Geometry, numerical models and revised slip rate for the Reelfoot fault and trishear fault-propagation fold, New Madrid seismic zone

The geometry and structural relief of late Holocene sediments folded across the Reelfoot scarp are characterized with trench excavations, shallow borings, a digital elevation model (DEM) of topography and Reelfoot Lake (RL) bathymetry. Results suggest the scarp forms by heterogeneous shear in the forelimb of a fault-propagation fold. Seismic data and the accompanying structural analysis support the argument that the Reelfoot fault does not offset the surface, and that the Reelfoot scarp is a fold limb. Structural relief measured across the Reelfoot scarp is slightly higher than previous determinations and varies from 7 to I I m along the western shoreline of RL. Numerical models based on trishear kinematics constrain growth of the Reelfoot monocline and indicate that the underlying Reelfoot thrust fault is steeply dipping. The shallowest portion of the central Reelfoot thrust segment dips 75 degrees where the fault tip has propagated upward from 911 rn to its current location 465 m beneath the surface. The models indicate that the shallowest section of the southern thrust segment in the Reelfoot thrust system dips even more steeply at 80 degrees but has propagated upward only a few tens of meters to its present location 1016 m beneath the surface. The thrust is flatter at deeper levels based on the location of earthquake hypocenters. Strain across the Reelfoot scarp is partitioned into two or three separate fold scarps that collectively record a late Holocene fault slip rate of 3.9 +/- 0.1 mm/yr. The slip rate is based on 9 m of structural relief, the 2290 +/- 60 ybp age of folded sediment and a 75 degrees dip for the fault. This implies a rate of horizontal contraction in the seismic zone of similar to1.0 mm/yr. This rate is strongly dependent on the dip of the thrust calculated by the structural analysis. The numerical models argue for reactivation of faults that previously offset Paleozoic strata. (C) 2001 Elsevier Science B.V. All fights reserved.