Ductile Opening-Mode Fracture During Combustion Alteration Of Siliceous Mudstone.

Peter EICHHUBL, and Atilla AYDIN,
Rock Fracture Project, Dept. of Geological and Environmental Sciences, Stanford University, CA 94305.

Fracture propagation in rock is traditionally considered a brittle process of mechanical bond breakage that may be assisted by chemical fluid-mineral interaction. Bond breakage leads to the nucleation and coalescence of micro-flaws in a small process zone ahead of the fracture tip. Outside the process zone, any strain associated with fracture opening and propagation is considered elastic. A different behavior is observed in materials that exhibit significant plastic deformation before bond breakage such as ductile metals where fractures propagate by nucleation, growth, and coalescence of voids ahead of the fracture tip, typically preceded by significant tip blunting.

Opening-mode fractures with blunt fracture tips are characteristic of clinker that formed by combustion alteration of siliceous mudstone. These fractures are inferred to result from growth and coalescence of pores that are inherited from the diatomaceous protolith. Pores grow preferentially in an en-echelon arrangement and coalesce to elongate pores and blunt-tipped fractures by thinning and rupture of bridges between pores. Coalescence of en-echelon pores causes fracture propagation in a zig-zag path that is considered characteristic of plastic flow. Less pronounced pore growth and coalescence along the sides of fractures lead to a concurrent increase in fracture aperture. This process of fracture formation by void growth and coalescence is inferred to result from diffusive mass transfer and possible bulk melt movement during partial melting of clinker allowing for significant non-elastic deformation in the surrounding host rock concurrent with fracturing. The preferred elongation of coalescing pores and local rupture of pore bridges is explained by a tensile sintering stress due to the thermodynamic tendency of the system for energy minimization of solid and liquid surfaces. It is suggested that ductile fracture processes may be significant in metamorphic and magmatic systems as well as in reactive diagenetic environments where chemical mass transfer provides a mechanism for significant non-elastic deformation in the surrounding host rock concurrent with fracture opening.