New structural data in the southwestern extension of the New Madrid seismic zone show that the Upper Cambrian strata of the Blytheville arch recorded at least three episodes of deformation following initial rift formation. The earliest deformation is represented by a phase of uplift and tilting of deepwater, anoxic mudstone truncated by a prograding sandstone that precedes the formation of the Blytheville arch. This episode is followed by progressive deepening and filling of the graben by late Cambrian time. Two sets of carbonate-filled veins identify subsequent deformational episodes. The older set, observed in three cores over a vertical distance of about 1000 m, strikes N35 °W ± 8° and dips 80°SW and probably formed during deformation of regional extent. The younger set is observed only in the top-most core and is typically stratabound, doubly terminated, and nearly vertical. This set probably developed in response to local faulting. The average orientation of the younger set is N41 °E ± 14°.

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