The Relationship Between Fracturing, Asymmetric Folding, and Normal Faulting in Lisburne Group Carbonates: West Porcupine Lake Valley, Northeastern Brooks Range, Alaska

By

J.R. Shackleton, C.L. Hanks, and W.K. Wallace (University of Alaska, Fairbanks)

The relationship between fracturing and asymmetric folding in carbonates in the western part of Porcupine Lake valley of the northeastern Brooks Range can be used to develop a predictive model for fracture density and distribution in asymmetric folds. Lisburne Group carbonates in this part of the northeastern Brooks Range are folded into strongly asymmetric NE striking and plunging folds characterized by short, steep to overturned forelimbs, and long (up to 1 km) gently dipping backlimbs. It is unclear whether these folds are detachment folds or fault propagation folds, although detailed geometric analysis may favor one kinematic model over another. Some of the folds are cut by NE- and NW-striking normal faults with relatively small displacements. Associated fractures consist of two major sets: a pervasive steeply dipping N-S striking set that is approximately perpendicular to fold axes, and a NE striking set that is approximately parallel to fold axes. Both sets have associated conjugate fractures, some of which have a component of shear. Other major mesoscopic-scale structures indicate some period of penetrative semi-ductile deformation, and include dissolution cleavage, deformed crinoid stems, sheared stylolites, and elongated and transposed chert nodules. Future work will be aimed at understanding the relationship between fracturing, faulting and folding in west Porcupine Lake valley. Important questions to be addressed are: did folds form as detachment folds or fault propagation folds, and how does each of these fold models influence fracturing? In addition, can we use fracture distribution to understand the kinematics of fold formation?

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.