Formation of joints in cooling plutons

The geometry, age, and mineralogy of steeply dipping joints in the Lake Edison Granodiorite of California indicate that thermal stresses played a key role in the formation of the joints. Joint traces curve to approach the pluton boundary at high angles at both large and small scales, and they generally terminate at or near the contact with an older pluton. Radiometric dates and the epidote and chlorite fillings of the joints tie jointing to the initial cooling of the pluton. A thermo-mechanical stress analysis assuming two-dimensional conductive cooling predicts thermal stresses of several tens of MPa, the same order of magnitude as plausible fluid pressures, regional stresses, and lateral normal stresses associated with the overburden. The orientation of the joints can be accounted for rather well by the stress field formed by superposing a uniform regional stress field on the predicted thermal stresses. The large-scale pattern of the early joints in many plutons should be predictable based on a plutonʹs geometry, its age relative to the adjacent rock, and knowledge of the regional stress at the time of initial cooling. These findings bear on issues pertinent to mining, petroleum recovery, nuclear waste repository siting, and ground water flow.