Abstract: Tectonic Analysis of Weakly Deformed Foreland Areas

S.E. Laubach, Roger Tyler, D. D. Schultz-Ela, C. M. Tremain

In slightly structured foreland areas adjacent to orogenic belts, the most reliable indicators of tectonic patterns commonly are fractures. Fracture orientation and style are related to far-field stresses generated during plate motion, and subsidence, uplift, and inversion of basins. Our ongoing study of fracture patterns in coal and associated sandstones in the Rocky Mountain foreland area shows that regional structural setting is a principal control on the style, dominant orientation, and overall architecture of coal fracture network patterns. Such patterns are useful for testing models for the kinematic evolution of adjacent orogenic areas and for tracking deformation patterns in the foreland. Our results suggest that the oldest opening mode fractures preserved in Cretaceous and Te tiary coals typically strike parallel to either the foreland basin axis or are parallel to overall shortening directions of adjacent orogene segments; the difference may be due to the timing of changes in coal properties during coalification relative to episodes of tectonic loading. As a consequence, fracture strikes may shift abruptly with depth. Pre-existing structures, especially those trending at moderate angles to tectonic shortening directions, can also modify fracture strike. Fracture patterns in sandstones adjacent to coal have significant similarities and differences to those in coal. Fracture patterns are illustrated by maps of fracture strike and style for the foreland area between Canada and Mexico and by finite element modeling of stress trajectories in this area. Fracture-p ttern information is useful for coal completion strategy design (e.g. cavitation vs. hydraulic fracture), and in sandstone, for formulating horizontal drilling strategy.

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