CONTRASTS IN BASEMENT FAULTS OF THE MISSISSIPPI VALLEY BASEMENT GRABEN AND THE NEW MADRID SEISMIC ZONE

The Cambrian-age Mississippi Valley basement graben trends northeast beneath the Mississippi Embayment of the Gulf Coastal Plain. The New Madrid seismic zone is along the northern part of the Mississippi Valley graben; but active fault surfaces, indicated by earthquakes, differ in location and geometry from the large boundary faults of the Mississippi Valley graben. Why the active fault displacement has not conformed to the pre-existing large faults remains unclear.

The northeast-striking boundary faults of the Mississippi Valley graben, as documented by seismic reflection profiles, deep wells, and magnetic map patterns, have as much as 2 km of vertical separation at the top of Precambrian crystalline basement. Possible secondary faults within the graben are not well documented. A thick succession of early Late Cambrian and older, dominantly fine-clastic sediment fills the graben, indicating synrift deposition during Ouachita rifting. The geometry of the graben is consistent with a northwest-southeast extension (sigma-3) direction. Middle Late Cambrian carbonate strata overstep the graben-boundary faults, recording evolution of a passive margin. Late Paleozoic normal reactivation of the graben-boundary faults resulted in approximately 0.5 km of vertical separation. Cretaceous strata in the broadly synclinal Mississippi Embayment overlie the boundary faults of the Mississippi Valley graben.

Earthquake hypocenters, at depths within the Precambrian crystalline basement, define a northeast-trending alignment that steps left across a northwest-trending alignment. The northeast-trending alignment parallels the boundary faults of the Mississippi Valley graben. The south segment of the northeast-trending alignment is near the mid-line of the graben. The northwest-trending alignment crosses the northwestern graben boundary, and the north segment of the northeast-trending alignment is northwest of the graben. Published data show that hypocenters within the northeast trend define a vertical plane, whereas those along the northwest trend define a southwest-dipping plane and compressional displacement. These data indicate east-northeast/west-southwest compression (sigma-1) along the northwest-striking southwest-dipping plane and strike-slip along the northeast-striking vertical plane.