USING THE SPATIAL ANALYSIS OF A STRUCTURE CONTOUR MAP TO COMPARE FOLD HINGE AND FAULT ORIENTATIONS WITHIN THE HARPETH RIVER FAULT ZONE, A CRATONIC FAULT AND FOLD ZONE IN CENTRAL TENNESSEE

The investigator used a digital structure contour map to estimate the orientation of macroscale fold hinges within the Harpeth River fault zone (HRFZ), a largely aseismic cratonic fault and fold zone on the northwest flank of the Nashville dome, central Tennessee. The structure contour map depicts the elevation of the top of the Ordovician Carters formation. Most strata on the northwest flank of the dome dip northwest, but HRFZ strata have dip azimuths of 22.5^o-157.5^o within four domains having areas of 3.7-34.4 km² and totaling 49.7 km². The investigator manually segmented the edges of the domains into 26 relatively linear curves, interpreted as fold hinges. Then he used the ArcGIS Spatial Statistics Directional Distribution Tool to fit standard deviational ellipses to each of these curves. The orientation of the long-axis of each ellipse provided an estimate of the orientation of the fold hinge, and the eccentricity of each ellipse provided a measure of the linearity of the hinge. Finally, he compared the orientation of the hinges with the published orientation of HRFZ faults. For the published study, the investigator had manually mapped four HRFZ normal fault zones on the basis of mesoscale structures and visual interpretation of the structure contour map.

Application of the Directional Distribution Tool showed that 8 of 26 hinges (31%) trend 335^o-350^o, closely approximating the published 331^o-358^o strike of macroscale HRFZ faults. Also, the 1^st and 4^th most linear hinges coincide with segments of the Arno fault zone, the zone associated with the greatest structural relief within the HRFZ. Two hinges (ranking 21^st and 22^nd in linearity) coincide with two of the five segments of the McDaniel fault zone, the zone associated with the 2^nd greatest structural relief. However, 11 of 26 hinges (42%) trend 000^o-030^o, including the 3^rd most linear hinge. Although these hinges do not parallel macroscale HRFZ faults, they share the published trend of many Midcontinent faults and folds, indicating that the folds may have formed through the reactivation of cratonic basement structures. Most of the rest of the hinges (23%) trend 055^o-085^o, and this group includes the 2^nd most linear hinge. These hinges are approx. orthogonal to the HRFZ faults and, therefore, folding may have accommodated along-strike variations in extension.