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 bori ngs, a digital elevation model (DEM) of topography and Reelfoot Lake (RL) b athymetry. Results suggest the scarp forms by heterogeneous shear in the fo relimb of a fault-propagation fold. Seismic data and the accompanying struc tural 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 determ inations and varies from 7 to I I m along the western shoreline of RL. Nume rical 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 dip s 75 degrees where the fault tip has propagated upward from 911 rn to its c urrent location 465 m beneath the surface. The models indicate that the sha llowest section of the southern thrust segment in the Reelfoot thrust syste m 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 thr ust is flatter at deeper levels based on the location of earthquake hypocen ters. Strain across the Reelfoot scarp is partitioned into two or three sep arate 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, t he 2290 +/- 60 ybp age of folded sediment and a 75 degrees dip for the faul t. This implies a rate of horizontal contraction in the seismic zone of sim ilar 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 react ivation of faults that previously offset Paleozoic strata. (C) 2001 Elsevie r Science B.V. All fights reserved.