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. TECTONIC DEVELOPMENT OF THE NEW MADRID SEISMIC ZONE by Lawrence W. Braile, William J. Hirze, John L. Sexton e-N Department of Geosciences Y Purdue University West Lafayette, IN 47907 G. Randy Keller Department of Geological Sciences University of Texas at El Paso El Paso, TX 79968 and Edward G. Lidiak Department of Geology and Planetary Sciences University of Pittsburgh Pittsburgh, PA 15260 ABSTPACT Geological and geophysical studies of the New Madrid Seismic Zone have revealed a buried late Precambrian rift beneath the upper Mississippi Embayment area. The rift has influenced the tectonics and geologic history of the area since late Precambrian time and is presently associated with the contemporary earthquake activity of the New Madrid Seismic Zone. The rift formed during late Precambrian to earliest Cambrian time as a result of continental breakup and has been reactivated by compressional or tensional stresses related to plate tectonic interactions. The configuration of the buried rift is interpreted from gravity, magnetic, seismic refraction, seismic reflection and stratigraphic studies. The increased mass of the crust in the rift zone, which is reflected by regional positive gravity anomalies over the upper Mississippi Embayment area, has resulted in periodic subsidence and control of sedimentation and river drainage in this cratonic region since formation of the rift complex. The correlation of the buried rift with contemporary earthquake activity suggests that the earthquakes result from slippage along zones of weakness associated with the ancient rif t structures. The slippage is due to reactivation of the structure by the contemporary, nearly east- west regional compress've stress which is the result of plate motions. 1) Now at: Arco DST 12104, PO Box 2819, 400 N. Olive, Skyway Tower, Dallas, TX 75201 8404270214 840403 PDR RES PDR e - _____ _ 4 . 2 INTRODUCTION The New Mad rid Seismic Zone has been the subject of increasing interest and a large number of geological and geophysical studies in the past several years. This interest has followed the recognition of the earthquake hazard in the New Madrid area as evidenced by the 1811-1812 series of earthquakes near New Madrid, Missouri (Nuttli, 1973; 1982) and of the significance of the New Madrid area as an example of intraplate seismicity. As the geological and geophysical data base in the New Madrid Seismic Zone has improved, more detailed interpretations of the subsurface structure and geologic history of the area have become c possible. In this paper, we review the tectonic development of the New Madrid Seismic Zone since its earliest known history in late Precambrian time. The interpretations of the geologic evolution of this area are based on geological data and geophysical models of the Phanerozoic sed imentary rocks and underlying structure of the crust, as well as related plate tectonic activities of the eastern North American craton. Interpretation of the Reelroot Rift beneath the upper Mississippi Embayment by Ervin and McGinnis (1975) was the first suggestion of a recognizable upper crustal feature which is correlative with the contemporary earthquake activity of the New Madrid Seismic Zone. Subsequently, Hildenbrand d & (1977, 1982) and Kane d & (1981) utilized gravity and magnetic data to better delineate the location of the buried s'eelroot Rift and infer a thick section of sedimentar/ rocks filling the graben associated with the rif t. In addition, they noted that the New Mad rid Seismic Zone was located near the center of the buried Reelfoot Rift. Braile d & (1982a,b) utilized local gravity and magnetic maps to infer extensions of the Reelroot Rift to the northwest and northeast. Soderberg and Keller (1981) described the suosurface structure of the Rough Creek Graben in western Kentucky adjacent to the northern end of the Reelfoot Rift. An integrated interpretation of these studies (Braile d &,1982b) indicates that a f ailed rift system exists beneath the upper Mississippi Embayment which these authors termed the 'New Madrid Rift Complex' (Figure 1). The rift , complex was delineated primarily on the basis of short-wavelength gravity and magnetic anomalies which are associated with Lho cdges of ' the rift. However, the _ regional gravity data of Cord ell ' (1977) , illustrated in Figure 1, also show a clear correlation with the rift complex. i The historical seismicity of the New Madrid Seismic Zone was described by Nuttli (1973, 1979, 1982) and microcarthquake activity since 1974 has been reported by Stauder n & (1977) and Stauder (1982). Earthquake activity within the New Madrid Seismic Zone and adjacent area is generally correlative with the configuration of the New Madrid Rift Complex (Braile d C,1982b,c) as illustrated in Figure 1. Additional important information from earthquake studies have been provided by Herrmann and Canas (1978) - who"have shown that* *Tocal''~ ~~~ ' mechanisms of earthquakes within the New Madrid Seismic Zone are consistent with right lateral strike-slip faulting along the primarily northeasterly trend of the microearthquake seismicity southwest of the town of New Madrid (Figure 1) . A variety of models have been proposed to explain the occurrence of earthquake activity in the intraplate region of midcontinent North America. Hinze d 1 (1980) reviewed the various models and grouped them into five different types. More recently, these authors (Hinze d & , 1984) have suggested that only two nf these mechanisms provide viable models for explaining the intraplate earthquakes in midcontinent . 3 North America. These models are termed the ' zone of weakness model' and the ' local basement inhomogeneity model' . The zone of weakness model was proposed by Sbar and Sykes (1973) and Sykes (1978) and was suggested by Zoback and Zoback (1981) and Braile d & (1982b,c) to be the cause of earthquake activity in the New Madrid region. According to this model, contemporary earthquake activity is due to a reactivation of ancient faults within the crystalline crust which are presently subjected to an appropriately oriented regional stress field. The orientation of the New Madrid Seismic Zone, the earthquake focal mechanisms, the correlation of the trend of seismicty with the buried Reelfoot Rift, and the nearly east-west compressive stress field of the New Madrid region are consistent with this hypothesis as an explanation for the earthquake activity in the New Madrid Seismic Zone. The local basement inhomogeneity model appears to best explain some small zones of earthquake activity which can be shown to be associated with local crustal inhomogeneities evidenced by pronounced gravity and magnetic anomaJies ( Hinze d 1, 1984). This mod el, originally suggested by Long (1976); Kane (1977); and McKeown (1978), may be the mechanism for a small percentage of intraplate earthquakes in midcontinent North America which appear not to be related to major buried structures. IDENTIFICATION OF THE NEW M4DRID RIFT COMPLEX The New Madrid Rift Complex can be broadly correlated with a linear positive gravity anomaly (Figure 1) trending roughly nortneastward along the axis of the upper Mississippi Embayment. The anomaly extends north of the Embayment and broadens near the junction of the arms of the New Madrid Rift Complex. Ervin and McGinnis (1975) and Cordell (1977) interpreted this anomaly to be due to a mass excess in the lower crust caused by. intrusion associated with late Precambrian rifting during the formation of the Reelfoot Rift. From Figure 1, it can be seen that near the northern end of the linear gravity anomaly, the anomaly splits into lobes approximately following the outline of the inferred rift complex. More detailed de.lnition of the configuration of the rift complex is provided by detailed gravity and magnetic anomaly maps as presented by Hildenbrand d & (1977, 1982) and Braile d & (1982a,b). The characteristic anomalies ~ defining the Rift Complex are correlative positive gravity and . magnetic anomalies (many of which are nearly circular) which are approximately coincident with the edges of the rift complex. In some locations, strong linear gradients in the gravity or magnetic field are also related to the edge of the buried rift. Within the rift complex, the gravity and magnetic anomaly expression is generally more subdued reflecting a deeper depth to basement (Hildenbrand n &,1982). Examples of gravity and magnetic anomalies associated with a portion of the New Madrid Rift Complex are shown in Figures 2 and 3 . The depth to-magnetic basement interpretation of Hildenbrand R & (1982) suggested the presence of boundary faults associated ' with the edges of the Reelroot Rift. Seismic reflection data in the Reelfoot area (Zoback d & , 1980; Hamilton and Zoback, 1982; Sexton R & , 1982) have also been used to identify faults within the rift. Recently, a seismic reflection profiling experiment conducted in the Wabash River Valley in southern Illinois and southern Irdiana near the margin of the inferred southern Indiana arm of the New Madrid Rift Complex (Figure 1) has provided clear evidence for late Precambrian to early Paleozoic faulting associated with the' New Madrid Rift Complex. An example of these data is shown in Figure 4 from Sexton d & (1984) { i _ . - _ _ _ _ _ _ _ _ _ - - _ _ _ _ _ - _ - _ _ _ _ _ _ - _ - - _ _ _ . * 4 for portions of two seismic reflection record sections centered on the Wabash River (Figure 5). The seismic sections (Figure 4) show good reflections from Paleozoic stratigraphic units and allow identification of the small-offset (20 to 50 meters) Wabash River Valley faults. In addition, the record sectione provide evidence for a thick section of pre-Mt. Simon layered rocks existing within a fault-bounded graben beneath the Illinois Basin. A schematic cross section based on the seismic reflection data of Sexton d & (1984), as well as gravity and nagnetic interpretations, across the Southern Indiana Arm of the New Madrid Rift Complex is shown in Figure 6. Two stages of normal fault activity are indicated by these data.
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