Strain and local heterogeneity in the forsterite–fayalite solid solution

Infrared powder-absorption spectra of nine natural and five synthetic olivine samples across the forsterite–fayalite join have been investigated at room temperature in the range 70–1400 cm^–1. Variations of peak positions as a function of Fe content are close to linear for those vibrational bands whose trend could be followed across the solid solution. Line-broadening has been quantified by autocorrelation analysis. Positive deviations from linearity of the line-broadening parameter, _corr, for groups of bands at low energies are consistent with the existence of local elastic strain heterogeneities at intermediate compositions in the solid solution. It also appears that the structure of forsterite is more homogeneous than Fe-rich olivines in relation to local elastic strain effects. Positive deviations from linearity of the line-broadening parameter for the low-energy regions scale linearly with calorimetric data for the enthalpy of mixing. This close correlation between line-broadening in IR spectra and calorimetric enthalpies of mixing has now been observed for four different binary solid solutions, and there is a further, qualitative correlation with bulk modulus.     

Free-surface formulation of mantle convection—II. Implication for subduction-zone observables

Viscous and viscoelastic models for a subduction zone with a faulted lithosphere and internal buoyancy can self-consistently and simultaneously predict long-wavelength geoid highs over slabs, short-wavelength gravity lows over trenches, trench-forebulge morphology, and explain the high apparent strength of oceanic lithosphere in trench environments. The models use two different free-surface formulations of buoyancy-driven flows (see, for example, Part I): Lagrangian viscoelastic and pseudo-free-surface viscous formulations. The lower mantle must be stronger than the upper in order to obtain geoid highs at long wavelengths. Trenches are a simple consequence of the negative buoyancy of slabs and a large thrust fault, decoupling the overriding from underthrusting plates. The lower oceanic lithosphere must have a viscosity of less than to²⁴ Pa s in order to be consistent with the flexural wavelength of forebulges. Forebulges are dynamically maintained by viscous flow in the lower lithosphere and mantle, and give rise to apparently stiffer oceanic lithosphere at trenches. With purely viscous models using a pseudo-free-surface formulation, we find that viscous relaxation of oceanic lithosphere, in the presence of rapid trench rollback, leads to wider and shallower back-arc basins when compared to cases without viscous relaxation. Moreover, in agreement with earlier studies, the stresses necessary to generate forebulges are small (∼ 100 bars) compared to the unrealistically high stresses needed in classic thin elastic plate models.     

Weak zone formation for initiating subduction from thermo-mechanical feedback of low-temperature plasticity

We have addressed the problem of subduction initiation with a solid-mechanical and fluid-dynamical approach, using a finite-element method. The setup has been extended by a rate-sensitive coupling at the bottom of a semi-brittle lithosphere and a fully coupled thermo-mechanical model. The central element of our model is a broad asymmetric sedimentary loading function at the passive continental margin, which grows with time to 15 km. Two fundamentally different modes of shear zone formation have been found depending on the rheology of the creep layer. Mode 1: For cases of low or absent yield stress in the creep layer only, the semi-brittle top develops a weak zone, while the rate-sensitive layer acts as a decoupling shear zone. Mode 2: For cases with a yield strength in the creep layer (strain rates above 10⁻¹⁵ s⁻¹ after yielding), the entire mechanical lithosphere fails on a major shear zone; mode 1 fails to model subduction initiation, while mode 2 creates a weak, major shear zone that severs through the entire lithosphere.     

Experimental analysis of ductile-slip rheology in shallow structural level faults

The authors select different grain size samples from granitic body in Weishan, central Hunan, to conduct mylonitization experiments under high temperature (HT) and high pressure (HP), by which the temperature and pressure conditions of mylonitization and microstructures of deformation have been obtained. Through the transmission electron microscopy (TEM) observations of experimental mylonite, we calculate its dislocation density (D), differential rheological stress (Pd) and speed rate of strain (Rs): 3.20x10⁹/cm², 139.32 MPa and 6.39x10^-10/s respectively. The results are in the same magnitude-order with those of natural mylonite from fault zones. By comparison and analysis of chemical compositions and microstructures of different structural environments, the authors establish approximate rheological parameters related to shallow structural level and also suggest the multiple rheological properties and total trend from deep structural level to shallow structural level.     

Study on the rheology of subducting slabs

We calculate thermal and phase structures of subducting slabs for different subducting velocities by a modified coupling code of the kinetic phase-transformation equations and the heat-diffusion equation with latent-heat release. Whereafter, we estimate their rheology structures based on the thermal and phase structures from the mineral physical point of view. At shallow depth, the upper layer has a high effective viscosity greater than 10³⁴Pa · s; while the lower layer has a relatively low effective viscosity, which is greater than 10²⁶Pa · s nevertheless. The effective viscosities below the kinetic phase boundary of olivine to wadsleyite decrease obviously, and reach a minimum of 10²²Pa · s. Small areas with higher effective viscosities exist above the depth of about 700 km in subducting slabs, which are produced by lower temperatures that are related with endothermic phase transformation of spinel to perovskite and magnesiowustite. The 1% and 99% isograds of spinel proportion delineate tortuous belts with low effective viscosities, which would affect the geodynamic behavior of subducting slabs.     

碧溪岭超高压石榴橄榄岩的流变学研究

从组构、位错、滑移系及古应力计算等方面综合研究了大别山超高压石榴榄榄岩的流变学特征（１）对橄榄岩中单斜辉石和斜方辉石的且构分析表明,碧溪岭地区超基岩经历了塑性变形,形成了ＬＰＯ组构：碧溪岭超基性岩的变形特征既淡类似于蛇绿岩,也不完全等同于幔源包体,它有十分独特遥变形的图象。（２）利用费氏台可观测到的滑移系有如下两种：「００１」（１００）为中高温滑移系,「０１０」（１００）为低温滑移系,低温滑移系和