OLD RIFTS NEVER DIE: CRUSTAL THICKENING ACROSS THE MIDCONTINENT RIFT AND ITS POSSIBLE ROLE IN POST-RIFTING TECTONICS

One of the remaining first-order problems in geoscience is the origin of large-scale, long-lasting tectonic deformation in the interior of stable tectonic plates. An example of such deformation is the Nemaha Ridge in Nebraska, Kansas, and Oklahoma. The 550-km-long, 30-50-km-wide feature is about 50 km east of the 1.1 billion years old Midcontinent rift (MCR) and its proposed southward extension. Spatial variation in the thickness of Paleozoic and younger sedimentary rock layers implies that the Ridge has been continuously uplifting for at least 600 million years. Its eastern border, the Humboldt fault zone, is still producing notable and sometimes damaging earthquakes.

The force that has been driving the uplift must be persistent over a period of 600 m.y. or longer. While mantle convection is considered as the source of most large-scale tectonic movement, it may not be the direct cause of the uplifting of the Ridge because of the abnormally-long time-scale which is several times longer than the 'over-turning' time of mantle convection. Thus the uplift might have a local or regional origin. Over the past several years we have been conducting a portable seismic experiment across Kansas. By stacking P-to-S converted seismic waves from the Moho, we have found that the crust beneath the MCR and the areas within about 120 km on each side of the rift axis is thickened by 5-12 km relative to the adjacent areas. The thickening was likely the result of the lateral compression during the closure of the MCR about 1.1 billion years ago. The Nemaha Ridge is located inside this zone of thickened crust, but outside the axis of the MCR, which is filled with a thick layer of high-density volcanic rocks that give characteristic gravity anomalies. Based on previous data and our new crustal thickness measurement, we hypothesize that the long-lasting uplift of the Nemaha Ridge is the result of the uplift of the Moho toward isostatic balance.