An Experimental Study On Strength And Deformation Mechanisms Of The Brittle-Plastic Transition Zone

Constraints provided by field observation, experiments in laboratory and seismicdata have led to a general consensus that shallow crust rocks deform by brittle faulting,while lower crust rocks deform by crystal plastic flow. These constraints provide thebasis for the two-mechanism model for the rheology of crust and lithosphere in whichthe strength of the upper brittle crust is limited by Byerlee’s law, while the strength ofthe lower plastic crust is limited by power law creep. The maximum depth ofmicroseismic activity is controlled by the broad zone of brittle-plastic transition thatlies between the two extremes brittle and plastic. While the two-mechanism model isso simple that overestimating the strength of rocks near the brittle-plastic transitionzone. Although many studies about the deformation mechanism of the brittle-plastictransition zone have been made, a ‘flow law’ representation which can describe itsstrength for the brittle-plastic transition zone has not been formulated. In addition,research on brittle-plastic transition usually focused on temperature effects, while theaspects of strain rate and fluid are relatively weak.The brittle-plastic transition of some faults occurs at the same depth（temperatureand pressure） from the evidence of deformation mechanisms of minerals in faults, andthis phenomenon which has been considered to be relevant to co-seismic loading andpostseismic creep in earthquake cycles and confirmed by distribution of focal depth isdue to the strain rate. The presence of high-pressure fluids in active faults at depth isproved by various characteristics of fault fluid, and the fluids which can evolve inpressure pertaining to fracturing and sealing processes play a key role during theseismic cycle. The formation of high-pressure fluids （cracks sealing） has severalmechanisms, but researches show pressure solution creep is one of the mainmechanisms which control crack sealing kinetics around active faults. Studies onpressure solution under the action of water can supplement and correct crustalstrength profile defined by traditional relations describing brittle/frictional behavior（Byerlee’s law） and dislocation creep, the strength and the deformation mechanismsof the brittle-plastic transition zone. As a consequence, it is necessary to further study the effects of strain rate and fluid on the brittle-plastic transition through highpressure-high temperature experiments.Samples of Carrara marble（60～100μm of initial grain size） were axiallycompressed at temperature ranging300℃～500℃, confining pressure600MPa～800MPa and strain rate1×10^-7/s～2.5×10^-6/s, with or without addition of water.Experiments of six groups（12times） on the Carrara marble were conducted, combinedwith the polarizing microscope and scanning electron microscopy andenergy-dispersive X-ray spectroscopy（EDAX） analysis, to explore pressure solutioncharacteristics of the Carrara marble and the influence of water, temperature, strainrate and confining pressure on rock strength and the main deformation mechanism ofthe brittle-plastic transition zone. Some conclusions can be drawn from theseexperiments:（1） The strength of dry or wet samples would reach the highest values of692±39MPa or639±25MPa under the conditions of300℃of temperature,600MPaconfining pressure, and5×10^-7/s of strain rate. The strength of Carrara marble wouldreduce with increasing temperature significantly. Water had little effect on the samplestrength, and the weakening degree could be smaller with increasing temperature.Confining pressure almost had no effect on it. The effect of strain rate on strength wasmore complex which could be related to the transition of deformation mechanism.（2） The main deformation mechanisms of Carrara marble include pressuresolution creep at the low temperature（300℃） and low strain rate(1×10^-7/s), pressuresolution+catacastic flow at low temperature（300℃） and middle strain rate(5×10^-7/s),cataclastic flow at the low temperature（300℃） and high strain rate(2.5×10^-6/s), andplastic deformation at the high temperature（500℃） and middle strain rate(5×10^-7/s).The transition of deformation mechanism under the experimental conditions wasmainly controlled by temperature and strain rate. However, at the sametemperature（400℃） and strain rate(5×10^-7/s), cataclastic flow+pressure solutiontranslated into plastic deformation with confining pressure from600MPa increasing to800MPa, which implies that increase of confining pressure could restrain cataclastic flow and pressure solution. In addition, at the lower-middle temperature （300℃、400℃）and lower-middle strain rate(1×10^-7/s、5×10^-7/s), the phenomenon of pressuresolution in wet samples was more significant than in dried samples. So the occurrenceof pressure solution can be promoted by water.（3） The seams and impurity diffusion were performances of pressure solution inCarrara marble at the low temperature and low strain rate in the experimental study.However, the microstructures of the field limestone showed that the seams wereperformances of pressure solution. The traces of dolomite and phlogopite in Carraramarble led to composition diffusion during pressure solution process which waspromoted by water and impurity （such as Mg, K）.The occurrence conditions and stress exponent of pressure solution in Carraramarble from this study was similar to previous results. Pressure solution could be oneof the main deformation mechanisms of the field faults which sliped by the low strainrate and had water in the brittle-plastic transition zone.