Topographic evidence of long wavelength active folding in the New Madrid seismic zone, intraplate North America

The northeast-striking Axial and Bootheel faults (AF and BHF) in the central USA are components of the New Madrid seismic zone (NMSZ) fault complex, source of M > 7 earthquakes in 1811-1812. Previous work shows Holocene displacement of the AF and BHF to be primarily right-lateral strike-slip movement west and south of the NMSZ restraining bend. In this study, treads of two Late Pleistocene alluvial terraces of the Mississippi River, the 14-16 ka Kennett and the 12 ka Morehouse terraces, were used as structural datums to assess any long-wavelength vertical component of deformation across these two faults. Using 10 m-resolution LiDAR, we con-structed a best-fit polynomial trend surface of these terrace treads. Our results reveal a subtle <= 4 m-deep, 40 km-wide, 190 km-long, topographic trough. In the south, it corresponds to the southern segment of the AF and parallels the BHF for 50 km. Farther north it corresponds to a syncline in subsurface Paleozoic strata 10 to 20 km northwest of and parallel to the BHF.The trough axis crosses obliquely down-valley from the younger Morehouse terrace to the older Kennett terrace and does not correspond to major drainages. Significantly, the Malden-Bernie fluvial scarp separating the two terraces continues into the trough but is down-warped, inconsistent with an erosional origin of the trough. Along the southern segment of the trough, the older Kennett tread is topographically lower than the Morehouse tread, but relict Kennett braided channels were not buried during deposition of younger Morehouse alluvium. This suggests the Kennett terrace tectonically subsided along the trough after Morehouse terrace deposition. We interpret this trough as a subtle syncline along the western margin of the NMSZ restraining bend that developed in response to down-on-the-northwest fault movement after deposition of Morehouse alluvium.