State gravity map of Kansas

Abstract

A gravity survey of the state of Kansas has been completed with compilation of 31,000 gravity data points covering the entire state. Repeat measurements at overlap stations indicate that the data are reliable to 0.1 mgal. A steep west-dipping gravity gradient in western Kansas corresponds to thickening of the crust to the west as evidenced by seismic-refraction results. Numerous residual anomalies remain after removal of the regional trend: a northeast-trending +60 mgal high with -30 mgal flanking lows in central Kansas, which is the southern end of the Midcontinent Geophysical Anomaly (MGA) and which reflects an aborted Precambrian rift; a +5 to + 15-mgal southwest-trending high which appears to extend the MGA to the Kansas-Oklahoma border; a +30-mgal high in Montgomery, Labette, and Cherokee counties, which coincides with rhyolitic basement; a northeast-trending +15-mgal high coincident with the Nemaha Ridge in southern Kansas; a triangular shaped -20-mgal low in Greenwood County, which overlaps part of the "Wichita magnetic low," which has been previously observed; +20 and +15-mgal highs over the northwest and southeast end of the Central Kansas uplift; and a +15-mgal high in the Salina basin west of the MGA. A seventh-order residual map enhances features such as the MGA, the anomaly over the Nemaha Ridge, a possible northwest-trending cross-state fault zone, and a northwest-trending fault zone near the southeast end of the Central Kansas uplift. Oblique illumination of the gravity map in eastern Kansas shows two sets of orthogonal lineaments, corresponding to the pre-rift northwest-trending fractures in the granitic basement and the northeast-trending lineaments associated with late Precambrian rifting. A difference in reflectance patterns seems to identify the east-west boundary between the two granitic basement terranes of different Precambrian age.