Tectono-thermal Numerical Modelling Of Passive Continental Margins

The passive continental margins record the complete geological information of the continental lithosphere from breakup to seafloor spreading and the study of its tectonic evolution is an important frontier field of earth system science.Numerous numerical studies have been done concerning tectono-thermal evolution of conjugate margins.However,there is no clear understanding of the impact of the duration of syn-rift phases,which is mainly controlled by the extension rate,on the evolution and architecture of rifted margins.More importantly,there is a lack of research on the differential influence of surface processes on the fault system,basement uplift and subsidence,along with temperature and heat flow evolution at different locations of the passive continental margins.So,based on previous studies,we systematically studied the influence of the extension rate and the rheological structure of the lithosphere on the tectonic evolution of the passive continental margin,especially on the initial mode of rifting and the symmetry structure of the final continental margins with numerical modelling.And then we have models coupled with surface processes to study the influence of surface processes on the development of faults,and evolution of basement subsidence,temperature and heat flows on different locations along rifted margins.The extension rate and rheological structure of the lithosphere are key factors for the continental breakup and rifting.With tectono-thermal numerical modelling,we analyzed the influence of extension rate and lithosphere rheology on the initial rifting mode and final architecture of rifted margins.The results showed that,with different lower crustal rheology/thickness and the extension rate,the initial rifting mode,the timing of continental breakup and the final architecture of rifted margins varied dramatically.For models with a lower crustal thickness of 15 km,experiments with ultra-slow extension rates(2-5 mm/year)generated symmetric conjugate margins,while experiments with higher extension rates generated asymmetric conjugate margins.However,for models with a lower crustal thickness of 20 km,experiments with different extension rates generated asymmetric conjugate margins.Meanwhile,for models with different lower crustal thicknesses,the bigger extension rate,the earlier continental breakup happened.In experiments with ultra-slow extension rates,due to long syn-rifting processes companied with lithosphere cooling,the upper crust mainly developed faults and the crust-mantle coupled,and ultimately hyperextended crust was generated.Moreover,in experiments with ultra-slow(2-5 mm/year)and ultra-fast(50mm/year)extension rates,the conjugate margins are more symmetric than experiments with intermediate extension rates.The extension rate and lower crustal rheology controls the symmetry of conjugate margins.The basement subsidence,temperatures and heat flow evolution of passive continental margins are closely related to rift deformation.Our results show that,the time for the peak of the basement subsidence,temperatures and heat flows are the time for the end of the deformation.When the fault activity stops,the horizontal stresses stored on both sides of the fault are released,causing the flexural uplift of the basement.For models without sedimentation,the stronger the lower crust,the bigger the flexural uplift,while for models with surface processes the situation is different for asymmetric margins.Moreover,the timing and magnitude of the peak basement subsidence,temperatures and heat flow are related to the structural positions along the margin: the nearer to the rifting center,the bigger the magnitude of basement subsidence,temperatures,and heat flows.We induce this is due to the crustal thinning and upwelling asthenosphere.The mechanisms that surface processes influence tectono-thermal evolution of rifted margins are flexural balancing effect and thermal blanketing effect.We use models coupled with surface processes to study the influence of surface processes on fault development,basement subsidence,temperatures and heat flow evolution along rifted margins.Results show that,as the thickness of the overlying sediment pile increases,flexural rebound for basement points in the symmetric margins decrease or even vanish.We induce that this is because higher basement subsidence leads to a higher basement temperature,so the flexural strength of basement decreases and results in decrease of flexural rebound.While for asymmetric conjugate margins,flexural rebound for basement points located in the proximal domains decrease and in the distal domains increase.We infer that this difference is because of the uplift of distal domains,which is the result of the flexural balancing effect of sediment load on narrow and wide margins.Basement temperature is a function of overlying sediment thickness and lithospheric cooling time.The thicker the overlying sediment pile,the higher the basement temperature.And,the shorter the lithospheric cooling time and higher the basement temperature.This is because in this experiment the closer it is to the riftcenter,the basement is more affected by the upwelling heat from the asthenosphere.