The New Madrid Seismic Zone is made up by a series of reactivated faults. The main fault of the shear zone is the Reelfoot Rift, which is a failed allocogen (continental rift zone) that formed during the breakup of Rodinia around 750 million years ago. During the Mesozoic ( 200 million years ago), the rifting was reactived, and intrusive igneous rocks formed. Today the fault accommodates some thrusting and strike-slip motion.
Summary
• 750 Million years ago—Continental Rifting, Reelfoot rift failed
• 200 Million Years Ago—Rifting Restarted, still fails
• Present day—Strike-Slip Motion and Thrusting.


Figure 1: Block Diagram illustrating the present configuration of the buried New Madrid Rift Complex. The structurally controlled rivers, Paleozoic rocks in cratonic sedimentary basins, and the Mississippi Embayment, all associated with the buried rift complex, are also shown. Dark areas indicate intrusions near the edge of the buried rift. An uplifted and possibly anomalously dense lower crust is suggested as the cause of the linear positive gravity anomaly associated with the upper Mississippi Embayment. ( Braile, 2004).

 

 

The center of the seismic zone is in the middle of the Mississippi Embayment, a southwest trending trough of fluvial and alluvial sediments.

Figure 2: The Mississippi Embayment location and approximate depths, showing the Mississippi River. http://www.ceri.memphis.edu/usgs/hazmap/resonance.shtml

 

The Mississippi Embayment was created by the rifting of Rodina, which left a large trough. The modern embayment developed after the sea level rise in the Cretaceous. The shoreline can inland and the downwarped basin was a natural course for the ocean to enter the continental interior. After sea level fell, the Mississippi River began depositing large amounts of sediment as the major drainage for the continent.