The Characteristic Of Crustal Weak Layer: Experimental Study And Numerical Simulation

Crustal weak layer is of great sifnificance for understanding China's present-day lithospheric structure,intra-plate orogenic dynamics,as well as big Earthquake's breeding background.In this thesis,high temperature(T) and high pressure(P) experiment,analogy emperiment and numerical simulation are carried out to explore basic physical properties and kinetic characteristics of crustal weak layer,and the results are as the followings:A series of elastic wave velocity(V_p) measurements under high T and high P are carried out on two-mica gneiss samples which are from high Himalayan crystalline rocks of Nyalam group of Tibetan Plateau.At low T section(＜600°C),V_p has linear relationship with P and T,while at high T section,V_p vs.T/P shows nonlinear charateristics.Rapid V_p decline caused by softenedα-quartz and muscovite dehydration(melting) revealed in our experiment is indicative of understanding the "reversal V_p structure" that has been observed within upper-mid crust at some high-T/P or thickened acidic crustal areas.This low-V_p characteristic is in sharp contrast with the V_p increase caused byα-βtransition of quartz,which is of great value for geophysical exploration. Additionally,in-situ measurement of V_p vs.depth along practical P-T gradient provide experimental evidence of the "reversal V_p structure",as well as "rapid V_p increase nearα-βtransition of quartz",which also provide direct evidence for explaining corresponding geophysical data.The Tibetan Plateau has a strong deep cruastal anisotropy and Lattice-preferred -orientation (LPO) of rock-forming minerals is not sufficient to explain the observed intensity.Our calculation shows that little amount of oriented-melt can contribute significantly to rock's anisotropy,and anisotropy induced by melt-preferred-orientation(MPO) is several times greater than that induced by LPO.Therefore,if partial melting does happen within crust,the contribution of MPO should be fully considered to account for crustal anisotropy.Experiment on anology material also shows that oriented distribution of melt can significantly enhance medium's anisotropy,and even with little amount of melt,the anisotropy can reach 1 to 2 times of the original state.Considering Qinghai-Tibet Plateau is propably undergoing low-degreed partial melting within upper-mid crust, MPO is a preferable reason to explain the observed anomalous anisotropy in the region.Crustal weak layer with soften mechanism can easyly develop into a detachment zone and harmonize the discrepant motion between the rigid shallow crust(block) and the lower crust (lithospheric mantle),thus making intra-decoupling possible under continental lithosphere.Here 3-D finite element method is applied to simulate crustal weak layer's influence on crustal motion/deformation,and to discuss factors that affects its decoupling effect.The results reveal that dynamical mode and mechanic strength of the weak layer is the decisive factor.The weaker the strength of this layer,the bigger the discrepancy of motion between upper and lower crust.Among the dynamical models proposed on the east margin of Tibetan plateau,the model that lower crust (lithospheric mantle) dragging the upper crust avail decoupling happening.This model is supported by a series of results gained from geophysical exploration.For Bayankala block and its surrounding areas,the existence of crustal weak layer and the decoupling caused by it has an important promotion effect to the uplifting and stress accumulation of Longmenshan area.The dragging force acting beneath the rigid block can preferably explain the stress state of two sub-block(Aba and Long menshan block),and the results of fault-across slipping rate and vertical deformation are also consistent with field observation or geological cognition.So,considering a big Earthquake's breeding background(like Wenchuan Earthquke),a decoupled crustal weak layer that directly drag beneath a rigid block is another important factor.