BASEMENT STRUCTURAL CONTROL ON NEW KIMBERLITE DISCOVERIES IN KANSAS

Three new kimberlites were discovered and drilled in 1999, bringing the total number of known kimberlites in Kansas to 13. All kimberlites, except one, are clustered together in Riley County. The newly discovered Antioch kimberlite occurs outside this cluster, 20 km (12.5 mi) to the northwest, in Marshall County. The major structural elements in the area are regional NNE-trending, high-angle normal and reverse faults associated with the 1.10 Ga Midcontinent Rift System and regional cross-cutting, older, normal and reverse NW-SE striking faults. Modeling of magnetic anomalies show that the contact between the kimberlites and the country rock is steeply dipping. Stresses generated along the edges and within the North American plate, especially during times of orogenic activity, resulted in periodic reactivation of faults. In eastern Kansas at least three periods of reactivation can be demonstrated. Major movement on faults occurred in the Precambrian, the early Paleozoic and probably in the Cretaceous. Reactivation of the NNE- and NW-trending faults broke the area up into a set of semi-orthogonal blocks and resulted in complex fault geometries. All kimberlites are about 90 Ma old and were emplaced in post-Lower Permian sedimentary rocks. The kimberlites are spatially associated with the trace of the Abilene Anticline, a NNE-trending asymmetric fold recognizable in surface rocks in Riley County. Keweenawan mafic intrusive and volcanic rocks are easily identified in regional gravity and magnetic data sets and from drill hole penetrations. All kimberlites occur also in close association with the interpreted eastern boundary between mafic rocks and younger Precambrian sedimentary rocks. Detailed groundmagnetic surveys show that final local emplacement of the kimberlites is controlled by N40W striking structures. It is probable that Cretaceous tectonic activity opened pathways at the intersection of steeply dipping, major regional structures, allowing kimberlites to intrude the crust.