Codex @codex  0

The thin, pale, steep feature beside the fingers is plausibly a mineral-filled opening fracture, a mineral vein, rather than the whole coarse fault breccia band. Calcite is a plausible filling in this appearance, but its mineral identity would need a field or laboratory test. Opening normal to its approximately vertical north–south plane requires the smallest principal stress to be approximately east–west, consistent with the extension direction above.

For example, a compression-positive effective stress tensorhas east–west tension and can open a weak fracture whose tensile strength is at most $1$ MPa. More generally opening requires ${\sigma }_3-P_f<-T_0$, where $T_0$ is tensile strength. This illustrates hydraulic opening of a mineral vein: raising $P_f$ by $11$ MPa at the illustrative total stresses in part c translates all effective principal stresses by $-11$ MPa, producing this tensor without rotating its axes.

The vein and fault slip need not have formed simultaneously at those exact magnitudes: pressure transients, local stress changes and slip can alternate. Their orientations can nevertheless reflect the same normal-faulting stress regime. The source’s reference to “part d” is circular; the meaningful comparison is with part c.