关键词: 脆塑性转化带, 地壳强度, 断层滑动稳定性, 定量研究

Abstract:

The strength properties of fault rocks at shearing rates spanning the transition from crystal-plastic flow to frictional slip play a central role in determining the distribution of crustal stress, strain, and seismicity in a tectonically active region. Since the end of the 20^th century, many experimental and modelling works have been conducted to elucidate the variation of the strength profile and mechanism of brittle-ductile transition(BDT)with temperature, pressure, and sliding rate. We review the substantial progress made in understanding the physical mechanisms involved in lithospheric deformation and refining constitutive equations that describe these processes. The main conclusions obtained from this study are as follows:

(1)The mechanical data and microstructure of friction and creep experiments indicated the transition from brittle to plastic deformation with the increasing crust depth, which not only controls the ultimate strength of the crustal profile but also limits the lower limit of the seismogenic zone. Moreover, based on the variation of rock characteristics, temperature, normal stress and sliding rate, the brittle-ductile transition zone distributes at different depths in the crust. The strength profile consisting of friction law and flow law is widely used to describe the strength and seismicity of the continental crust. However, this profile model is oversimplified in the BDT zone because this area involves a broad region of semi-brittle behavior in which cataclastic and ductile processes occur. At the same time, the model also lacks characterization of the transient dynamic properties of faults. Rate-and-state friction(RSF)law stipulates that the occurrence of slip instabilities(i.e. earthquake)can be linked with the velocity dependence of friction. Therefore, the RSF equations, when applied to the kilometer-scale of fault zones, models incorporation RSF equations can reproduce several important seismological observations, including earthquake nucleation and rupture, earthquake afterslip, and aftershock duration. However, these key microphysical processes of fault gouge evolution are unknown to this model.

(2)During numerical model-fitting experimental observations, the Friction-to-flow constitutive law merges crustal strength profiles of the lithosphere and rate dependency fault models used for earthquake modelling on a unified basis, which is better than controlling the boundary of BDT using the Mohr-Coulomb criterion, Von Mises criterion and Goetze’s criterion. The Friction-to-flow constitutive law can predict the steady-state and transient behavior of the fault, including the response of shear stress, sliding rate, normal stress, and temperature, in addition to simulating the transition of fault sliding stability from velocity-weakening to velocity-strengthening. It also solved seismic cycles of a fault across the lithosphere with the law using a 2-D spectral boundary integral equation method, revealing dynamic rupture extending into the aseismic zone and rich evolution of interseismic creep, including slow slip before earthquakes. However, these constitutive models do not base on microphysical behavior. Furthermore, at low to intermediate temperatures, the ductile rheology of most crystalline materials are different from those at high temperatures.

(3)A recent microphysical model, which treats fault rock deformation as controlled by competition between rate-sensitive(diffusional or crystal-plastic)deformation of individual grains and rate-insensitive sliding interactions between grains(granular flow), predicts both transitions well, called the CNS model. Unlike the numerical model, this model quantitatively reproduces a wide range of(transition)frictional behaviors using input parameters with direct physical meaning, which is closer to the natural strength of the fault. This mechanism-based model can reproduce RSF-like behavior in microstructurally verifiable processes and state variables. However, the major challenge in the CNS model lies in capturing the dynamics of micro- and nanostructure formation in sheared fault rock and considering the different processes of rock deformation mechanisms.

Since it is microphysically based, we believe the modelling approach can provide an improved framework for extrapolating friction data to natural conditions.

Key words: brittle-ductile transition, crustal strength, slip stability of fault, quantitative analysis