Paper No. 13
Presentation Time: 11:15 AM
IMAGING THE ILLINOIS BASIN: THE OZARK ILLINOIS INDIANA KENTUCKY (OIINK) FLEXIBLE ARRAY EXPERIMENT
The overall objective of the Ozark Illinois INdiana Kentucky (OIINK) Flexible Array experiment is to improve our understanding of lithospheric scale structures of North America's stable craton. The array spans the Ozark Plateau, the southern Illinois Basin, the Rough Creek Graben, and the Grenville front. It can be viewed as a 3-fold oversampling of the Transportable Array (TA) rolling from west to east using a maximum station count of 70. The array was deployed in stages beginning July 2011 to a full complement during the past year. It is presently in the process of rolling into Kentucky. We merged our data with data from TA and regional network (NM) stations within this study area. The combined data lowered the earthquake detection threshold by approximately 0.5 magnitude units in the Mississippi Valley region, and allowed us to refine characterization of local seismicity. Results highlight the boundary between the Illinois Basin and the Ozark dome, emphasizing that this is a weak crustal feature. Estimated depths were found to be approximately 5 km deeper than the well-constrained distribution of New Madrid region. Analysis of these data to define crust and upper mantle structure is underway. Receiver function stacks indicates that the crust is greater than 40 km thick in the region spanning from the Ozark Plateau into the Illinois Basin. The area of thickest crust within the OIINK study area (near 50 km) appears to be located near the Ozark-Illinois Basin boundary region. The pulse width and amplitude of the receiver function phase marking the Moho in the Illinois Basin differs from that in surrounding areas. Current efforts are directed at removing basin effects to better define true Moho geometry. Preliminary teleseismic P wave tomography results suggest slower upper mantle velocities east of a transitional area defined by the Ohio River boundary of Illinois and Kentucky. These result, however, are likely biased because they do not include a crustal thickness or basin correction. An update with these corrections will be presented at this meeting. A digital shaded-relief map of basement topography developed during this project provides a context for interpreting the results.