"Tectonic Development of New Madrid Seismic Zone."

TECTONIC DEVELOPMENT OF THE NEW MADRID SEISMIC ZONE

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

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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)

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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. Major graben-bounding faults formed in late Precambrian to early Cambrian time. The grabens are filled with pre-Mt. Simon layered rocks. A generally conformable sequence of Paleozoic sedimentary rocks follows with the upper boundary of the sequence being a post-Pennsylvanian unconformity. Relatively minor faulting evidenced by the Wabash Valley Fault System followed. Although we do not have equivalent, detailed seismic reflection data in other areas, the representation of one of the arms of the New Madrid Rift Complex, as illustrated in Figure 6, is consistent with the interpretations in the leelfoot area and we expect that other sections of the rift complex will have a similar configuration. Seiemic refraction data are also available which support the interpretation of a buried rift complex beneath the northern Mississippi Embayment area. McCamy and Meyer (1966) discovered that an anomalous, basal high-velocity layer existed in the crust beneath the upper Mississippi Embayment. Recent refraction (Mooney n L , 1983) and surface wave ( Austin and Keller, 1982) studies have better defined the extent of this high-velocity layer and correlated it with the excess mass in the crust required to explain the gravity data. Mooney d & (1983) also have interpreted a low-velocity layer beneath the Paleozoic carbonates in the Reelfoot Rift ar ea which they suggest is due to late Precambrian to early Paleozoic graben-filling sedimentary rocks. Baldwin (1980) utilized refraction data in southern Indiana to suggest a similar increased thickness of sedimentary rocks in the southern Indiana Arm of the rift complex. He also found that the basement beneath one of the rift margin gravity and magnetic anomalies exhibited an anomalously high seismic velocity.

TECTONIC EVOLUTION OF THE NEW MADRID SEISMIC ZONE Hinze d & (1980) and Braile d i (1982b) argued that the New Madrid Seismic Zone is related to a late Precambrian rift and that the origin of this rift and its subsequent tectonic evolution has been intimately related to the plate tectonic activity of the North American continent. This view has provided a plate tectonics framework for the contemporary intraplate seismicity of the New Madrid Seismic Zone, as well as the basis for understanding the geologic history of the region. Although the basic conclusions of these studies are not changed here, much additional evidence for this tectonic evolution has been gathered in the past few years and a more detailed picture of the geological history of the midcontinent region of the North American craton and its relationship to plate interactions along the eastern and southern margins of the continent are now available. The data indicate a causal relationship between plate tectonic activities of the North American craton and tectonism and sedimentation in the New Madrid area at the interior of the craton. The New Madrid Rif t Complex, a failed rift or aulacogen, influenced the geologic history of this region since late _ _ .

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Precambrian time. It has controlled sedimentation, drainage of major river systiems, and contemporary earthquake activity, as well as possibly localizing ore deposits. This control appears to have been exercised by two properties of the rif t complex which were produced at its formation. Firstly, a mass excess, probably due to intrusion of mafic rocks, exists within the crust along the rift zone. This mass excess is clearly reflected in the linear positive gravity anomalies (Figure 1). Under * appropriate thermal and stress conditions, this maes excess has resulted in periodic subsidence of the craton in the New Madrid region, thus 4 influencing sedimentation and the drainage of major river systems. Secondly, the deep-seated normal faults associated with the initial rif ting of the New Madrid Rift Complex have served as zones of weakness and have localized intrusive activity in late Paleozoic and Mesozoic time. Heyl (1972) showed that intrusives and ore bodies in the mideontinent area are related with structures which we now associate with the buried rifts. Faults within the rift p rovided a means for reactivation in later phases of faulting, such as in the Wabash Valley Fault System in post-Pennsylvanian time. They may also be the fault planes related to the contemporary earthquake activity in the New Madrid Seismic Zone. The stratigraphy of Paleozoic rocks in the Illinois basin and interpretation of deep seismic reflection results (such as those shown in Figure 4) indicate that the rifting of the New Madrid Rift Complex occurred prior to deposition of the basal clastic sedimentary rock that is characteristic of midcontinent North America known as the Mt. Simon formation of late Cambrian time. Thus, the initial rifting event was either latest Precambrian or earliest Paleozoic. Pre-Mt. Simon rocks are largely confined to the area within or immediately adjacent to the rift complex and are interpreted to be contemporaneous with rifting of the Ne'w Madrid Rif t Complex. Adjacent areas were largely topographic highs as evidenced by the fact that Mt. Simon sedicentary rocks typically rest on crystalline basement (Schwalb, 1982). Subsequently, the mass excess in the crust beneath the New Madrid Rift Complex caused subsidence during Paleozoic time resulting in sedimentary basins above the rift complex. Figure 7 shows isopach data for early Paleozoic sedimentary units which indicate a significant thickening of the formations approximately coincident with sections of the New Madrid Rift Complex. Subsidence of the area generally persisted throughout Paleozoic time with thickened sedimentary units in the Illinois basin and the Reelfoot Basin - a trough between the Arcoma and Black Warrior basins underlying what is presently the Mississippi Embayment (Figure 8). After a period of uplift and erosion in Mesozoic time, subsidence of the southern part of the New Madrid Rift Complex has continued adjacent to the downwarping coastal plain causing the Mississippi Embayment ( Figure 8). The subsidence and structural control of the underlying faults within the rift complex have also served to control the drainage of major river systems for tens to hundreds of millions of years. As illustrated in Figure 8 and demonstrated by Potter (1978), the lower Mississippi River has been nearly in its present position approximately coincident with the Reelfoot section of the New Madrid Rift Complex since early Mesozoic time. Potter also showed that the predecessor to the Ohio River, which he called the Michigan River System flowed southwestward approximately down the center of the inferred Southern Indiana Arm of the New Madrid Rift Complex (Figure 8). The lower part of the Ohio River was part of this river system, which like the lower

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Missi ssippi , has followed approximately its present course for the last 250 million years. The locations of the present upper Ohio and Wabash Rivers are due to disruption by Pleistocene glaciation. The upper Mississippi River, which flows down the center of the St. Louis Arm of the New Madrid Rif t Complex, has been interpreted by Flint (1941) to be structurally controlled and approximately in the same position since at least mid-Tertiary time. The tectonic evolution of the New Madrid Rift Complex and its relationship to adjacent plate tectonic activity since the late Precambrian are illustrated in Figures 9 and 10. Plate reconstructions of a portion of the North American craton and adjacent plates which have influenced its history are shown in Figure 9. Schematic cross sections ( Figure 10) through the New Madrid Rift Complex (approximately northwest-southeast) illustrate the geologic activity of the rift complex related to plate interactions of the Horth American craton. The configuration of the New Madrid Rif t Complex through time and the plate reconstructions (Figure 9) are modified and updated from the presentations of Hinze n & (1980) and Keller d & (1983). The local geology and structural evolution of the area surrounding the New Madrid Rif t, Compl3x are based primarily on stratigraphic data (Schwalb, 1982) and the interpretat, ion of the rift complex determined from geophysical data as described previously. The plate reconstructions are based primarily on paleomagnetic (Lefort and Van der Voo, 1981; Van der Voo, 1982) and geological and geophysical (Keller and Cebu11, 1973; ' Morris, 1974; Cook d & , 1980; Burke, 1980, Lillie d 1,1983) data related to the plate interactions of the North American craton during the past 600 million years. The tectonic development of the New Madrid Rif t Complex (Figures 9 and 10) indicates the important control of the rift complex on the geologic history of thic portion of the North American craton, as well as the connection between cratonic tectonism and plate margin interaction through time. After formation of the rift during continental breakup (Figures 9A and B, and 10 A and B), the increased mass of the crust beneath the rift complex caused subsidence during times of compression associated with either continent-ocean subduction or continent-continent collision along the eastern and southern margins of the North American craton. During early Mesozoic rif ting of the continents (Figures 9E and 10E), the craton was undergoing uplif t and erosion resulting in the broad unconformity which is observed in the cratonic basins of North America and erosion of considerable thicknesses of sedimentary rocks over intracontinental arches such as the Pascola Arch centered near southeastern Missouri. Reactivation of faults associated with the New Madrid Rif t Complex caused structural uplifts and intrusion of plutons near the margins of the rift complex. Since Cretaceous time (Figures 9F and 10F), the eastern margin of the North American craton has been the trailing edge of a rifted continental margin and the continent has been under a regional compressive stress. The New Madrid Rift Complex has continued to exert control on depositional patterns as evidenced by the subsidence of the Mississippi Embayment and the location of the drainage of major river systems. The correlation of earthquake epicenters with the location of the buried rift complex suggests that the earthquakes in the New Madrid Seismic Zone are the result of slippage along pre-existing zones of weakness inherited from the late Precambrian rift. The fault planes are reactivated by the contemporary stress field which is approximately eant-west compression in the New Madrid area (Haimson, 1976; Zoback and

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Zoback; 1980, 1981). The correlation of cratonic basin subsidence with compression of the continental crust caused by continental margin subduction and of cratonic uplift with extonsion in the c rust (Figures 9 and 10) is consistent with a model for cratonc basin subsidence suggested by DeRito n & (1983). These authors show that many cratonic basins occur over ancient rift zones and that these rift zones are recognizable by a positive linear gravity anomaly. DeRito d & (1983) suggest that the driving force for subsidence of these basins is the isostatically uncompensated ancient mass excess. Their numerical modeling of lithospheric flexure for a crust containing a mass excess indicates that periods of subsidence due to the mass excess will be correlated with periods of horizontal compressional stress in the lithosphere and that relaxation of the compressional stress, such as during a period of rif ting along a continental margin adjacent to the craton, may allow the cratonic region to rebound producing uplift and erosion. Their model appears to qualitatively fit the observations for the New Madrid area and helps explain the continued influence of the rift complex through time. Although not all ancient intracontinental rifts are reactivated as contemporary seismic zones, cratonic basins are commonly underlain by rifts (DeRito d i , 1983). Other Precambrian rifts beneath cratonic basins have 'eeen identified in southern Alberta (Kanasewich, 1968), the Michigan Basin (Hinze d &, 1975; Brown d & ., 1982), the Rome trough (Ammerman and Keller, 1979), the southern Oklahoma aulacogen (Burke and Dewey,1973; Brewer d d.,1983), and the Lake Superior basin and midcontinent geophysical anomaly (King and Zietz, 1971; Ocola and Meyer, 1973; Wold and Hinze, 1982). These rifts may have had a similar tectonic history to that displayed in the New Madrid Rift Complex. Each has certainly had a continued influence on the geological evolution of the associated cratonic basin.

CONCLUSIONS The New Madrid Rift Complex has had a significant and consistent influence on tectonism of the North Americar. craton since its formation in late Precambrian time. It has controlled sedimentation by causing subsidence in cratonic basins, influenced the location of major river systems, localized intrusive activity during reactivation of rift structures, and is probably the cause of contemporary earthquake activity. The geologic and tectonic activity of this intracratonic region are also correlated with plate interaction events of the North American plate. The history of the New Madrid Rift Complex through time provides a plate tectonic framework for understanding the formation of cratonic basins associated with the rift complex and intraplate seismicity of the New Madrid Seismic Zone. Thus, the New Madrid Rift Complex, formed by plate tectonic interactions about 600 million years ago, has influenced the geologic activity of the midcontinent in response to plate interactions at the margin of the North American plate. And , it is plate motions which are presently producing the compressive stress within the North American craton (Zoback and Zoback, 1980) which probably cause the reactivation of faults within the New

, Madrid Seismic Zone. , , q 3 3, .

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ACKNOWLEDGEMENTS This research was supported by the U.S. Nuclear Regulatory Commission under Contracts No. NRC-04-81-195-01 and NRC-04-80-224. We are grateful to many graduate students who have assisted in the collection, compilation and processing of data upon which many of the interpretations contained in this paper are based. We thank Paul Morgan for a number of helpful comments. We greatly appreciate the many useful discussions with colleagues of the 'New Madrid Study Group' concerning the seismotectonico of the New Madrid area.

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Mooney, W.D., H.C. Andrews, A. Ginzburg, D. A. Peters and R.H. Hamilton, Crustal structure of the Northern Mississippi Embayment and a comparison with other continental rift zones, Tectonochysics, 21, 327-348, 1983.

!!or ris , R.C., Sedicentary and tectonic history of the Ouachita Mountains, in Tectonics and Sedimentation, W.R. Dickinson ( ed . ) , Soc. of Econ. Paleontologists and Mineralorists Soccial Publication No. 22, 120-142, 1974.

Nuttli, 0.W., The Mississippi valley earthquakes of 1811 and 1812, in- tensities, ground motion, and magnitudes, Bull. Seismol. Soc. Am. , 61, 227-248, 1973.

Nuttli, 0.W., Scismioity in the central United States, Geol. Soc . Am. Rev. Eng. Geol., 1, 67-93, 1979

Nuttli, 0.W., Damaging earthquakes of the central Mississippi valley, in Investigations of the New Madrid, Missouri, Earthquake Region, F. A. McKeown and L.C. Pakiser (eds.), U.S. Geological Survey Pro- rosnional Paner 1216, 15-20, 1982. Ocola, L.D. and R.P. Heyer, Central North American rift system: 1. Structure of the axial zone from seismic and gravimetrio data, J. Georhys. Res., 23, 5173-5194, 1973

Potter, P.E., Significance and origin of Big Rivers, J. Geol., Ai, 13- 33, 1978.

Rankin, D.W., Appalachian salients and recesses: late Precambrian continental breakup and the opening of the Iapetus Ocean, 24. Geophys. Res. H1, 5605-5619, 1976. Sbar, H.L. and L.R. Sykes, Contemporary compressive stress and seismioity in eastern North America: An example of intraplate tectonics, Ggol. Soc. Am. Bull., A1, 1861-1882, 1973

Schwalb, H.R., Paleozoic geology of the New Madrid area, U.S. Nuclear Regulatory Commission Report, NUREC/CR-2909,1982.

______- _ _ _ - _ _-

. 13

Sexton, J.L., E.P. Frey and D. Malicki, High-resolution seismic- reflection surveying on Reelroot scarp, northwestern Tennessee, 111 Investigations of the New Madrid, Missouri, Earthquake Region, F. A. McKeown and L.C. Pakiser (eds.) , U.S. Geologi 2a1 Survey Pro- fessional Pacer 1216, 137-150, 1982.

Sexton, J.L., L.W. Braile, W.J. Hinze and M.J. Campbell, Evidence for p a buried Precambrian rift beneath the Wabash Valley Fault Zone from seismic reflection profiling, submitted to Geophysics, 1984. Soderberg, R.K. and G.R. Koller, Geophysical evidence for deep basin in Western Kentucky, Am. Assoc. Pet. Geol . Bull . , ii. 226-234, 1981.

Stauder, W., M. Kramer, G. Fischer, S. Schaefer and S. Morrissey, Seismic characteristics of southeast Missouri as indicated by a regional telemetered microearthquake array, Bull. Seiemol. Soc. Am , ii, 1953-1964, 1977. Stauder, W. , Present-day seismicity and identification of active faults in the New Madrid seismic zone, in Investigations of the New Madrid, Missouri, Earthquake Region, F. A. McKeown and L.C. Pakiser (eds.), U.S. Geological Survey Professional Pacer 1216, 21-30, 1982.

Sykes, L.R., Intraplate seismicity, reactivation of pre-existing zones of weakness, alkaline magmatism, and other tectonism postdating continental fragmentation, Rev. Geophys. Scace Phys., 15, 621-688, 1978. Van der Voo, R. , Pre-Mesozoic paleomagnetism and plate tectonics, Anam Rev. Earth Planet. Sci., la, 191-220, 1982.

Wold, R.J. and W.J. Hinze, (eds.), Geology and Tectonics of the Lake Superior Basin, Geol. Soc. of Am. Memoir 156, 1982. Zoback, M.D., R.M. Hamilton, A.J. Crone, D.P. Russ, F.A. McKeown and S.R. Brockman, Recurrent intraplate tectonism in the New Madrid seismic zone, Science, 2Da, 971-976, 1980.

Zoback, M.D. and M.L. Zoback, State of stress and intraplate earthquakes in the United States, Science, 211, 96-104, 1981. Zoback, M.L. and H.D. Zoback, State of stress in the conterminous United States, J. Geochvs. Res., 81, 6113-6156, 1980.

w . _ _ _ . _ _ _ _ . _ _ _ _ . ______. _ __..____._._m._ _ _ . _ . . . _ . _ . _ _ _ _ . . _ _ . _ _ _ . ______. . _ _ _ . _ _ . . _ _ . _ . ______. . . . _ . ______. _ . _ _ _ _ . _ _ . _ . . __ . _ . .

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. ...,o0. - ! a *^3 ** . . ,asua _,NE,W MADRID g | om = _ ' - ,~h* f "'aa h*'* RIFT COMPLEX) ,. g ,s'. ps. INDIAN A .= *- - - aau . .p; 3,. , ; 1 3s ' * f f:.g .. :! - - 4 0 A.sr. tams. - . ;,; : .h * - - . 4 : -20= '. : :?- ./ .- / " ) , ')* ..., . :e 4 : . fili '?.\ $* p : :. : .!'f wi =s -|0 $ E m- / | 77 - #! .! . ::::+7 ~%h.:: 2 0i, he : ::eu , e %a.,w% sa- | . 2:%.:34 :. % s , O & h ( f :' A < fj f t-- g'O'Ohh/%* j' '1 / %. % n! ' ' '%

I . . A!g" .,!h:..f %}-| ' Y.r:. ... g ~,b O,,s $*** N :q .- - : ,: .i N N ' [ ' ?g ** ENK y . -40 3,. - f(E0i y @ .' l J\_4 0 * . g 37- ' Jy:/N' '" : , 4pf)ne y- g, - hm* , -30 O W d * J/ V* "*ca"ajc"d E K - - . . , . . h f;'f )- . ,C . , . - =s,su _ 7 . #,- g ' 20 . * * .w. , 5'. v . .w .% [oc h.e g). . . S* 3 k# II ..,,:*f h . . s. %;g /* , tumiams: truantins <=-em . m[ :g, +- .,i . -3 .y g> o . f .,,.l ,%,, n.ur mm / $ $. 1 L '# i , , [ 4 ss ' YJ Oy$"j"'~ef'K'/, r M .,. ,. h _ () 80*- es- es. ee-

Figure 1. Index map of the New Madrid Seismic Zone and surrounding regions. The outline of the New Madrid Rift Complex is from Braile A.t. I ab.(1982b). Major faults (Pre-Cenozoic) are shown from the work of Heyl (1972); Heyl and McKeown (1978); Bristol and Treworgy (1979); and Ault d ab (1980). Circles are earthquake epicenters from the data file provided by Otto W. Nuttli. Locations of epicenters have been " randomized" by adding a random number uniformly distributed between 220 to the latitude and longitude. This randomization prevents an artifical alignment of epicenters along even lines of latitude and ! longitude caused by round-off of the original epicenter locations to g The solid line in the vicinity of New Madrid | the nearest 0.1 . indicates the location of the linear trend of microcarthquake | ! epicenters reported by Stauder g A L,(1977) and Stauder (1982). The | arrows indicating strike-slip and thust fault mechanisms along these linear trends of epicenters are inferred from the focal mechanisms of Herrmann and Canas (1978). The contours shows the Bouguer gravity anomaly (in mGal) from the geologic corrected regional gravity map pre'sented by Cordell (1977).

| |

i . I3 .

92* g g. 90 89* 88* 1 40* - ~ L g , ( y Jh

i \L - k{ ; a h n v .@|(%bx $ o ,M. j f 39 + &e . v c,(J, c~~ -, /. ,-np .e(p . as j V h @ < C Ss,I / . . .' ' St L ,e .- a n\, s[j a-[)ma,g/ ILLINOIS

38- M / - ' ~' ' i s , %g a m f-so n . - 5 )c;%) <|h' j~'- ,q * / J \ t ) 3{,Jda e {s - 37' - " -so ' 9( (- ).3 - ~$Nj] 'x) ' UCKY! :"t9 B j

. 4- , .. ,

BOUGUER GRAVITY (MGAL) <-50 -50 to -30 -30 to -lO -10tolO >10 o So loo km * 1 1 e t ie l l !

Figure 2. Simple Bouguer gravity anomaly map of the upper Mississippi

; Embayment area. Contour interval is 5 mGal. Shading interval is 20 mGal. !!aavy lines are faults assooiated with the Cottage Grove and St. Genevieve Fault Zones. Short-wavelength positive anomalies and high gradient zones delineate the approximate edge of the St. Louis Arm of the New Madrid Rif t Complex. The gravity data are from Keller .tti, & ( 1980 ) . Figure from Braile .cl 1, ( 1982b) .

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40* J | ; i | W .q * @h,s a A ty - %gs NW i i ) - i 39' 3 * 0 (4

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n '%i_ k . >1000 TOTAL FIELO < 400 400 600 600-800 800-1000 VERTICAL FIELD <0 0-200 200 400 400 600 >600 (GAMMAS)

o So too km It ie i i I .

. Figure 3 Magnetic anomaly map of the upper Mississippi Embayment area. Data east of the Mississippi River (border between Illinois and Missouri) are total field aerosagnetic values from Johnson d AL, (1980). Data in Missouri are averaged - (over a ten by ten km grid), vertical field, ground magnetic data from Buehler (19111). Contour interval is 100 nT (gammas). Shading interval is 200 nT. Ileavy lines are faults associated with the Cottage Crove and St. Genevieve Fault Zones. The prominent short. wavelength anomalies, approximately 100 km on either side of the Mississippi River, mark the approximate edge of the St. Louis Arm of the New Madrid Rift Complex. Figure from Braile d AL, ( 1982b) .

__- . _ _ _ _ - . - - _ . . - - - - _ _ . - - _ - - _ _ - - - _ _ _ . . _ _ _ _ ------_. . . _ . . . . - - .. . _ - - - _ . - - . - . . - _ ..__ - ..- _ . _ _ - _. - _ -- - . - - -._- . . . - . _ _ - . . - ~ _ . -,c.-- _ | * I I , ! ? . |

1 . I , f EAST END-grain."LLE LINE WEST END-NEW HARMONY LINE j =cc.., rm.s t CRAYvtLE. LUNOIS gg mettw as.se rauty ace a%, raar , [ - -..-- - . . - . < .. .. .n . . . g < . f stum 4, ...t. ..,..;. O- - . , c % %c g.- g -o g _ 1 / ,

. . . . , HERALD-PHLLIPSTO*N MY'N M i

r ., .. [: , ~ ~ .~-- _ .. / my : : : - - t aw~ w - u.,; :,. ~_:. i . . _ , , ...... - S ~ V- m - =m- , . --___ , ,. . . .~ - y . 7- | ' I \ ,_. ~ ~K a, A M . z, W GVi~ gggg&m" --W_~ S' " P' J -* * ~ * - -- u ( o, - ,f w o.3 0.S . * C ~'*s**-"""nJf'Z~y . %s1 n _ mg,, v .'s " -- ) ~ .~. ru - . 2. - .is-s v ,. Mcnntsswrxa-- ; . ~ m- : -

_ _--- _. _.w. .. - . . a -. -_ q .h----- m.- w- .- gg _ _ __ - - -- , -- - - .: .. ygm _ . m -- - = - , - =_ _y _ _ _ . -

, w= - ; - . m_,v mm: - v . m ..w. . , .______m_ - __ ._ , _ m mm : - . gmw, .._:_DENN _ ; - ,, . 1 ,. N -~T_i # D h [ , __ - - - - - . . . . .b ______. pm b.1 d. | 1_ _ - ~ - . . - , . _ , . . -. | _- g , x ._ - ' ' W NM SM I' 1 s.s T_. ~ ' $ hW'. ~ . 3 h-* . ------, 74_ [ r -.., . . - " - - - + ,, p g _ w,_ _ _ _ _.~p _ _ _ _i A-. . i.3 r - L _. _ f . L* S y g r- WQ-- ~ " ,=_ --W q=j:~:-? m~ Q m_ =pr%_ - 2 | , w_ ~ . _ . j . \ yp ,. = = - - - , % .:.z m ~.- . ~ . . w . , -- ~ m .. , . ,. m..g. ,.e. _ ,- ' ~ \ N fff.-~ f - ! M~.$h& ~ + --- ..g . . ~~1 t_ =Ncm.W, =- i - "*** . w ] . . m- .- % * . ',g: j 4;. e _ %- ^ | ; .*. M t ..,.. 1- 1 m t - i ; ... T- _ - . ___ m _ _ . . - -- ,, ~ & ,- -- ,, _ . ~ , . , ,, i , , ...... ; , .. - , , . _ , ; ; . . .._ . ..

. | GRAYVILLE GRABEN i I 4 t i Figure 4. Seismic reflection record section (Sexton d al. 1984) for | I an east-west line across the Wabasa River in southern Illinois and | ; southern Indiana. The surface location of the Wabash Valley Faults | | ( Alnion-Ridgway. Harold-Phillipstown, Ribeyre Island, and New Harmony ! l' Faults) is from Bristol and Teeworgy (1979) and Ault n 31 (1980). *-- ! ' " | The depth to the Eau Claire reflector is approximately 3.6 km below r j sea level and to the prominent faulted reflector below the pre-Mt. ( layered , Vertical Simon rocks approximately 5.2 to 6.2 km. |- i exaggeration varies, but is approximately 1.2. | ! ! i 4 i,, I

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f , k * i GRAYVILLE LINE o.4,vuc / onessviu * " ! < 200 _J.00 38*:54 , 4o + | | e a'i s' 9EW HARMONY"f * i |w/4 > LINC '- / t

1 ' o a wee i ; /[ ( , i jj New ManwoNf o 'O**

|- | .

! Figure 5. Index esp of the southern Illinois and southern Indiana area surrounding the Webnah River for the seismic refloation record . sections illustrated in Figure 4. ! ! The seismic reflection lines are I ' shown by the solid lines with seali dots every 100 source point ' locations. The heavy lines indicate the locations of the sections

wnloh are shown in Figure 4. ,

_ _ _ - - . ______- ______. _ _ _ . ______. ______. ______. _ _ _ . _ _ _ - _ _ _ _ _ . . ______. _ . _ _ _ . ______. ______. _ _ _ . ______. _ .- . -. _ _. .- - _ _ . . _ - - - _

! . ; * * 19 | i ,

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* 1 i e

: a !

. >- f ^ NW SE O 4 .J [ < C C' . - ( gO2 O w, GRAVITY /O -1000' ' / *~~, - > -10~ / - - 900 -\ La- / *--MAGNETICS / / . I / # \ , -20 ' , , , , f / - 800 g ~ ! '' ' # 5 2-30 700 # : Wayne Co. II. V.') stem Rift * Eastem Rift Deep Well Soundary Webeeh Rivec SEISMIC REFl.ECTION Boundary / :: :: !,: p:;PROFIL.E$- (PMGJECTED) | | ' | WAma8 . - s L A1E 9 Alt 0tOIC CL A C TICS f 4;;'**,oosa rasgosorc ce2& ten;-

- a - - . 1 na rr + ! .1 a 2- .,. i r w evec,e,1,s. 3. , 3 . ' . . m. ''' #. # '#,,,,,g,o,o,c cameomares,- 4' a mr. ==wns 7 , 7 Jz y ggg, I$ Y ' . . . e L,. . o s- ~gav et aes -.~" .:.. .;::.: : | , . .!! *; ~ .. .::::..y. . . . w . . . ; . . .J ...... :....

. :.:.'n }|. :;:::..::|: . : is ' ' ...... ,,,t, .:.5:':... . . :::. . . : :.:.. ::.:.'.:::::.;;.; ; ? s ...... : ...... , . . se ...... :. . a . . . . ,. . : : . : . ., . . .c. .. . :: . . . :c ...... :.:...... ,...... ::...... :. . . v c ru a : , o. .:. : . ;ne. . . .:..:: .a r- ,...: . o c ". . : . . .:. . .:. . : . . ~i,:: : , . , , , . * ...... < ...... ::: . . . . ::: : : :: .. :: ...... ' ...... ::-/.. ; ; '. f,* ,,. . !!!u !!R":!P ',!!!!!!! '''ddfrc rayrpag_ . . . :f,. * ;...... j...,. 12,_i . 1 i i , , . , , , , , , , , , , , i . .n 0 40 80 120 DISTANCE ( KM )

Figure 6. Schematio diagras illustrating the geologically and geophysically determined configuration of Phanerosoio mediseratary rocks and crystalline baseeent beneath the Southern Indiana Arm of the New Madrid Rif t Complex (Sexton el.,, AL.,19811). The cross section is along a northwest to southeast profile through the area shown on ! ' Figure 5.

_ - ______- ______- ______- ______-__.______-_. ______- _ _ _ _ - ______- ______- _ _ _ - _ - - - - _ _ _ _ - _ - . a ' 20

w w. .o. * * w w . .y .. . * . . |x. ks . 1 , * 3 4. Leos a . p g .,,,.. s. t . s .e ~ ~ . /os I (J "' " * * ' " - ['DT, f , jj h_ a -

t - ~.d)f[ . .s ..v.7, . . sy, . / " \ % \a . s' / M .5* - / \! i T fARil .L SCC,T.I'0I'**N ' . S 3e. O. ,N,L - y.n q rn .s * ' ':r)W..,j# ..c'I ' * y ""'NIN kHIS ANC 4 M 6[ N : m 'th 4 'i yx . e ouch.o $n'piro,y,,.y' sp.>,7 ...:.'. ' ...q. /*r% :., p,,> ~ v. %,M ., . w . n ... i u, w wn, . . - . , . ,, :.. . 6%wn. a/,,;i ,,, . . , - : . .um ^*.===.=*n . # // / ://,6, ,. s .. .e e . . . .

' * . e /' , . . . :. .:.x s. . . * * !.! , !. ,. .e rs;;;.;; nam i 4r .'. 9:%::j.-Q.{!:h,[,x,. .::p , , / g.m.4 - , . . . . 'A? , ,w,/ 5 . .; ;::::b.t -c ,w, / 4yo e _ , (, (g An j/ ' . . n. wf;~*.!!.D.Eh@ _ , ==- J - ~-- ~ . = -~-..- a Vi. go.~ . _Y//[mu'gge tv ~ ge*

ISOPACHS OF C AMLY PALCO2010 . SCDlWClif ARY UNif 8 [ APPMOWIWATC OUTLINC OF SURICO MIFT COMPLCX WT. SIMON (UPPER CAW 8MIAM /[ THICKNCSS OF SCDlWENT ARY UNITS Q'y'c1 C AU CL AiRC AND 80NNCTCMRC g (UPPCM C AWMRIAN) f IN WCTCRS CVCMTON = MNOW (C AW8MIAN - OM00VICIAN) (ISOP ACH D AT A FM0W SCHW AL8. I 882) E@%L-

Ptgure 7. Index sap of the New Madrid area showing isopachs of early PAleosoto sedimentary rocks after Schwalb (1982). Additional features are as described in Figure 1.

. ______.,

|| 1 . 21 , .- ! ! :

, i 1 ! 4 r SEDIMENT ARY BASING. MAJOR RIVER SYSTEMS, REGIONAL j , POSITIVE GRAVITY ANOMALY AND THE NEW MADRID RIFT COMPLEX I I ~ ' . 'fLUNOIS '| h/ ! * * + . I i t h ;

40* ;enemm =ynneamamm- m poxu . ,'/< , "' . 1 _ g ' ! ILLINOIS E ASIN % | .:n gun, gun. ' - . .. tt.. l a / I ; L-. (y 1 ._/ i &RW MADMID _0 :a ? Q g** .g0 , ! ! MIF T COMPL - Ky,$ =' .3 : _ 3 - fr,Q=r,:

\ .I |- *,a z- ' \ W = :d===.r- = hn- o< { f=:. .- - i . | , . om.ou., . %=m, w gj J' f P '' k m ;.- . b -- =J ~ - > /'~ n o t. - - m> Jf ; + ,, ;,:: =q q ~ q p., t | ~,-m es. ' > : , - - - ' s g - - - - .: g p /%e. -- ' . 3.NTU.C.MY j ;i,.p. _-m . . '" . , , , I | uN,asste l ..--..,o ,. -.p . g g - - 7 j ARMANSA$f - ~ g [, t

k%, .|* k W AO ?fjh]s I ! - - + , , s q.. q q# ny. . yh Yf4 \ e .uw!g. ', h'. t : ;rhW A v,a . ,,. ,o. ... ee. ; :

< Figure 8. Map of the upper Mississippi Eebeyuent area showing falaosoto sedimentary beatna (contours are depth to basement in ks for , ' the Paloosolo sedimentary rocks and depth to base of the Cretaceous sediments for the Mississippi Babayments the sero contour marks the edge of the embayment), the geologic corrected Douguer gravity anomaly data of Cordell (1977), the looetions of major river systems which have been structurally controlled and the approatsete outline of the New Madrid Rift Comples. The sodioentary basina containing Paleosolo rocks are shown with the anali dot pattern. Cretaceous to present sedimentary rocks in the Misalsaippi Embayment are shown with the large dot pattern. Stratigraphic data are from Schwalb (1942) and , Busohbach (1983).

______. ______. _ _ _ _ . _ . ______. _ _ _ _ - . _ . . _ _ _ _ . . _ _ . ______- ______. - . . _ _ . _ _ _ _ . _ _ _ . _ _ . ______. _ _ _ _ _ . _ _ . ______. _ . . . _ _ . _ . _ _ _ . - _ _ -

e ' U * A. D. '*' ' '' * * + * * * /* ' &* - , S ,

. .. 'I' ' >' | i' , p, |s W..._/ s w. J g/ k ~ % ah;.:.4). , 7'' .y,4 3 0', * J o* g.-4 m +30 p ,,g,k # ' * - ''). ' ' ..b -[* L ATC PH C MOHiA'N - INCIPICNT' RIFTING. * V ' COLLISIONL ATI P ALC020lc, AC A0l AN 0,R00ENY, - CONTINENT , ,, | AL,j '/, .. FORIA ATION OF TRIPLC JyhCTl'dNS') , *a * ' AND AUL AC00 ENS. \ CR ATONIC O ASINS. v, .

B. E.

' * * 'e -)|| ', '| T * '' | 'n*YG - ' ' ? ,* * t C OM P L E X ,'.., '$ '. .. ' /,; y % 4 *** . . , , .. ^ ,o ' - * * * * Q' . \ ,b ' : ,* * , ' , . | 'YWY ^: : f' p//\f ... . ' ...- : . . . ,..... x ; . . . ,:::::;9.. r ...... ,, , .. .r.. :.gy);s .x

' ' ' :. % .O ,

. ' h' '- - 1t,.7,D g , . ' ' h #e . ..- #.,.d \\ L A T , PHCCAMBRI AN - C AHLY C AMBRIAN* u'E S 0Zm OIC - IF ING&*b AT ATL ANTIC M AR0lN, 'N CON flNCNT AL SEP AR Afl0N. IGNCOUS INTRUSIONS AND RE AC(IVATION ' F Allt 0 INTR ACONTINENT AL HIF TS._}0F F AULTS IN AUL AC00 ENS. >

F. ~ C. ' - J i o +- . . .",..confiset=tast ] ..., a ..1 ..t, comeiom te * ' , pl. '{. ., .86 f ,' ' ,.s, '*. = f= ./, ,. ' . ** */ * e ', ' ,* .'l , '.. 7 *g1-. ,fML,'80*fM,, ) f . , . * ' ' ' '' ' ' - *! - , ' , ' , 7 [. M. s , ' ,'[[

Fispire 9. dobematto diagrams illustrating the plate reconstruction of the North Amerto.sn oraton and interactions with adjacent plates and geologio activity of the New Hasleid flif t Complex during the last 600 million years. The outline of the State of Hissouri is shown for loostion and approstmate scale.

_ _ _ _ _ - _ _ - - _ . _ _ _ . . . ______- _ - - - _ _ . - ______. ______. _ - . _ _ . ______. _ _ _ _ . . . _ - - . _ _ . . - _ . - - * 9 , 23

__ _ - - - .. 1

A. tare pRecauBRian: uncuP\eNr RsF,\Na ,,v - 4 vw v y n x f '4 ,w *4 ,>4 v v >f4 >u > * ' \, a 2 '' '' .:v '. CRUST.w A. a ."ww^ " ' y"n' * * ' EXTENSION' ~ g :z _ UPPER, ' M,A,@h q NN \ g k\ B. LA Te PRECA'bfBRIAN-EARL~' Y CAMBRIAN:, \r o -- * ' ''\ y , RIFTfNG s s 3 2 o v 'y 4 c v4 n N i * >a r,2 * s nas ',. . . . , r r. , ' ' E, , , ~ . . v ' ~ . , ** n * ',~)' r; C#'s 'u ,' . % . . < > ^ ' ' ' ' , , {{ ' *(, , ., .,\ , ,

' b '' ~ \ .. , .. g

*s' C, D. PALEOZOIC: SUBSIDENCE

<,a v ,. ,, y v4 2 v (- , v , v.COMPRES3 ION

$~$, ~itidd..e. dd's>$'-- _ ws . .. ME4: '' , h \\\\ M M $ $ , 'bi E. Mesozoic: UPLIFT. REACTIVATION , w ,[ 'y ! 4 eT =- r, * * < , ! ',

=hS'y F. iAre uesozoic-rRessur: SuessOeNCes i \ fuississione mven **

, COMPRESSION i * .- * ,e n, a 44 5, ,s ~.

- ,

,,V'n . . ' . . . ;, ~Qg

- .', ; i s Figure 10. Schematic cross sections for a northwest-southeast' profile through the New Madrid Rift Complex illustrating the evolution of the !. ' rift complex and associated cratonic basins through time.10tages of development shown in parts A through F are related approximately to 4 the map views illustrated in the corresponding diagrams in F.igure 9. ( .

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MICHIGAN RIVER SYSTEM ' ; N IPR,E-at A CIA t> ILLINOIS t OHIO - INDIANA 'j/ i - s sV M.s e E' 4 # j O ~ OUTLINE Gi's. .W . */ |**.* ' ' ' , Y' /. RIFTOF BU.?tED COMPLEX \. z,**,*$*i'\* $ i * , ( *. . ***'.. .. * aG _ } ' ' MIS SOURI * *d i; KENTUCKY i , .. .J.'* ,'' w ", TENNESSEE ARKANSAS .. .. 99.# :; .. .g g f REGIONAL 1 . .g%[.* e, 4:4* , N{ ggay,0ENT COMPRESSIVE ' ', ' ****h , STRESS 4 x x' ; Yp , ' ' ."4 '', \ ; '- - (1 s >4 c .s . ., d s ~W' _ -p' ',4 , ,,a4,, ,, ,4,2*, , '' , > , , a a > \ s ( s,", % >a>a'^>,L ,,1< * 4 ANCIENT ~ ~;- gU'RI'E * D''< * ' *' $ ^v' * ' y ' u|| >e , , FAUL TS v * - , , - , <, a se- RIFT ' < , U"PPER C'RUS T > s ' 1 < > e,a v . . - < ^< ^a v ,* 7 ., y ,4 c v ' * ' " > , < " w, c , < 3'< , , ,a" ' v,a c>c, 4 4' , , = ' - ' p ,u ' < ,e v . EARTHQUAKES ' < , ' , u ,, L ' > n 's I *d ' ' >< ^>< f bh t 4

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' [t s, Figure 11. Block diagram illustrating the present configuration of the buried New Madrid Rift Complex. The structurally controlled '- \ rivers, Paleozoic rocks in cratonio sedimentary basins, and the Mississippi Embayment, all associated with the buried rift complex, \.4 - 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.

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