Strain Analysis of Elliptical Grains from a Fold and Thrust Belt, Kavik River Area, Northeastern Alaska

P. R. Delaney, A. M. Loveland, J. G. Clough, and M. A. Wartes
Alaska Division of Geological & Geophysical Surveys, Fairbanks, AK

Strain analysis of elliptical markers provides an estimate of the finite strain ellipse shape. Subsequent strain integration parallel to principle transport direction provides an estimate of shortening due to penetrative strain. Ongoing geologic studies in the petroleum province of northern Alaska aim to characterize and quantify shortening in the Brooks Range fold and thrust belt. Balanced cross-sections are commonly used to estimate shortening, but this method ignores penetrative strain. Strain markers from the Mississippian-Pennsylvanian Lisburne Group and the overlying Permian Echooka Formation were analyzed by the Rf/φ method to quantify the penetrative component of shortening in these units. Samples with appropriate random and anticlustered grain distributions were also evaluated using the center-to-center Fry technique. Thin sections from oriented field samples included ooids, calcispheres, and crinoid columnals from the Lisburne and quartz sand grains from the Echooka Formation. Results for three orthogonal sections from each sample were incorporated to determine the local three-dimensional strain ellipsoid shape and orientation. A symmetry test assessed the assumption of original symmetry of grain distributions and identified samples likely to have had pre-strain sedimentary or tectonic fabrics. Statistical confidence limits on ellipticities of strain ellipses are given. Integration of strain values across local structures provides an estimate of penetrative shortening that can be added to values from balanced cross sections, providing a closer approximation of total shortening.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas