During the second phase,sub-sags are mainly controlled by high-angle faults,showing sheet structure and the fault rotation is characterized by slight intensityWe have summarized two basic rift structure,namely wedge-shaped half-graben and sheet-shaped half-graben.The former one always shows that the steep margin of sags is strictly controlled by boundary fault,which result that the thickness of strata is biggest near the fault,thinning towards gentle slope belt.Meanwhile,the degree of fault rotation is rather high and the proportion of lake is a little small.This kind of structure is the representative structure style during rift phase I.The second rift structure style is characterized by that the strata expand cross faults in spite that boundary faults control the margin of sags in early time.The whole strata are isopachous,overlapping towards the distant margin,forming a wedge-sheet complicated shape.At that time,the proportion of sags is rather big.This kind of structure is the representative structure style during rift phase II.We have summarized the tectonic evolution and diversity of boundary fault during multiple phase rifting on the basis of systematic analysis of geometry and kinematics of the NE-NEE striking fault,NW-NWW striking fault and complicated fault system.Our research shows that during rift phase Ia and Ib,the first fault system shows multiple wave peak in D-L map,revealing the activation in segment,which means several depocenters form along fault strike.The second EW-NWW fault system shows single wave peak in D-L map,forming single depocenters along fault strike.The last complicated fault system shows multiple wave peak in the D-L map.However,the biggest displacement locates at the corner of the fault.During rift phase II,these three faults system share a common characteristic,showing simple single wave peak in D-L figure.By means of contrastive analysis of the sag's architecture and the evolution of the controlling faults,we reveal that there are two tectonic transition in the Zhu I Depression during the evolution of the multiphase rifting: the tectonic transition happened between the rift phase Ia and Ia and the tectonic transition between the rift phase I and phase II.The first one is characterized by the transition from the south to the north and the migration zlong the trend of the boundary fault.The second one is characterized by the transition from the NE-NEE system to the EW-NWW system,the sustaining transition from the south to the north and the architecture transition.