Slip and stress in block-in-matrix shear zones: 2. numerical modelling of a serpentine-filled dilational jog

Abstract

We use 2-D numerical models to explore slip and stress dynamics in a fault-bound dilational jog, informed by a field example from the Dun Mountain Ophiolite in New Zealand. The jog is hosted in a metre-scale phacoid of massive serpentinite embedded in a “block-in-matrix” creeping serpentinite shear zone. The models show how periodic exceedance of the tensile strength of the contact between the sealed jog and host rock leads to episodic opening and deposition of a new crack-seal band, with a thickness limited by the release of stress around the tips of the stepover-bounding faults. Jog stress release in the model is lower than that predicted from linear elastic fracture mechanics because additional crack opening can occur slowly due to post-failure creep on the bounding faults. For 10 km overburden and constant high fluid pressure, we predict event stress release of ca. 16 MPa and total crack opening of ∼22 μm on either side of the jog, consistent with crack-seal band widths in the field example. Because our models show that total crack width reflects both initial cracking and subsequent creep, we suggest caution when using crack-seal band widths to directly infer stress release in similar shear zone settings.