Numerical Simulations On The Tectonic Deformation Of Collisional Orogens And Related Seismicity

Since Cenozoic era,the rapid and continuous northward migration of the rigid Indian plate has lead to the rapid uplift of the giant Tibetan Plateau,and has formed a number of marginal active fold-and-thrust belts.The formation and evolution are closely related with the uplift mechanism of marginal plateau and the occurrence of seismic harzards.Hence,we choose two typical areas:the deformation front of the Tibetan Plateau-the Nepal Himalaya,and a classical intracontinental orogenic belt of the Tibetan Plateau-the Longmen Shan fold-and-thrust belt.By adopting sequential limit analysis method,for the former,we investigate the geological earthquake potential during the long-term evolution of the Nepal Himalaya.For the latter,we adopt limit analysis to analyze the mechanical status of present-day Longmen Shan fold-and-thrust belt,and investigate the influence of optimal failure mechanism in association with fault geometry,rock strength and gravitional variations in layered models.Model results have greatly deepened insights in seismogenic environment and uplift mechanism of these two areas.The convergence rate of Nepal Himalaya,located in the middle of the arcuate shaped orogenic belt,is released by frequent large earthquakes along the megathrust,the Main Himalaya Thrust.Present-day seismic cycle behaviors along MHT have been clearly characterized.However,in a relative long time scale,especially in the mountain building period,how to systematically extract large earthquake statistical characteristics remains unsolved.The available seismic cycle behaviors suggest that the full activation of the decollement from seismogenic zone to the deformation front is a important factor for distinguishing large(M<8)or great(M>8)earthquakes,and the binary interseismic coupling pattern reveals that the rear area of seismogenic zone is fully creeping and aseismic.Hence,in this eontribution,we propose a mapping projection approach by using the loeations of the refined Cenozoic thrusting sequences with accuracy reaching to large earthquake cycle scale(500yr),to avoid the coupling computation between geological elastoplastic mountain building process and short-term visco-elasto-plastic seismic cycle estimation.Model results exhibit,after a 200-km growth of the initial wedge,apparent thrusting concentration phenomenon exists with four thrusting clusters entitled as thrusting families to facilitate the development of the sub-Himalaya,the Lesser Himalaya,the Higher Himalaya and the Tethyan Himalaya,respectively.Besides,two large periods of fault activities in the Higher and Tethyan Himalaya extending to 2-2.5 Myr are within 30-28 Ma and 15.8-13.35 Ma respectively.This is approximate with the timing of the emplacement of Higher Himalaya which belongs to creeping behaviors based on mapping projections and absorbs around 10%of total geological shortening.The rest thrusting clusters are located in the seismic stick-slip area.The active period of faults decreases rapidly from the rear to deformation toe with least period of 0.5 Ma.Shortening of each thrusting family is roughly equal with 30%of total shortening.Thrusting family located in the sub-Himalaya activates the whole decollement which belongs to the scope of great earthquake;whereas Lesser Himalaya contains two thrusting families and the latter thrusting family results in the duplex development in the Lesser Himalaya.Finally,this approach is applicable in the whole Himalayan wedge,due to the similar interseismic pattern and can also be promoted to the other continental collisional zones.Whereas within the oceanic subduction zones,because of the non-one-to-one correspondence between mountain building and effective convergence rate under strongly heterogeneous interseismic coupling pattern,basically,it is invalid to conduct mapping operations in oceanic subduction context.An out-of-sequence thrusting rear-wedge fault activation pattern is clearly identified by recent large earthquakes and the associated aftershocks with synchronous rupturing of listric-type splay faults,the Beichuan fault and Pengguan fault in southern Longmen Shan fold-and.thrust belt.To date,the comprehensive mechanical understanding of such fault failure mode in Longmen Shan fold-and-thrust belt and the adjacent Sichuan Basin remains unclear where it is critical for seismotectonies.The geometry of active 'flat-ramp-flat' style decollement and the preexisting faults within the structural wedge have beyond the scope of critical taper theory deduced by Dahlen which can not be used for mechanical analysis of the entire Longmen Shan and Sichuan Basin.In this thesis,we rely on maximum strength theorem,the kinematic approach of limit analysis,to systematically conduct stability analysis associated with fault geometry,rock strength and lateral material variations.Four basic failure modes,or collapse mechanisms by limit analysis are proposed to identify the component of the calculated fault activation pattern,including the activation of Wenchuan fault and Beichuan fault from Wenchuan earthquake,the rupture of Range Frontal Blind fault from Lushan earthquake and the activation of the upper decollement beneath Sichuan Basin via Range Frontal Blind fault.By comparative study through homogenous and layered models,the listric geometry of Beichuan fault and Pengguan fault,rather than rock strength or layered gravity difference,predominantly controls the present-day rear fault rupturing mechanism.The synchronous rupturing of Beichaun fault and Pengguan fault requires remarkable strength weakening on the Beichan fault which associated with dynamic rate-dependent weakening.Favorable deformation towards Sichuan Basin is jointly influenced by large back-ground ramp strength,large upper decollement strength,low density and low rock strength of the foreland molasse sedimentation since Mesozoic in the western Sichuan Basin.Due to the relative long-term stability of fault geometry,we propose that the eastward active convergence across the southern Longmen Shan could be mainly absorbed by recurring large earthquakes at the hinterland of the Longmen Shan,especially the Beichuan fault and Pengguan fault,where holds inactive seismicity in Sichuan Basin.By validating the topographic relief response of flat-ramp-flat style decollement,more deformation will be concentrated in the rear and central ramp decollement leading a great increase of topographic slope angle.The sharp topographic transition within the Longmen Shan range front and Nepal Higher Himalaya is probably resulted by the mechanical influence induced by flat-ramp-flat decollement.When combined with the recent deep seismic reflection profile,Cenozoic mountain building of Longmen Shan is followed as folding and thrusting in the upper crust which is consistent with the Cenozoic outward growth of marginal Tibetan Plateau by thrusting and folding.