Major Scientific Questions and Hypotheses
What has driven the subsidence of the ME from late Cretaceous time?
We hypothesize that the lithosphere is thinner or has properties that will be reflected by lower seismic velocities within the ME than in the surrounding region. Although initial formation of the Reelfoot rift did not involve voluminous magmatic intrusions, the lithosphere experienced extension. Continued weakness of the lithosphere below the ME is suggested by reactivation of the rift in Permian and/or Cretaceous times and by the emplacement of intrusions during these periods of renewed activity.
Is the “rift pillow” related to early rifting or is it a relict structure?
We hypothesize that the Reelfoot rift is associated with an ancient (Proterozoic) crustal structure and that Early Cambrian rifting occurred along this previously established zone of weakness. The existence of a major crustal boundary below the axis of the MVG is suggested by the change in trends of gravity and magnetic anomalies on either side of the graben. It was also noted that the margins of the MVG are exceptionally straight (more like the boundary of a transform fault than a rift) and attributed this to strong control from a preexisting basement feature. A change in upper crustal velocity across the axis of the MVG is found in recent local earthquake tomography studies; slower P- and S-wave velocities are found to the SE of the axis and faulting associated with the NMSZ does not disrupt this pattern. A similar change in P-wave velocity, extending to the base of the crust (40 km), is imaged in a teleseismic tomography study centered on the northern ME. The ancient structure may or may not affect mantle rocks; Pn tomography indicates a uniform high velocity (8.3 km/s) of the uppermost mantle below the northern ME. Evidence for an ancient crystalline basement fault below the MVG axis is also present in reinterpreted COCORP data.
How have possible thermal events in the pre-Cambrian, Permian, and late Cretaceous affected the lithosphere?
We hypothesize that the lithosphere is thinner or has properties that will be reflected by lower seismic velocities within the ME than in the surrounding region. Although initial formation of the Reelfoot rift did not involve voluminous magmatic intrusions, the lithosphere experienced extension. Continued weakness of the lithosphere below the ME is suggested by reactivation of the rift in Permian and/or Cretaceous times and by the emplacement of intrusions during these periods of renewed activity.
What is the relationship of lithospheric structure with the NMSZ?
We hypothesize that the existence of the NMSZ is related directly to lithospheric structure beneath the ME. We suggest that the mantle lithosphere may be thinner under the embayment than in surrounding regions, producing an increase in overall lithospheric stress within the NMSZ. Thinning may have occurred during original rifting or in response to the Late Cretaceous thermo-tectonic event that produced renewed uplift of the Pascola arch. Deciphering lower crust and upper mantle structure below the Pascola arch is critical to our understanding of the NMSZ; the Pascola arch is the location of the most active portion of the NMSZ and appears to be underlain by the thickest portion of the mafic pillow. Another important question involving seismic hazard evaluation is why NMSZ seismicity does not extend further north along faults that appear to be direct extensions of MVG structure. The upper mantle S-wave velocity model determined by Bedle and van der Lee in 2006 indicates that low S-wave velocities beneath the northern ME and the Illinois basin are separated by a region of high velocity. They speculate that the high velocity upper mantle effectively isolates the NMSZ from structure further north. Our experiment is designed to provide detailed velocity structure through the NMSZ and north, into the Wabash Valley seismic zone, to answer questions regarding the connectivity of these seismogenic features. For example, the existence of the rift pillow has been proposed as a defining structure, intimately connected with New Madrid seismicity. We will be able to map the continuity and variations of this layer with joint inversion of surface wave, transfer function, and gravity data to see if it is truly unique to the NMSZ.