THE LITHOSPHERIC SIGNATURE OF THE REELFOOT RIFT AS REVEALED IN THE SHEAR VELOCITY STRUCTURE BENEATH THE CENTRAL UNITED STATES

We investigate the lithospheric structure beneath the US Midcontinent by measuring Rayleigh wave phase velocities from teleseismic earthquakes recorded at USArray Transportable Array and OIINK (Ozarks-Illinois-INdiana-Kentucky) Flexible Array stations. The study area encompasses the Illinois Basin (IB) and Reelfoot Rift (RR). We find that high mid- to lower-crustal velocities (varying from >4.15 km/s to 4.3 km/s) characterize the RR and the southern IB. The observed high crustal velocities beneath the two basins may correspond to high-density mafic intrusions that were emplaced during the rift development associated with initiation of both basins. The additional mass of these intrusions would have contributed to subsidence within the southern IB and Mississippi Embayment. The distribution of faults within the IB aligns with the northern edge of the high crustal velocities, suggesting crustal-scale lithological control on the development of surface structures. Beneath the high crustal velocities in the rift area, we also observe low mantle velocities at depths between 75 and 125 km, with the lowest velocity (4.35 km/s) >7% lower than the average shear velocity of 4.7 km/s outside of the rift. Examining the sensitivity of shear velocities to physical parameters points to the need for a combination of thermal and compositional (presence of water and variations in the Fe/Mg ratio of mantle olivine) heterogeneities to reduce upper mantle velocities of the midcontinent to this degree. The heterogeneous upper mantle would act as a weak zone, and the New Madrid seismic zone, which annually hosts an average of 200 earthquakes of magnitude greater than 1.5, may correspond to the location of concentrated deformation above a weak mantle. The tectonic history of the area included rifting and the passing of a mantle plume, both of which could introduce the compositional and thermal heterogeneities that led to the observed reduced velocities. A similar NE-SW orientation of the low velocity zone and the RR suggest that a rift origin may be responsible for the reduced velocities. In addition, we observe a similar mantle velocity reduction beneath the Wabash Valley seismic zone, but at a reduced amplitude of 3%. This similarity suggests that the contemporary seismicity in both zones may result from reduction in lithospheric strength.