Effect of quartz on antigorite gouges: friction decrease by talc formation or pressure solution.

Abstract

Normally conditions in the forearc mantle wedge would cause stick-slip behaviour. However, the forearc mantlewedge shows aseismic behaviour. Sections of the San Andreas fault also showaseismic behaviour. Previous work suggested that serpentine was the cause of this aseismic behaviour, in both forearc mantle wedges and the San Andreas fault. However, serpentine normally experiences temperature strengthening and causes stick-slip in laboratory experiments, which means that other weakening mechanisms are causing the aseismic behaviour.D E Moore et al., 2013 showed that when serpentine is sheared against quartz it experiences decreasing friction with increasing temperature. One explanation for this decrease could be the formation of talc due to its low friction in comparison to serpentine. A reaction of serpentine with silica rich sources will form talc which is stable over a wide range of temperature–pressure conditions and has a very low friction coefficient. A second explanation is the activation of pressure solution which is activated by the presence of quartz. Up till now, research on this topic has only been done up to temperatures of 350°C and few experiments have been conducted with quartz incorporated in the antigorite gouge. In this research I show the effect of different weight percentages of quartz on the friction of antigorite at temperatures up to 500°C. Experiments were conducted with the rotary shear aparatus at different velocities. The experiments may give an insight in the frictional strength, expressed using the rate and state parameters (a-b). Microstructural data of the experiments can give an insight in the active deformation mechanisms in antigorite mixed with quartz during shearing. Experiments conducted with antigorite containing quartz, show a significant decrease in friction and positive (a-b) values which suggests that shearing is aseismic. Results from the microprobe, EDX and FTIR prove the formation of talc during the experiments at temperatures of 500°C and the presence of pressure-solution by pressure shadows and the dependence on grain size. In nature, these two processes could be the cause of the aseismic behaviour within forearc mantle wedges and the San Andreas fault.