CONSTRAINING THE ORIGIN AND EMPLACEMENT OF OFF-CRATON KIMBERLITES IN RILEY COUNTY, KANSAS, WITH GRAVITY DATA

Cretaceous-age kimberlites in Riley County, Kansas, are unusual in that they erupted through Proterozoic rather than Archean crust. Their occurrence, along with other localities in Wyoming and Northern Canada, have been linked to a lithosphere-scale structure called the “kimberlite corridor” and attributed to low-angle subduction beneath North America. Riley County kimberlites represent the southern-most kimberlite locality along the corridor, with the next nearest kimberlite locality over 2000 km to the north, which has raised questions about the nature of their association, if any, with the kimberlite corridor. 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 are undertaking a combined petrological, geochemical, and geophysical study to better understand the origins of these rocks. The regional gravity map shows the kimberlites of Riley County are located on a high gravity gradient. The free air anomaly is indeed transitioning from positive west of the kimberlites to negative east of the kimberlites. This would indicate that lithospheric extension may have facilitated the ascent of kimberlitic lavas. To better constrain the phenomena at the origin of the kimberlite emplacement, we also conducted land gravity surveys mapping the Stockdale, Winkler Crater, and Bala localities. The gravity data provide information on density variations in the subsurface and are therefore used to constrain the subsurface structure of the kimberlites, mainly of the sills and dykes composing them. These structural orientations provide insights on the stress field at the time of their emplacement. Our preliminary results show that the gravity signature of the Stockdale kimberlite has an NE-SW orientation, parallel to the mid-continental rift, close to our location, and to the Abilene Arch, a major structural feature in the region. Most of the kimberlites are located on the Abilene anticline. All these observations suggest lithospheric extension helped the kimberlites emplacement. Detailed study of the gravity and geomorphology of the kimberlites fields will bring new insights on the emplacement processes.