Cambrian to Recent Structures around the New Madrid Seismic Zone

In the region of the New Madrid seismic zone (NMSZ), upper crustal structures indicate a long history of deformational events and persistent weak crust. Deep wells and seismic profiles document prominent structures: Cambrian northeast-striking Mississippi Valley graben (MVg), intersecting the east-striking Rough Creek graben (RCg); a late Paleozoic arch and reactivated faults; and Mesozoic-Cenozoic Mississippi Embayment syncline (MEs). MVg extension parallels that of the late stages of Iapetan rifting of Laurentia, oblique to strike of the RCg. A Middle Cambrian and older clastic succession thickens >1 km across the MVg boundary faults, and is ~8 km thick within the RCg. In the RCg, the west-striking northern boundary faults curve into southwestward splays; stratigraphic units dip northward into the northern boundary fault system and northwestward into the southwest-striking splays, suggesting pull-apart basins along strike-slip faults. Broad subregional thickening of Upper Cambrian-Lower Ordovician carbonate rocks indicates anomalous downwarp along the RCg during post-rift thermal subsidence; a regionally average carbonate thickness accumulated across the MVg. Low gradients of stratigraphic thickness change suggest little fault movement in the Late Cambrian-Early Ordovician. Palinspastic restoration of the pre-Cretaceous unconformity shows a broad south-plunging arch in upper Paleozoic and older rocks along the southern part of the MVg. Reactivated normal faults have >500-m displacement in the upper Paleozoic rocks on the limbs of the arch and aggregate as much as 2 km of vertical separation at the top of Precambrian crystalline basement. Farther north near the intersection with the RCg, a high-amplitude short-wavelength diapiric anticline within the MVg has a core of ductilely deformed Middle Cambrian shale beneath the Upper Cambrian-Lower Ordovician carbonates. Geometry of the shale diapir suggests contraction approximately perpendicular to the graben. Stratigraphic variations along the RCg boundary fault systems indicate minor fault reactivation from Middle Ordovician through Mississippian, followed by dextral strike slip during or after Pennsylvanian deposition; a late Paleozoic synclinal sag accompanied fault reactivation. The distinct post-Paleozoic angular unconformity and overlying Cretaceous strata are warped down into the MEs, which is co-axial with the south-plunging Paleozoic arch and with the mid-line of the MVg. The up-plunge northern end of the Cretaceous MEs covers the northern end of the MVg near the intersection with the RCg, but no post-Paleozoic record is preserved along the RCg. Beneath the northernmost MVg, the distribution of NMSZ hypocenters, at depths near the top of Precambrian crystalline basement, defines two segments of northeast-striking near-vertical strike-slip faults that step left across a northwest-striking southwest-dipping reverse fault. The south segment of the northeast-striking fault is parallel with and near the mid-line of the MVg. The northwest-striking fault intersects the boundary faults of the MVg. The north segment of the northeast-striking fault is along the northwest boundary. Determining the cause of current seismicity remains a challenge.