|
Geometric Analyses |
Robertson Research International Ltd, Tyn-y-coed Site, Llanrhos, Llandudno LL30 1SA, UK
Although veins and joints are both extension fractures, they should be treated separately, especially in field analyses. For example, they commonly form at different times and under different conditions. Excellent exposures of the Liassic limestones at Kilve, Somerset, UK, are used as a case study. These rocks show calcite veins related to the Mesozoic extension and Alpine contraction of the Bristol Channel Basin, and joints that appear to have been caused by late- or post-Alpine stress release. The veins at Kilve show the following characteristics that distinguish them from joints. (1) They are mineralized. (2) Veins have measurable widths (up to tens of millimetres wide) and represent measurable strain. En echelon veins are common at Kilve, and indicate shear strains. (3) Vein widths may obey a power-law scaling relationship. (4) The veins were open for long enough and under the correct conditions for calcite to be deposited, indicating high fluid pressures. They are also typically en echelon. (5) Veins form sets with a narrow range of strikes, whereas the joints typically form complex networks. (6) The veins at Kilve are commonly clustered around faults, and are precursors to, or synchronous with, the faults.
