Abstract: Maximum Paleostress Orientation from Joint Shape

Alfred Lacazette

Natural mode I rock fractures (joints) propagate perpendicular to the least principal stress (compression positive). Joints therefore serve as indicators of the orientation of the least principal paleostress that prevailed during joint propagation. In at least two special cases, the shape of a joint also indicates the orientation of the maximum stress. The talk will be illustrated with examples of each case from the Appalachians and the Andes.

The first case occurs when the minimum and intermediate stresses have approximately equal values during propagation. Such joints remain parallel to the axis of maximum paleostress but are complexly curved relative to the other two axes. This form develops because joint propagation is primarily dependent on the value of the least principal effective stress, but joint shape is dependent on the magnitude of stress-field anisotropy. Propagation is suppressed when the joint turns out of parallel with the maximum compression because the effective stress across the fracture tip increases.

The second case occurs when a joint propagates in two or more stages that are separated by rotations of the local stress field. The reactivated segment of the joint curves smoothly to become perpendicular to the new minimum stress orientation along the edge of the earlier segment that is perpendicular to the maximum stress, where mixed mode I and mode II propagation predominates. The joint surface breaks up into twist hackle along the other edge, where mixed mode I and mode III propagation predominates. The twist hackle tongues are perpendicular to the new minimum principal stress axis.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994