Strike‐slip faulting at Thebes Gap, Missouri and Illinois: Implications for New Madrid tectonism

Abstract

Numerous NNE and NE striking strike-slip faults and associated normal faults, folds, and transtensional grabens occur in the Thebes Gap area of Missouri and Illinois. These structures developed along the northwestern margin of the buried Reelfoot rift of Precambrian-Cambrian age at the northern edge of the Mississippi embayment. They have had a long-lived and complex structural history. This is an area of recent moderate seismicity, approximately 45 km north of the New Madrid seismic zone. Stratigraphic evidence suggests that these faults were active during the Middle Ordovician. They were subsequently reactivated between the Early Devonian and Late Cretaceous, probably in response to both the Acadian and Ouachita orogenies. Deformation during this period was characterized by strongly faulted and folded Ordovician through Devonian rocks. In places, these deformed rocks are overlain with angular unconformity by undeformed Cretaceous strata. Fault motion is interpreted as dominantly strike slip. A still younger period of reactivation involved Late Cretaceous and Cenozoic formations as young as the Miocene or Pliocene Mounds Gravel. These formations have experienced both minor high-angle normal faulting and subsequent major, rightlateral strike-slip faulting. En echelon north-south folds, E-NE striking normal faults, regional fracture patterns, and drag folds indicate the right-lateral motion for this major episode of faulting which predates deposition of Quaternary 1oess. Several nondefinitive lines of evidence suggest Quaternary faulting. Similar fault orientations and kinematics, as well as recent seismicity and proximity, clearly suggest a structural relationship between deformation at Thebes Gap and tectonism associated with the New Madrid area. Introduction Previous work in the Thebes Gap area [McQueen, 1939; Stewart, 1942; Stewart and McManamy, 1944; Pryor and Ross, 1962; Johnson, 1985] indicated that strata as young as the Miocene or Pliocene Mounds Gravel and perhaps Quaternary 1oess were faulted and folded. These generalized studies identified some of the structural complexities in the area; however, they did not provide a comprehensive structural analysis or tectonic history. The goals of our study were (1) to define the structural framework of the area with field data and kinematic indicators, (2) to determine the tectonic history of the area, and (3) to compare our findings to present models of New Madrid tectonism. This paper is not subject to U.S. The Thebes Gap area is located at the northern edge of the Mississippi embayment (Figure 1), where the Mississippi River flows through a relatively narrow gorge cut into Paleozoic, Cretaceous, and Cenozoic rocks. Just 45 km north of the New Madrid seismic zone, these rocks are the closest exposures to the seismic zone accessible for structural analysis. It is also an area of recent moderate seismicity [Stauder, 1982; Rayat et al., 1987; Stover, 1987, 1988; Stover and Brewer, 1991; Chiu et al., 1991]. Geophysical data by Hildenbrand and Hendricks [ 1994] indi...

Late Cretaceous and Cenozoic Geology of the New Madrid Seismic Zone

“…Jackson Formation thickness is quite variable because its upper contact is an unconformity overlain by Quaternary Mississippi River alluvium within the valley and by Pliocene-Pleistocene Lafayette Formation (Upland Gravel) on the bluffs east of the Mississippi River (Autin et al, 1991). With the exception of Miocene gravels (Olive, 1980;Harrison and Schultz, 1994), there are no Oligocene or Miocene sediments above the Jackson Formation in the NME. Thus, it appears that the NME has been above sea level since Oligocene Jackson Formation time.…”

“…This fault has been reactivated a number of times since the Paleozoic (Sexton and Jones, 1986;Van Arsdale et al, 1998). Bedrock exposures in the Benton Hills of southeastern Missouri reveal strike-slip faulting (Harrison and Schultz, 1994) that has been episodic throughout the Cenozoic with from 4 to 6 faulting events within the Quaternary (Harrison et al, 1997).…”

Interpreting the Earthquake Source of the Wabash Valley Seismic Zone (Illinois, Indiana, and Kentucky) from Seismic-reflection, Gravity, and Magnetic-intensity Data

“…Ravat (1997) andHildenbrand et al (2002) suggest that the CGL is related to structures older than the Reelfoot-Rough Creek system and that it was probably reactivated during rifting. Along much of its length, the CGL seems to correlate with moderate-magnitude historic earthquake epicenters and large prehistoric earthquakes (Harrison and Schultz, 1994;Langenheim and Hildenbrand, 1997;Hildenbrand et al, 2002). Near the Mississippi River in southeastern Missouri, the CGL locally corresponds to the Commerce fault zone, which displaces sediments as young as Holocene .…”

Integrated Geological and Geophysical Study of Neogene and Quaternary-age Deformation in the Northern Mississippi Embayment

“…The best documented case is the northeast-striking Commerce Fault zone (regionally, the CGL, Figure 2), which displaces Wisconsinan loess and Holocene sediments in southeastern Missouri (Harrison and Schultz, 1994;Harrison et al, 1999;Stephenson et al, 1999;Odum et al, 2002). The CGL, expressed as a northeast trend of gravity and magnetic anomalies, seems to correlate spatially with moderate-magnitude earthquake epicenters between northeastern Arkansas and southeastern Illinois (Harrison and Schultz, 1994;Langenheim and Hildenbrand, 1997). In order to explain the discrepancy between the seismically active and inactive areas of southeastern Missouri and southern Illinois north of the NMSZ, we suggest that active deformation north of and including the NMSZ has migrated from place to place over recent geologic time.…”