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The AAPG/Datapages Combined Publications Database

Alaska Geological Society

Abstract


Alaska Geological Society 2001 Geology Symposium, 2001
Page 36

The relationship between fracturing, asymmetric folding, and normal faulting in Lisburne Group carbonates: West Porcupine Lake Valley, northeastern Brooks Range, Alaska - Abstract

J. R. Shackleton,1 C. L. Hanks,2 W. K. Wallace3

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 (NEBR) 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 in Porcupine Lake Valley are cut by NE and NW striking normal faults with relatively small displacements. Fractures associated with folding and faulting 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.

Normal faulting in West Porcupine Lake Valley is atypical for the NEBR, and may have influenced fracture character and distribution. Cross cutting relationships suggest that NE striking faults occurred after thrusting, whereas folds truncated by hinge sub-parallel normal faults suggest that normal faulting may have occurred during folding, or may have significantly modified fold geometries after a previous phase of compressional deformation. Changes in fold geometry were observed across NW striking normal faults, suggesting that either the normal faulting modified fold geometries, or that these faults originated as transverse structures during folding and were reactivated as normal faults.

Future work will be aimed at understanding the relationship between fracturing, faulting and folding in West Porcupine Lake Valley. Some important questions to be addressed are: did folds in the field are form as detachment folds or fault propagation folds, and how does each of these fold models influence fracturing? Conversely, can we use fracture distribution to understand the kinematics of fold formation? Another important question is how normal faulting has affected fracturing and folding in the area, and whether or not fractures related to folding can be distinguished from those related to faulting. In order to answer the previous questions, it will be important to understand how lithology and bed thickness affect fracturing, since changes in these two variables affect fracture spacing within the stratigraphy.

Acknowledgments and Associated Footnotes

1 J. R. Shackleton: Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK

2 C. L. Hanks: Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK

3 W. K. Wallace: Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK

Copyright © 2014 by the Alaska Geological Society