A DETAILED ANALYSIS BETWEEN GROUND LOCAL GRAVITY KIMBERLITE SURVEY AND STATE-WIDE GRAVITY SURVEY WITHIN KANSAS

The Kansas kimberlites intrude Permian strata along the crest of the Nemaha uplift, a reverse-faulted structure with fault planes on the eastern margin that dip to the west and is believed to be related to the timing of the Mid-Continental Rift. Their occurrence, along with other localities, have been linked to a lithosphere-scale structure called the “kimberlite corridor” and attributed to low-angle subduction beneath North America. Mantle plumes, continental extension, or melts originating in the mantle transition zone or deeper are alternative explanations for the origins of these rare rock types in an off-craton location in the middle of North America. We use gravity to try and constrain the processes involved in the kimberlite emplacements. Regional gravity data show that the kimberlites of Riley County are located on a high gravity gradient in the area. The free air anomaly transitions from positive values west of the kimberlite field to negative values east of the kimberlites, suggesting that lithospheric extension may have facilitated the ascent of the kimberlitic magmas. We are undertaking ground gravity surveys at three of the eleven kimberlites located within Riley County (Bala, Fancy Creek and Stockdale), with a Worden gravimeter. The gravity signature of the Stockdale kimberlite has an NE-SW orientation, parallel to the Abilene Arch and Mid-Continental Rift. Whereas the gravity signature from the Bala kimberlite has a NW-SE trending orientation, perpendicular to the Abilene Arch and presents the sharpest topographic relief out of all kimberlites in this study. Our preliminary results show that the gravity signature of the kimberlites and their associated structures (e.g. sills) correlate with the regional stress field, which may indicate that their emplacement has been facilitated by lithospheric and crustal extension.