Geophysical Setting of the Reelfoot Rift

and Relations Between Rift Structures

and the New Madrid Seismic Zone

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1538-E
Cover. Gray, shaded-relief map of magnetic anomaly data. Map area includes parts of 

Missouri, Illinois, Indiana, Kentucky, Tennessee, and Arkansas. Illumination is from the 

west. Figure is taken from figure 5 in this report.
Geophysical Setting of the Reelfoot Rift 

and Relations Between Rift Structures 

and the New Madrid Seismic Zone

By Thomas G. Hildenbrand and John D. Hendricks

INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

Edited by Kaye M. Shedlock and Arch C. Johnston

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1538-E

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

For sale by U.S. Geological Survey, Information Services

Box 25286, Federal Center

Denver, CO 80225

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government.

Library of Congress Cataloging-in-Publication Data

Hildenbrand, T.G.

Geophysical setting of the Reelfoot rift and relations between rift structures and the New 

Madrid seismic zone / by Thomas G. Hildenbrand and John D. Hendricks.

p. cm. — (Investigations of the New Madrid seismic zone ; E) 

(U.S. Geological Survey professional paper; 1538)

Includes bibliographical references.

Supt of Docs, no.: I 19.16: 1538-E

1. Seismology—Missouri—New Madrid Region. 2. Geology, Structural— 

Missouri—New Madrid Region. I. Hendricks, J.D. II. Title. 

III. Series. IV. Series: U.S. Geological Survey professional paper ; 1538. 

QE535.2.U6I59 1995 vol. E 

551.2'2'09778985 s—dc20

[551.2'2'09778985] 93-46275

CIP
CONTENTS

Abstract........................................................................................................................... El

Introduction and Geological Background....................................................................... 1

Magnetic and Gravity Data............................................................................................. 3

Interpretive Techniques................................................................................................... 3

Magnetic Basement Depths .................................................................................. 3

Local Magnetic Sources........................................................................................ 6

Magnetic and Density Boundaries........................................................................ 6

Maximum Depth to Gravity Sources.................................................................... 7

Models................................................................................................................... 9

Profile X-Y (the MGL)................................................................................... 11

Profile W-Z (the PGL)................................................................................... 11

Geophysical Setting........................................................................................................ 11

Southern Terminus................................................................................................ 11

Rift Development............................................................................................ 11

Intrusions......................................................................................................... 15

Central Section...................................................................................................... 15

Reelfoot Graben.............................................................................................. 15

Missouri Gravity Low..................................................................................... 16

Graben-Margin Intrusions............................................................................... 18

Axial Intrusions............................................................................................... 18

Northern Terminus................................................................................................ 19

Rift Zones........................................................................................................ 19

South-Central Magnetic Lineament (SGML)................................................. 19

Paducah Gravity Lineament (PGL)................................................................. 20

Fault Zones...................................................................................................... 21

Seismicity........................................................................................................................ 21

Discussion....................................................................................................................... 25

Summary and Conclusions.............................................................................................. 26

Acknowledgments........................................................................................................... 27

References Cited............................................................................................................. 27

FIGURES

1. Reference map of the northern Mississippi Embayment region ................................................................................. E2

2. Color shaded-relief, reduced-to-pole magnetic anomaly map .................................................................................... 4

3. Complete Bouguer gravity map................................................................................................................................... 5

4. Maximum depth to magnetic basement map............................................................................................................... 6

5. Shaded-relief map of the first-vertical derivative of the reduced-to-pole magnetic anomaly data.............. ............... 7

6. Pseudogravity field map.............................................................................................................................................. 8

7. Map of gradient maxima of the pseudogravity field or magnetization boundaries..................................................... 9

8. Ideal body analysis...................................................................................................................................................... 10

9. Simultaneous inversion of magnetic and gravity data across the source of the Missouri gravity low........................ 12

10. Simultaneous inversion of magnetic and gravity data across the source of the Paducah gravity lineament.............. 13

11. Map showing regional geophysical features............................................................................................................... 14

12. Color shaded-relief map of the gravity field, upward-continued 6 km....................................................................... 17

13. Reduced-to-pole magnetic anomaly map of the New Madrid seismic zone with earthquakes epicenters and

	boundaries of major geophysical features................................................................................................................... 22

14. Complete Bouguer gravity map of the New Madrid seismic zone with earthquake epicenters and boundaries of

	major geophysical features.......................................................................................................................................... 23

15. Earthquake occurrence map ........................................................................................................................................ 24

III

GEOPHYSICAL SETTING OF THE REELFOOT RIFT

AND RELATIONS BETWEEN RIFT STRUCTURES

AND THE NEW MADRID SEISMIC ZONE

By Thomas G. Hildenbrand and John D. Hendricks

ABSTRACT

Magnetic and gravity data in the northern Mississippi 

Embayment and surrounding region provide a geologic picture of the subsurface that indicates a long and complex tectonic and magmatic history. The Reelfoot rift possibly 

developed along a preexisting shear zone in early Paleozoic 

time and extends from east-central Arkansas to western Kentucky, where it seems to merge with the Rough Creek graben. At the juncture of the Reelfoot and Rough Creek 

grabens, interpreted northwest-trending faults and a zone of 

intrusive rocks form the Paducah gravity lineament, which 

may represent a block-faulted region. Most of the magmatism associated with the Reelfoot rift may have occurred during Cretaceous reactivation, in the sense that the graben's 

margins apparently provided the channelways for the 

ascending magma. Older igneous events may have emplaced 

dense, magnetic intrusions near the axis of the Reelfoot graben, particularly during the late Paleozoic. A southeasttrending gravity low may delineate a 100-km-wide Precambrian batholith that intersects the Reelfoot graben.

Several potential-field features seem to focus earthquake activity. On a regional scale, the intersection of the 

Reelfoot graben and the interpreted Missouri batholith may 

represent a weak zone susceptible to seismic activity. The 

proposed granitic weak zone would contrast with more competent, flanking metamorphic terranes. In our model for the 

origin of the New Madrid seismic zone, the rift axis acts as a 

crustal flaw along which present-day earthquakes concentrate. The presence of the granitic weak zone may be a contributing factor in restricting the lateral extent of the seismic 

zone along the rift axis. Local changes in the trend of seismic zones within the New Madrid system are related to 

obstacles that divert strain along paths of less resistance. In 

particular, seismic zones change trend in response to preferred directions of strain release related to intrusions and 

older faults.

INTRODUCTION AND 

GEOLOGICAL BACKGROUND

The importance in understanding the structural development of the Reelfoot rift stems from its spatial correlation 

with the New Madrid seismic zone (fig. 1). The graben associated with this rift contains the area of principal present-day 

seismicity in the upper Mississippi Embayment region 

(Hildenbrand, 1985a; Himes and others, 1988) and, in particular, the epicentral line of the devastating (magnitude 8) 

1811-12 New Madrid earthquake series (Fuller, 1912; Nuttli, 1982). Earthquake statistics indicate that an earthquake of 

6 or greater magnitude is expected on average at least once 

every 100 years. Risk studies indicate that an event of magnitude in the 6 to 7 range could cause today about $3.6 billion 

in property losses (Hamilton and Johnston, 1990). Thus, the 

study of rift structures and their relation to strain release is an 

integral part of the concerted effort to reduce earthquake 

hazards in the Mississippi Embayment region.

Because Cenozoic sedimentary rocks fill the Mississippi Embayment, the structure and geology of underlying 

rocks are known only from drill-hole records and geophysical data. This paper reviews previous geophysical (primarily 

potential-field) studies and considers the tectonic and magmatic development of the study area and their interrelationships. With an understanding of the geophysical setting of 

the Reelfoot rift, structures near seismic zones are described 

in detail and investigated as to their influence on the release 

of seismic energy

We define "rift" here as a fundamental flaw in continental crust along which the lithosphere has ruptured under 

extension. A clear distinction is made between rift and graben: the graben is one near-surface manifestation of the rift. 

Thus, a rift is a large tectonic system with many related 

structures that may occur anywhere in the crust with horizontal dimensions of several hundred kilometers (see also 

Cordell, 1978).

El
E2

92C

INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

91° 90° 89° 88° 87°

Illinois ,M,w) >$

/&{£>. *••*..

Indiana

Northern limit of coastal-plain material 

of the Mississippi Embayment

EXPLANATION

Mafic or ultramafic intrusions within the 

Mississippi Embayment identified in 

drill-hole cuttings. Intrusions labeled 

PO and LO are also locations of the 

Pure Oil McGregor No. 1 and the Lion 

Oil Company No. 1 wells, respectively

Mafic intrusion within the Mississippi

Embayment interpreted from the magnetic 

field. Approximate boundaries of 

intrusions determined from zero contour 

of associated anomaly on a second vertical 

derivative map

City or town

Long dashed lines show principal magnetic 

lineaments reflecting faulting and lithologic 

contrasts in magnetic basement, a and b 

are lineaments discussed in text

Fault -Dashed where inferred

Possible or hypothetical fault. Locations based 

on subsurface data or exceptionally strong 

lineaments from aerial photographs

Approximate margins of the Reelfoot graben

Locations of the Big Chief (BC) and Dow 

Chemical 1 Wilson (DW) drill holes

Earthquake epicenter

Figure 1. Reference map of the northern Mississippi Embayment region (after Hildenbrand, 1985a).
GEOPHYSICAL SETTING OF THE REELFOOT RIFT, NEW MADRID SEISMIC ZONE E3

The Mississippi Embayment is a trough filled with Late 

Cretaceous and Cenozoic clastic sediments that reach an 

aggregate thickness of roughly 1.5 km near the Mississippi 

River at the southern boundary of the study area. Due to late 

Paleozoic and Mesozoic uplift, Cretaceous sedimentary 

rocks (maximum thickness of about 0.3 km) rest uncomformably on Late Cambrian to Ordovician limestone, shale, 

and sandstone. In southeast Missouri, Precambrian rocks 

crop out in the St. Francois Mountains and are composed of 

unmetamorphosed volcanic and related epizonal intrusive 

rocks that are about 1.5 to 1.3 Ga (Bickford and others, 1981; 

Kisvarsanyi, 1981). The St. Francois Mountains are within 

the Ozark Plateau physiographic province that extends primarily over southern Missouri and northern Arkansas. Precambrian basement also rises to the east along the northsouth-trending Cincinnati arch in central Kentucky and Tennessee. To the north, basement descends beneath the Illinois 

Basin.

Burke and Dewey (1973) suggested that the Mississippi 

Embayment originated as a Mesozoic failed rift. Ervin and 

McGinnis (1975) synthesized gravity with seismic, stratigraphic, and petrologic data and suggested that the rift, 

which they called the Reelfoot rift, formed in Late Proterozoic or early Paleozoic time and was reactivated during the 

Cretaceous. Cordell (1977) suggested that the Reelfoot rift 

extends northward into southern Illinois and western Kentucky, where broad gravity highs are interpreted as evidence 

for fossil rift cushion at the crust-mantle boundary. Subsequent potential-field studies (Hildenbrand and others, 1977; 

Kane and others, 1981; Hildenbrand and others, 1982; Braile 

and others, 1982; Ravat, 1984; Hildenbrand, 1985a; Hildenbrand and others, 1992) have provided greater details of 

Reelfoot rift structures and its geophysical setting. An eastwest trending graben (Rough Creek graben), defined with 

gravity, magnetic, and drill hole data (Soderberg and Keller, 

1981) meets the northeast-trending Reelfoot rift in southern 

Illinois and western Kentucky. Both the Reelfoot rift and 

Rough Creek graben (fig. 1) probably developed in Cambrian time (about 550 Ma) in conjunction with the formation 

of a passive continental margin to the south of the study area 

(Thomas, 1991).

There is no evidence for major Cambrian volcanism in 

the embayment related to the initial phases of rifting 

(Hildenbrand, 1985a). Igneous rocks as young as Late Cretaceous, however, intrude Precambrian basement and 

younger formations within the study area. For instance, 

emplacement of intrusions during the Devonian (Zartman 

and others, 1967) may have been accommodated by the Ste. 

Genevieve fault zone (fig. 1). In the Illinois-Kentucky fluorspar district, near lat 37°30' N. and long 88° 15' W., Permian 

mica peridotite dikes and diatremes are restricted to northwest-trending fractures (Watson, 1967). Permian lamprophyric dikes have been encountered in drill holes in rocks of 

Cambrian and Ordovician age on the Pascola arch (fig. 1) 

(Zartman, 1977). In wells in southwest Tennessee, Upper 

Cretaceous sediments overlie weathered syenite (Moody,

1949; Kidwell, 1951; Caplan, 1954) that intrude Early 

Ordovician sediments. The Magnet Cove complex of ring 

dikes in east-central Arkansas (Erickson and Blade, 1963; 

Hendricks, 1988) and the nepheline syenite intrusions near 

Little Rock (Gordon and others, 1958) are composed of Late 

Cretaceous alkalic rocks (Zartman, 1977). Large plutonic 

bodies (fig. 1) were emplaced along the margins of the Reelfoot graben, possibly during Cretaceous time (Hildenbrand, 

1985a).

MAGNETIC AND GRAVITY DATA

The digital set of aeromagnetic data for the study area 

was compiled by merging four previously compiled databases of Illinois (Hildenbrand and others, 1993), Missouri 

(Hildenbrand and Kucks, 1991), east-central Arkansas 

(Hildenbrand and others, 1981), and the remaining parts of 

the study area (Hildenbrand and others, 1983). A 1-km grid 

of magnetic values was prepared for each aeromagnetic survey within these four regions using minimum curvature 

(Briggs, 1974) and a computer algorithm developed by 

Webring (1982). The grids were analytically continued to a 

consistent 305 m above average terrain before compositing 

to create the four compatible data sets. The four data sets 

were then merged to generate the final data set. Data were 

reduced to the north magnetic pole, a procedure that shifts 

the anomalies to positions above their sources. The resulting 

magnetic anomaly map is shown in figure 2.

The gravity data (fig. 3) were taken from the Department of Defense (DOD) database. All data were tied to the 

IGSN-71 gravity datum and reduced to complete Bouguer 

anomaly values using a reduction density of 2.67 g/cm3 and 

the 1967 formula for theoretical gravity (Cordell and others, 

1982).

INTERPRETIVE TECHNIQUES

Before investigating the geophysical setting of the 

Reelfoot rift, the potential-field data were filtered and 

inverted to facilitate interpretation. Techniques used 

include: calculation of magnetic gradient widths to estimate 

basement depths; derivative filtering to enhance near-surface, local magnetic sources; mapping maximum-horizontal-gradient magnitudes to locate magnetization and density 

boundaries; applying ideal body theory to determine maximum depths to gravity sources; and 2'/2 -dimensional modeling to find various details of magnetic and gravity source 

distributions.

MAGNETIC BASEMENT DEPTHS

Magnetic basement is defined here as crystalline rocks. 

Phanerozoic sedimentary rocks are assumed to produce little 

or no regional effect on the magnetic field.
E4

38°

36°

34°

INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE 

92° 90° 88° 86°

-400 -300 -200 

100

-100 0 

200

100 200

300

300 NANOTESLAS 

400 KILOMETERS

Figure 2. Color shaded-relief, reduced-to-pole magnetic anomaly map. Models along profiles X-Y and W-Z are shown in figures 9 and 

10, respectively. Anomalies A through V are discussed in the text. ATF and SGML represent the Arkansas transform fault and the southcentral magnetic lineament. Illumination is from the west.

An interpretational method developed by Vacquier and 

others (1951) was employed to roughly estimate the maximum depth to magnetic basement (fig. 4X In this method, we 

compared observed anomalies with theoretical anomalies 

produced by vertical prisms of various dimensions and

different orientations of the Earth's ambient field. It is 

assumed that all magnetic sources are uniformly polarized in 

the same direction as the Earth's present field. When the 

areal shape of a theoretical anomaly closely matches that of 

the observed anomaly, maximum depth to the magnetic
38°

36°

GEOPHYSICAL SETTING OF THE REELFOOT RIFT, NEW MADRID SEISMIC ZONE 

92° 90° 88°

E5

86°

34°

-60 -50 -40 

100

-30 -20 

200

-10 0 

300

10 MILLIGALS 

400 KILOMETERS

Figure 3. Complete Bouguer gravity map. Models along profiles X-Y and W-Z are shown in figures 9 and 10, respectively. Anomalies 

A through S and V are discussed in the text. Profiles labeled SI. S2, S3, Nl, and N2 are used in the ideal body analysis shown in figure SB. 

Similarly, profiles PI, P2, and P3 are related to the ideal body analysis shown in figure 8C. ATF, MGL, and PGL represent, respectively, 

the Arkansas transform fault, Missouri gravity low, and the Paducah gravity lineament. Dots denote gradient maxima or density boundaries.

source is determined by comparing lengths of horizontal gra- causative body generally has sloping or irregular sides;

dients associated with the two anomalies. The method yields therefore, its actual depth of burial would be shallower than

maximum depths to magnetic sources because the theoreti- the computed depth.

cal models used in the curve-matching process are assumed In the study area, 800 estimates of maximum depth to

to have vertical sides. In real geologic situations, the magnetic basement, relative to mean sea level, were
E6

92°

INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

90° 88° 86°

38'

36C

34°

EXPLANATION

Maximum depth to magnetic

basement, in kilometers

-6.00

100

-3.75 -1.50 0 KILOMETERS

200 300 400 KILOMETERS

Figure 4. Maximum depth to magnetic basement. Small triangles denote locations of depth estimates. Depths shown are relative to 

mean sea level.

determined. The depths reported here must be regarded as 

rough estimates. Magnetic basement clearly reaches to depths 

greater than 5 km in the Reelfoot and Rough Creek grabens.

LOCAL MAGNETIC SOURCES

In areas of steep, broad magnetic gradients, low-amplitude and spatially restricted anomalies (related to near-surface features) and other subtle features or trends tend to 

escape notice on the reduced-to-pole map. To resolve these

short-wavelength anomalies, a first-vertical-derivative filter 

(Bhattacharyya, 1965) is applied to the reduced-to-pole data. 

The first-vertical-derivative map (fig. 5) thus enhances subtle local and shallow features and reduces the effects of broad 

regional gradients. The expression of the Reelfoot graben is 

apparent in figure 5.

MAGNETIC AND DENSITY BOUNDARIES

Maxima in the gravity horizontal gradient occur near 

steep or vertical boundaries separating contrasting densities
GEOPHYSICAL SETTING OF THE REELFOOT RIFT, NEW MADRID SEISMIC ZONE

92° 90 88C

38

34

Figure 5. Shaded-relief map of the first-vertical derivative of the reduced-to-pole magnetic anomaly data. Illumination is from the west.

(Cordell, 1979). In a similar way, magnetization boundaries 

can be defined by first making a pseudogravity transformation (fig. 6) of the magnetic field (Baranov, 1957) and then 

calculating the horizontal gradient (Cordell and Grauch, 

1982). On these gradient maps, lines drawn along ridges 

(i.e., continuous maxima) of high-horizontal-gradient magnitudes correspond to these steep boundaries. The lines are 

drawn automatically with the aid of a computer (Blakely and 

Simpson, 1986).

For the present study, the density boundaries determined by this technique are superimposed on the gravity 

anomaly map in figure 3, and magnetization boundaries are 

shown in figure 7. If the boundaries have shallow dips or if 

contributions from adjacent sources are significant, the

maximum gradient will be shifted a certain distance from the 

uppermost part of the boundary (Grauch and Cordell, 1987). 

To make the pseudogravity transformation, the direction of 

the total magnetization was assumed to have an inclination 

of 66° N. and a declination of 0°.

MAXIMUM DEPTH TO GRAVITY SOURCES

For a given gravity anomaly, fundamental associated physical properties can be constrained for an entire 

set of reasonable sources (Parker, 1974, 1975). For example, densities can be calculated for various source depths 

to produce a plot shown in figure 8A. On the basis of
E8 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

92° 90° 88° 86°

38°

36°

34°

-125 

100

-50 25

200 300

100 MILLIGALS

400 KILOMETERS

Figure 6. Pseudogravity field. Northwest-trending, dashed lines bound the source of the Missouri gravity low as defined in the gravity 

field (figs. 3 and 12). (Note that this southeast-trending source may not cross, the Reelfoot rift and, thus, may not extend far into Tennessee.)

rock-property studies (i.e., a geological bound), the maximum source depth can be determined by considering the 

range of permissible source densities. Huestis and Ander 

(1983) described a computer algorithm to find the greatest lower bound on density. The body with this minimum density is called the ideal body. The ideal body is 

unique in that a higher density leads to an acceptable

source (limited only by a reasonable maximum density) 

and a lower density is physically unacceptable.

The ideal body method was applied to two prominent gravity features (fig. 3). One feature, called the Missouri gravity low (MGL) (Guinness and others, 1982), 

extends more than 700 km across Missouri southeast to 

western Tennessee. The other feature, called the Paducah
38°

36°

34°

GEOPHYSICAL SETTING OF THE REELFOOT RIFT, NEW MADRID SEISMIC ZONE 

92° 90° 88°

E9

86°

100 200 300 400 KILOMETERS

Figure 7. Gradient maxima of the pseudogravity field or magnetization boundaries.

gravity lineament (PGL) (Hildenbrand and others, in 

press), is a northwest-trending zone of gravity highs near 

the juncture of the Reelfoot and Rough Creek grabens in 

southern Illinois and western Kentucky. The resulting density-depth trade-off curves for selected profiles across 

these gravity features are shown in figures 8S and 8C. 

Assuming a maximum density contrast of 0.15 g/cm3 , the 

source of the Missouri gravity low lies at a maximum 

depth of about 3 km (fig. SB). Similarly, the top of the 

body expressed as the Paducah gravity lineament is shallower than about 6 km for a maximum density contrast of

0.25 g/cm3 (fig. 8C). It is interesting to note that maximum depths to the source of the Paducah gravity lineament are comparable to the maximum depths to magnetic 

basement in figure 4. Both results indicate that basement 

deepens westward into the Rough Creek graben. The 

implications of these results are discussed below.

MODELS

The principal goal of potential-field studies is to detect 

and quantify changes in magnetic and mass properties at
E10 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

depth. To translate observed magnetic and gravity anomalies 

into a meaningful geologic picture of the subsurface requires 

inversion or modeling programs. We used a 2 lh -dimensional 

modeling program, SAKI (Webring, 1985), based on generalized inverse theory to derive upper-crustal models. The 

program requires an initial estimate of model parameters 

(depth, shape, density, and magnetization of sources) and 

then varies selected parameters in an attempt to reduce the 

weighted root-mean-square error between the observed and 

calculated gravity and magnetic fields. Due to the lack of 

information on remanent magnetization, total magnetization 

was assumed to be in the direction of the Earth's present-day 

magnetic field (inclination = 66° N. and declination = 0°). 

Two selected profiles (X-Y and W-Z, figs. 2 and 3) attempt 

to characterize, respectively, southeast-trending structures in 

Missouri and the upper crust at the juncture of the Reelfoot 

and Rough Creek grabens in western Kentucky and southern 

Illinois.

Knowledge of several parameters facilitated the selection of the initial estimate for the models. Several hundred 

measurements of physical properties of rocks from drill 

holes and outcrops in the St. Francois Mountains (Eva Kisvarsanyi, Missouri Department of Natural Resources, written commun., 1990) were used to assign a value for density 

and susceptibility of the Precambrian basement granite-rhyolite terrane. Statistical measurements on these laboratory 

results (Joseph Rosenbaum, written commun., 1991) indicate that unweathered St. Francois granite-rhyolite rocks 

have an apparent average density of 2.67 g/cm3 and susceptibility of 0.009 SI (4rt cgs = SI). Metamorphic basement was 

assigned a susceptibility of 0.006 SI (Carmichael, 1982).

Geophysical logs from drill holes suggest a wide range 

of densities for Paleozoic units. For example, average densities range from 2.4 g/cm3 for the Lamotte Sandstone to 2.73 

g/cm3 for the Eminence Dolomite. A value of 2.65 g/cm3 

was obtained for the average density of the Paleozoic section 

(based on visual inspection of density logs) and agrees with 

the estimated density of similar rocks at depth in Oklahoma

Figure 8 (facing column). Ideal body analysis. A, Trade-off 

curve for ideal body. The curve represents the greatest lower bound 

on density. The horizontal line denotes the maximum density permitted by geologic constraints. The intersection of the curve and 

line leads to the maximum depth to the body (Zmax). B, Ideal body 

theory applied to gradients along the Missouri gravity low shown in 

figure 3 (Nl, N2, SI, S2, S3). Assuming a maximum density contrast of 0.15 g/cm3 , the associated source lies at a maximum depth 

of about 3 km. C, Ideal body theory applied to gradients of the Paducah gravity lineament shown in figure 3 (PI, P2, P3). For a maximum density contrast of 0.25 g/cm , the associated sources are 

shallower than about 6 km.

C/0

Geological bound

0.2-

B

o.iDEPTH,IN KILOMETERS

r i F i i 

12345 

DEPTH, IN KILOMETERS
GEOPHYSICAL SETTING OF THE REELFOOT RIFT, NEW MADRID SEISMIC ZONE Ell

(based on density-depth data from drill samples—Athy, 

1930; Cordell, 1977). This value was used in our models. A 

more detailed model for the Paleozoic is considered inappropriate due to the lack of information on depths and densities 

of the Paleozoic units within the broad study area. The estimated depths to magnetic basement (fig. 4) were used to 

define the base of the Paleozoic sedimentary rocks.

The interpretation of potential-field data yields nonunique solutions because an infinite number of geometrical models will have an associated field that closely 

matches the measured field. Available drill-hole information, simultaneous inversion of gravity and magnetic data, 

and geological reasoning have aided in deriving a suitable 

geophysical model to represent the geologic situation in 

the study area. Increasing the density while decreasing the 

thickness of a proposed intrusion, with no change in 

mass, will generally not produce an appreciable change in 

the computed fields.

PROFILE X-Y (THE MGL)

Because Precambrian basement along this profile lies at 

shallow depths or crops out, the surface geology along the 

200-km-long profile (fig. 9) is known (Kisvarsanyi, 1979) 

and consists primarily of unmetamorphosed St. Francois 

granite and rhyolite except for a 75-km-wide zone of granitic 

gneiss and gneissic granite. Lower density granitic rock, 

which contrasts with flanking, denser, metamorphic rock, 

appears at depth in our model to account for the prominent 

gravity low along the profile. Because the ideal body analysis above leads to a maximum depth of 3 km for this granitic 

body, surface terranes are assumed to extend to a depth of 2 

km. A mafic intrusion may lie along the southwest margin of 

the buried granitic body. Because profile X-Y was selected 

to understand the source of the Missouri gravity low, no 

attempt was made to model many of the short-wavelength 

features along the profile.

PROFILE W-Z (THE PGL)

Before inverting the data along profile W-Z (figs. 2 

and 3), individual anomalies related to inferred intrusions 

were modeled to provide reasonable estimates of their 

thicknesses. A prominent gravity anomaly (R, fig. 3) was 

modeled using a three-dimensional inversion algorithm 

(GI3, Cordell, 1968). Assuming this is an intrusion and 

has a mafic composition (e.g., gabbro) with a density of 

2.85 g/cm3 , the modeling results suggest that the bottom 

of the intrusion lies at about 15 km below the surface. A 

similar modeling exercise was carried out for magnetic 

anomalies T and U, figure 2 (using the algorithm SAKI). 

If the depth to the bottom of the intrusions is placed at 10

km, calculated susceptibilities were unreasonably high (> 

0.15 SI); therefore, the base of these intrusions is 

assumed to also lie at 15 km.

Profile W-Z crosses a prominent magnetic low. In the 

Midcontinent, other linear magnetic lows of similar intensity 

and length exist but are uncommon. The low may reflect a 

trough filled with low-density, reversely magnetized rocks 

(e.g., an old rift filled with volcanic rocks). Ravat (1984) 

modeled a reversely magnetized body in the midcrust to 

explain the presence of this pronounced magnetic low. The 

existence of reversely magnetized rocks is plausible but not 

necessary to explain the steep gradient between the magnetic 

low and adjacent highs. We assume here that the crystalline 

rocks beneath the sedimentary sequence in the region of this 

magnetic low have negligible magnetic properties (i.e., zero 

susceptibilities).

In the resulting model (fig. 10), the interpreted northwest-trending unmagnetic Precambrian zone is flanked by 

mafic intrusions. As will be discussed later, the intrusions 

are related to the south-central magnetic lineament north of 

this zone and to the Paducah gravity lineament to the south.

GEOPHYSICAL SETTING

Different tectonic processes during and subsequent to 

the formation of the Reelfoot rift have produced three distinct regions with comparatively different structures. For 

example, near its southern terminus in central Arkansas, the 

tectonic development of Ouachita fold belt greatly influenced the rift's evolution. Along the central part of the rift, 

notable post-rift geologic processes include thick accumulations of sediments, uplift, emplacement of massive intrusions, and present-day active faulting. At its northern 

terminus in southern Illinois and western Kentucky, the rift's 

evolution appears to be intimately related to the development 

of the Rough Creek graben and mapped fault systems. Due 

to these significantly different structural domains, discussions on the geophysical setting of the Reelfoot rift are subdivided into descriptions of associated structures at its 

termini and in its central section.

SOUTHERN TERMINUS 

RIFT DEVELOPMENT

The Reelfoot rift developed during the Late Proterozoic-early Paleozoic opening of the lapetus (proto-Atlantic) 

Ocean. Along the resulting North American continental margin, the Appalachian-Ouachita erogenic belt formed later, in 

late Paleozoic time. Hendricks (1988) reviewed several 

plate-tectonic evolutionary models of the Ouachita system 

and proposed one that satisfies many fundamental geologic
E12 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

P o

§< 

oz

1,000

500

_ Calculated anomaly 

__ \

25 50 75 100 125 150 175 200

Observed anomaly

^^

+ 

_+

Calculated anomaly .

i i i i I i i i i I i i i i ff

X(SW) DISTANCE, IN KILOMETERS Y(NE)

i i i i I i i i i I i i I i ii i l I i l i l I i i l i I i i l i

(2.91 g/cm3, 0.049 SI)

EXPLANATION

St. Francois granite-rhyolite terrane (2.67 g/cm3,0.009 SI) 

Granitic gneiss and gneissic granite (2.74 g/cm3,0.006 SI)

(2.79 g/cm3, 0.038 SI) (2.74 g/cm3, 0.057 SI)

Intermediate and mafic hypabyssal intrusions 

Buried granitic batholith (2.67 g/cm3,0.009 SI)

Figure 9. Simultaneous inversion of magnetic and gravity data across the source of the Missouri gravity low showing computed and actual magnetic and gravity fields (upper diagrams) and model (lower diagram) for profile X-Y in figures 2 and 3. Numbers in model are 

density contrast (g/cm3 ) and susceptibility contrast (SI). In the magnetic field diagram, open circles represent actual (interpolated) magnetic 

data; the line represents the computed magnetic field of the model. In the gravity field diagram, crosses denote actual (interpolated) gravity 

data; the line represents the computed gravity field of the model.

and geophysical constraints in Arkansas. In his model, the 

proto-North American continent began to breakup near the 

beginning of Cambrian time to form a passive margin along 

the eastern and southern edge of the present-day craton. Passive rifting and strike-slip faulting produced microcontinents 

(rifted blocks). Of interest here is that, in southwestern 

Arkansas, a triple junction formed at a corner of a rifted 

block (Hendricks, 1988) in Cambrian time (about 550 Ma). 

At this junction, the Reelfoot rift and Oklahoma aulacogen

represented two of the failed arms and extended into the craton at high angles from bends in the North American 

continental margin. Syn-rift igneous rocks associated with 

the Oklahoma aulacogen have radiometric dates between 

577 and 525 Ma (Thomas, 1991).

In eastern Arkansas, a southeast-trending transform 

fault (referred to here as the Arkansas transform fault, fig. 

11) marked the boundary between a rifted block and the craton. Later, in Mississippian time, this major structure played
GEOPHYSICAL SETTING OF THE REELFOOT RIFT, NEW MADRID SEISMIC ZONE E13

200-

125

00

&

B

W (SW) DISTANCE, IN KILOMETERS

0 25 50 75 100 125

I l I I I I l I I I I I l I I I I I I I I l I I I I I

Z (NE)

10-

15-

EXPLANATION

Paleozoic sedimentary rocks (2.65 g/cm3 , 0.0 SI)

Precambrian sedimentary rocks (?) (2.72 g/cm3 , 0.0 SI)

Igneous intrusion

Igneous intrusion consisting of rocks with varying physical properities

Precambrian metamorphic rocks (2.74 g/cm3 , 0.006 SI)

Figure 10. Simultaneous inversion of magnetic and gravity data across the source of the Paducah gravity lineament showing computed 

and actual magnetic and gravity fields (upper diagrams) and model (lower diagram) for profile W-Z that crosses the juncture of the Reelfoot 

and Rough Creek grabens (figs. 2 and 3). Numbers in model are density contrast (g/cm3 ) and susceptibility contrast (SI). In the magnetic 

field diagram, open circles represent actual (interpolated) magnetic data; the line represents the computed magnetic field of the model. In 

the gravity field diagram, crosses denote actual (interpolated) gravity data; the line represents the computed gravity field of the model.
E14

38°

36°

INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE 

92° 90° 88° 86°

34°

100 200 300 KILOMETERS

Figure 11. Regional geophysical features. Black bodies represent dense, magnetic plutons of Ordovician-Cretaceous age 

(except the Bloomfield pluton, which may be Cambrian or older). Gray bodies denote intrusive complexes of Cambrian 

through Cretaceous age. Igneous bodies A-P and V produce anomalies that are labeled with identical letters on figures 2 

and 3. SGML, south-central magnetic lineament; PGL, Paducah gravity lineament.

an important role in the formation of the Ouachita fold belt 

because it bounded the Ouachita mobile belt on the northeast 

(Hendricks, 1988). Northwest movement of the rifted block 

due to compression during the closing of the lapetus Ocean 

continued into Pennsylvanian time. Sediments were thrust 

over cratonic platforms to form the Ouachita mountains. 

Wide-angle reflection and refraction data show the continental crust beneath southward-dipping thrust sheets in the 

southern part of the Ouachita Mountains (Keller and others, 

1989). At the end of the Ouachita orogeny, the North and 

South American and African continents were juxtaposed and 

constituted a portion of the Pangea supercontinent (Van Der

Voo, 1988). In Late Jurassic time, Pangea began to separate, 

eventually forming the Gulf of Mexico.

The geologic history of Arkansas therefore suggests 

that the triple junction, and thus the ancestral southern terminus of the Reelfoot rift, was essentially destroyed or obliterated during thrusting of the Ouachita mobile belt onto the 

craton. Inspection of regional gravity and magnetic anomaly 

maps provide no evidence for the southwestward continuation of the Reelfoot graben beyond its juncture with the 

Arkansas transform fault. This fault and flanking, dense, 

magnetic intrusions are clearly expressed in the potentialfield maps (figs. 2, 3, and 5).
GEOPHYSICAL SETTING OF THE REELFOOT RIFT, NEW MADRID SEISMIC ZONE E15

INTRUSIONS

Hendricks (1988) suggested that the magnetic and gravity highs along the northwest-trending Arkansas transform 

fault (A and C-F, figs. 2 and 3) delineate mafic or ultramafic 

alkalic intrusions of Cretaceous age. Drill holes in the vicinity of anomaly F (fig. 2) encountered nepheline syenite at a 

depth of about 1.5 km directly beneath Cretaceous sediments 

(Nockolds and Alien, 1954). Peridotite at a depth of 1.1 km 

was encountered beneath Upper Cretaceous strata near 

anomaly E. Magnetic basement (fig. 4) sharply rises to 

depths shallower than 1.5 km in regions intruded by these 

Cretaceous plutons.

Two major igneous complexes, Magnet Cove and Little 

Rock (respectively, C and D, fig. 2), formed in Late Cretaceous time near the juncture of the Arkansas transform fault 

and the northwest margin of the Reelfoot graben (Erickson 

and Blade, 1963; Malamphy and Vallely, 1944). The 

exposed portion of the Magnet Cove complex is a 12-km2 

alkalic ring-dike complex that is divided into three zones: an 

ijolite core, an intermediate ring of trachyte and phonolite, 

and an outer ring of nepheline syenite and jacupirangite 

(Erickson and Blade, 1963). Assuming a laccolith-shaped 

body that extends to a depth of about 8 km, Hendricks (1988) 

calculated susceptibility and density contrasts of 0.314 SI 

and 0.3 g/cm3 to produce the 8,000-nT (at 300 m above 

ground) and 30-mGal positive anomalies of Magnet Cove. 

The Little Rock complex, however, is expressed as a 25- 

mGal gravity low that Hendricks (1988) modeled as a 20- 

km-wide nepheline syenite body. He proposed that a 

regional-gravity-high ridge, extending from southeast 

Arkansas to Louisiana and central Mississippi, delineates a 

dense parental pluton at depth from which the low-density 

nepheline syenite was derived by fractionation and differentiation. This deep parental pluton and related smaller, shallow intrusions (anomalies A, and C through F) probably 

intruded crust along the Arkansas transform fault.

CENTRAL SECTION 

REELFOOT GRABEN

From east-central Arkansas to southwestern Kentucky, 

the Reelfoot graben is clearly expressed (fig. 5) as a northeast-trending, 70-km-wide feature with exceptionally linear 

margins (Kane and others, 1981; Hildenbrand, 1985a). The 

subdued anomaly pattern over the graben reflects the relatively greater depth of crystalline basement beneath the thick 

sedimentary section. Magnetic basement on the northwest 

and southeast flanks of the graben (fig. 4) is approximately 

1.2 and 2.5 km deep, respectively. Here, magnetic basement 

primarily coincides with Precambrian basement. Within the 

graben, the estimated average depth to magnetic sources of 

about 4 km indicates a sedimentary section that is roughly 2

km thicker than that along its flanks. However, in several 

regions within the graben, igneous intrusive complexes may 

have been emplaced in Phanerozoic rocks, resulting in 

underestimates for the thickness of the sedimentary section. 

These intrusive complexes are characterized by magnetic 

and gravity highs (anomalies G, N, O, and P, figs. 2 and 3). 

For example, seismic reflection data over the igneous intrusions expressed as anomaly O indicate that Precambrian 

basement is roughly 5 km deep (Richard L. Dart, written 

commun., 1991). In contrast, the regional magnetic basement lies at only 3 km. Hildenbrand and others (1992) also 

obtained a 3-km depth in a model (using SAKI) for the top 

of an intrusion in this region. The average increase in sediment thickness associated with the Reelfoot graben is therefore closer to 4.5 km.

Although the graben margins are shown as lines (fig. 

11), the associated fault zones probably have substantial 

widths and extend outward for some distance from these 

lines (which represent the boundaries between the floor and 

margins of the graben). Models derived from ground-based 

magnetometer data (Hildenbrand, 1982) indicate that the 

southeast margin, near Memphis, Tenn., is a 5-km-wide fault 

zone. In this region, seismic reflection studies by Crone 

(1992) indicate similar widths (4-8 km) for the margin of the 

graben. Elsewhere, the margin of graben may be substantially wider. For example, a linear magnetic feature crossing 

the Arkansas-Missouri State line at long 90°W. (fig. 5), 

which may be a rift-related fault, parallels and lies 25 km 

from the northwest margin of the graben.

Geophysical basement that flanks the graben may have 

different tectonic histories (Kane and others, 1981; Hildenbrand, 1985a). Southeast-trending features northwest of the 

graben (figs. 2 and 7) represent fracture systems in an old 

metamorphic terrane (about 1.6 Ga; see Kisvarsanyi, 1974, 

1984). Some of these features terminate abruptly along the 

graben's northwest margin. Southeast of the graben, the geophysical grain is predominantly northeast. This difference in 

structural trends prompted Hildenbrand (1985a) to propose 

that the graben developed along a zone (shear zone?) separating contrasting basements.

On the other hand, Nelson and Zhang (1992) suggested 

that this change in magnetic grain across the rift is due to a 

westward offset in the south-trending Grenville front in central Kentucky. They proposed that the Reelfoot rift developed along the Grenville front and separates a 1.5- to 1.3-Ga 

granite-rhyolite terrane on the northwest from a younger 

metamorphic terrane on the southeast. The westward displacement of the Grenville metamorphic terrane provided an 

explanation for the metamorphic rock encountered in a drill 

hole in the Reelfoot graben (DW, fig. 1), dipping reflectors 

deep in the crust (a characteristic of Grenville terrane), and 

the lack of prominent upper-crustal stratification (typically 

observed in the 1.5- to 1.3-Ga granite-rhyolite terrane). The 

granitic gneiss recovered from the drill hole in the graben, 

however, may be related to the 1.6-Ga metamorphic terrane
E16 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

that crops out or lies at shallow depth in southeast Missouri 

(see Kisvarsanyi, 1979). We propose, below, that immediately north of this drill hole, an upper-crustal granitic block 

(1.5-1.3 Ga) crosses the Reelfoot graben. The seismic lines 

analyzed by Nelson and Zhang (1992) lie mostly within the 

horizontal limits of this granitic block The absence of uppercrustal reflectors in this region, as observed by Nelson and 

Zhang (1992), may therefore be due to the presence of the 

proposed granitic batholith, which may be more homogeneous than the nearby St. Francois volcanic terrane. 

Although our interpretation does not favor a westward offset 

in the Grenville terrane to explain the change in magnetic 

grain across the Reelfoot rift, we agree with Nelson and 

Zhang (1992) that the deep seismic reflectors may delineate 

stratification associated with a metamorphic layer.

Utilizing modeling results of four gravity profiles (constrained by seismic refraction data), Hildenbrand (1985a) 

compiled contour maps of the thickness of and depth to 

anomalous crust (i.e., the fossil rift cushion). An anomalously dense, lower crustal layer had been previously interpreted from gravity interpretations (Ervin and McGinnis, 

1975; Cordell, 1977), seismic refraction data (McCamy and 

Meyer, 1966), the correlation of gravity and seismic refraction data (Mooney and others, 1983), and the analysis of 

Rayleigh wave dispersion (Austin and Keller, 1982). As 

expected (fig. 6 in Hildenbrand, 1985a), depth to anomalous 

crust decreases to 26 km beneath the graben. The crust-mantle boundary in this part of the Midcontinent is normally 40 

km deep. Anomalous crust reaches a maximum thickness of 

18 km beneath the New Madrid seismic zone.

Based on seismic-reflection interpretations (Hamilton 

and McKeown, 1988; McKeown and others, 1990), a 

10-km-wide arch and complexly faulted zone, named the 

Blytheville arch, follows the axis of the Reelfoot graben and 

probably represents a structural feature in Paleozoic sedimentary rocks above a deeper seated fault zone. McKeown 

and others (1990) proposed that the arched sediments reflect 

a late Paleozoic diapiric structure formed by gravitationally 

intruding sedimentary rocks. Rodriguez and Stanley (this 

volume) suggest an alternative origin of the arch. Their 

model, based on magnetotelluric data, invokes differential 

uplift over a lateral distance about equal to the length of the 

arch. Internal antiforms arise from diapirism related to differential uplift on horizontal layers of varying thickness. 

Although there is no magnetic expression of the Blytheville 

arch, a subtle gravity high correlates with the arch over most 

of its extent (Langenheim, this volume).

MISSOURI GRAVITY LOW

Guinness and others (1982) described a linear gravity 

feature, called the Missouri gravity low (MGL), as a 120- to 

160-km-wide gravity low that extends from the Midcontinent rift system in southeast Nebraska 700 km southeast to

the Reelfoot graben in northwest Tennessee. They interpret 

the MGL as a failed rift that was directly related to the formation of the granite-rhyolite terrane of southern Missouri. 

Rifting reportedly may have formed a low-density crustal 

inhomogeneity at the base of the crust or in the middle crust 

(between 4 and 8 km). An alternative explanation for the 

source of the MGL given here is a granitic batholith that contrasts in density with adjacent, relatively dense, metamorphic rocks. In our model (fig. 9), mafic intrusions were 

emplaced at or near the margins of the buried granitic 

batholith, which Guinness and others (1982) also pointed 

out.

To investigate the southeastern terminus of the MGL, 

the gravity data were upward-continued 6 km to enhance 

long-wavelength features (fig. 12). A close inspection of 

figure 12 reveals several northwest-trending features that 

suggest the southeastward extension of the MGL to, at least, 

the southeast margin of the Reelfoot graben. Although the 

gravity effect of dense Phanerozoic material raise the level 

of the regional gravity field along this extension as it intersects the Reelfoot rift, gravity intensities are lower compared 

to values northeast and southwest of this juncture. A southeast-trending feature on the pseudogravity map (fig. 6) also 

supports the continuation of the MGL source into Tennessee. 

Along the MGL in western Tennessee, the Big Chief well 

(BC, fig. 1) encountered predominantly feldspar-bearing 

basement rock (ranging in density from 2.65 to 2.68 g/cm3) 

that has been intruded by dense mafic intrusions (about 2.9 

g/cm3). Adjacent to the MGL within the Reelfoot graben, 

the Dow Chemical 1 Wilson well (DW, fig. 1) penetrated 

about 140 m of granitic gneiss (Denison, 1984) with densities greater than 2.7 g/cm3 . From the ideal body analysis 

(fig. 8fi) and modeling results (fig. 9), the top of the source 

of the MGL in Missouri must be shallower than 3 km and its 

bottom may extend to a depth of 15 km.

Other information on the source of the MGL is that it 

trends in the same direction as the fracture systems in the old 

metamorphic terrane (about 1.6 Ga) underlying Missouri 

(Kisvarsanyi, 1974). We propose that the MGL represents 

low-density granitic rocks intruded into this old gneissicmetamorphic terrane, probably during the magmatic events 

that formed the St. Francois granite-rhyolite terrane (about 

1.5-1.3 Ga). An analogous linear batholith may be the 

Sierra Nevada in California, which extends approximately 

700 km—a distance comparable to the length (about 900 

km) of the interpreted batholith in Missouri and Tennessee. 

The 110-km-wide, low-density zone expressed as the MGL 

is named here the Missouri batholith.

Of importance here is the structural relation of the Missouri batholith and the Reelfoot graben. The batholith (about 

1.5-1.3 Ga) does not appear to be offset as it crosses the 

younger graben (about 0.55 Ga). Focal mechanisms of 

present-day earthquakes indicate right-lateral movement 

along northeast-trending planes along the axis of the graben. 

Due to the linearity of the margins of the Missouri batholith
GEOPHYSICAL SETTING OF THE REELFOOT RIFT, NEW MADRID SEISMIC ZONE

92° 90° 88°

E17

38°

36°

86°

34-

-48

300

MILLIGALS 

400 KILOMETERS

Figure 12. Color shaded-relief map of the gravity field, upward-continued 6 km. Illumination is from the northeast. MGL. Missouri gravity low.

(figs. 11 and 12), accumulative northeastward lateral offsets 

associated with faulting in the Reelfoot rift must be necessarily small (less than 10 km). The Bloomfield pluton (fig. 11) 

lies along the northeast margin of the Missouri batholith, and 

the Jonesboro and Covington plutons (anomalies J and I,

respectively, figs. 2, 3, and 11) lie near the intersections of 

the southwest margin of the Missouri batholith and the margins of the Reelfoot graben. The margins of the batholith and 

graben and particularly their intersections may have been 

favorable channelways for magma.
E18 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

GRABEN-MARGIN INTRUSIONS

Along the margins of the Reelfoot graben, prominent 

magnetic and gravity highs have been interpreted as large 

plutons by Hildenbrand and others (1977). Based on the new 

magnetic anomaly maps (figs. 2,5, and 7), the boundaries of 

these intrusions (fig. 11) have been modified slightly from 

those shown by Hildenbrand and others (1977). Although 

not identified on figures 2 and 11, prominent magnetic highs 

lying between anomalies H and D along the northwest margin may reflect either plutons or intrusive complexes (plugs, 

dikes, and sills). Magnetic features B and V (figs. 2 and 3) 

represent intrusive complexes (fig. 11) lying along the southeast margin of the Reelfoot graben.

Limited drill-hole data indicate an Ordovician-Cretaceous age for intrusions along the southeast margin of the 

Reelfoot graben in western Tennessee. The Lion Oil Company No. 1 Bateman (LO, fig. 1) passed through Upper Cretaceous sedimentary rocks and into normal syenite (Caplan, 

1954) at 900 m. At the base of Upper Cretaceous sandstone, 

normal syenite (Caplan, 1954) was encountered in the Pure 

Oil Company McGregor No. 1 well (PO, fig. 1). These syenites intrude Early Ordovician sediments. Nearby, the prominent gravity high associated with the Covington pluton 

(anomaly I, fig. 3) indicates mafic igneous source rocks with 

densities significantly higher than the density of syenite 

(2.52 g/cm3 ; see Malamphy and Vallely, 1944). The need for 

higher densities in the gravity modeling prompted Ervin and 

McGinnis (1975), Hildenbrand and others (1982), and Hendricks (1988) to propose a primarily felsic composition for 

the upper part of the pluton and a more mafic rock (such as 

gabbro) at depth. The Covington pluton may also vary laterally in composition (anomaly I, figs. 2 and 3). Its central core 

is both very dense and magnetic but an outer ring is characterized by dense, low-susceptibility rock (Hildenbrand, 

1985a; Langenheim, this volume).

Other indirect information suggests the emplacement of 

the intrusions along the margins of the Reelfoot graben subsequent to the formation of the rift. For example, Tertiary 

doming occurs over the Newport pluton (Click, 1982). A 

poorly constrained age estimate of post-early Mesozoic was 

determined for the Bloomfield, Paragould, and Covington 

plutons (anomalies M, K, and I, respectively) by calculating 

its remanent magnetization direction (by modeling magnetic 

anomalies) and comparing its corresponding paleomagnetic 

pole position with the polar wandering path for North America (Hildenbrand and others, 1982). This indirect evidence 

and drill-hole data together suggest the emplacement of large 

volumes of mafic rock along the border faults of the Reelfoot 

graben and Arkansas transform fault in Cretaceous time. It 

should, however, be emphasized that the igneous intrusions 

along the margins of this graben northeast of the Arkansas 

transform fault could be much older, as old as Ordovician.

The Bloomfield pluton may, however, be much older. A 

drill hole (C. Barnett No. 1; Grohskopf, 1955) over the center

of this pluton encountered a typical sedimentary section to its 

total depth of 1 .4 km. The drill hole bottomed immediately 

below the unaltered Cambrian Bonneterre Formation. Potential-field models of the Bloomfield pluton (Ravat and others, 

1987) estimate its depth range from 2 to 17 km and width of 

about 35 km. A Cretaceous emplacement of this voluminous 

pluton should have altered the Bonneterre carbonates lying 

only 600 m above the pluton. Thus, the Bloomfield pluton 

may well be pre-Late Cambrian. This conclusion was also 

reached by Eva B. Kisvarsanyi (Missouri Department of Natural Resources, written commun., 1992). An erroneous 

younger age, obtained by comparing its calculated paleomagnetic pole position with the polar wandering curve of 

North America above (Hildenbrand and others, 1982), could 

have resulted from a destruction of remanent magnetization 

due to elevated temperatures over its long existence. This 

destruction of magnetization would have changed the direction of the total-magnetization vector to a vector more 

closely aligned with the present-day inducing field.

Compared to other plutons along the northwest margin 

of the Reelfoot graben (fig. 11), the Bloomfield pluton is 

located at a greater distance from the margin and is elongated 

more in a northwest direction rather than northeast along the 

trend of the graben. The Bloomfield pluton appears to have 

intruded the northeast margin of the Missouri batholith, and, 

if so, it must be younger than 1.5 Ga. Because the pluton is 

deeper than the Precambrian surface at 1.5 km (Buschbach, 

1986), its possible age ranges from 1.5 to 0.55 Ga.

AXIAL INTRUSIONS

Two intrusive centers within the Reelfoot graben (O 

and N, fig. 2) geophysically resemble ring-dike complexes 

(Hildenbrand, 1985a). Distinct short-wavelength magnetic 

highs (fig. 5) form two, ringed, magnetic-anomaly patterns 

near the axis of the Reelfoot graben. The regional magnetic 

basement map (fig. 4) and a model of one prominent high of 

anomaly O (Hildenbrand and others, 1992) both indicate that 

the tops of the axial intrusions are about 3 km deep. Seismic 

reflection data at about lat 36°15'N., however, place the Precambrian surface at a depth of 5 km (Richard L. Dart, written 

commun., 1991). This difference in the depths between Precambrian basement and magnetic basement suggests the 

presence of Phanerozoic intrusions. The spatial relation 

between igneous intrusions and the axis of the graben suggests the presence of axial faults. The depth of emplacement 

and proximity to the graben's axis are indirect evidence for 

the development of axial faults during rift formation in Middle Cambrian time. On the other hand, a drill hole near the 

northern intrusive complex (anomaly O) encountered micaperidotite dikes or sills that were dated at 267 Ma by the KAr method (Zartman, 1977). The intrusions may therefore be 

younger (e.g., Permian or Cretaceous) than the Reelfoot rift 

but were emplaced only at the stratigraphic level of early 

Paleozoic rocks.
GEOPHYSICAL SETTING OF THE REELFOOT RIFT, NEW MADRID SEISMIC ZONE E19

Farther south, near lat 35°N., magnetic and gravity 

highs (anomaly G, fig. 2) probably express mafic intrusions 

emplaced in a large region between the graben's axis and 

southeast margin. Because this intrusive complex lies near 

the Cretaceous intrusions along the Arkansas transform 

fault, it may also be Cretaceous in age. It is also possible that 

this intrusive center near the graben's axis formed during the 

initial phases of rifting (Martin F. Kane, oral commun., 

1991).

NORTHERN TERMINUS 

RIFT ZONES

Hildenbrand and others (1982) and Braile and others 

(1982, 1986) suggested that the graben related to the Reelfoot rift continues northeastward in the Mississippi Embayment, eventually merging with or intersecting the Rough 

Creek graben (Soderberg and Keller, 1981) in southern Illinois and western Kentucky. Braile and others' (1982, 1986) 

interpretation includes a quadruple junction in the study 

area—the Reelfoot and Rough Creek grabens representing 

two failed rift arms. A third rift arm extends northeastward 

along the Wabash Valley fault system, while the fourth arm 

extends northwest to about St. Louis. They named this entire 

system and its quadruple junction the New Madrid rift complex. The Reelfoot and Rough Creek grabens are clearly evident in the magnetic basement map in figure 4. Nelson 

(1990), however, suggested that there are geologic data (e.g., 

lack of graben fill) and proprietary seismic data to dispute 

the existence of the other rift arms of the proposed quadruple 

junction. The strongest arguments for a rift arm in the 

Wabash Valley are seismic reflection data, which reveal a 

15-km-wide graben (Sexton and others, 1986). However, 

Kolata and Nelson (1991) proposed the development of this 

graben during late Paleozoic extension, long after the formation of the Reelfoot and Rough Creek grabens.

A less complex rift structure was proposed by Hildenbrand and others (1982). This interpretation describes the 

southeast margin of the Reelfoot graben bending to the east 

along the Pennyrile fault zone, where it merges with and 

becomes the southern margin of the Rough Creek graben 

(fig. 1). The nature of the northern extension of the northwest 

margin of the Reelfoot graben beyond the Bloomfield pluton 

(fig. 1) is equivocal. Northwest-trending geophysical features described below may mask the more subdued gradients 

of the graben's northwest margin. Close inspection of 

regional magnetic and gravity anomaly maps (e.g., fig. 5) 

suggests that the northwest margin does not continue northeastward into southern Indiana. This margin may therefore 

bend to the east and merge with the northern margin of the 

Rough Creek graben, roughly paralleling its southern margin. Such an interpretation is easily visualized in the depth to 

magnetic basement map (fig. 4). Although a sketch of the

juncture of the two grabens in the study area is shown in figures 1 and 11, the reader should be aware that this geometry 

was based on little geophysical information and represents 

an estimate of the largest region structurally associated with 

this juncture. Therefore, the Reelfoot graben probably does 

not widen at his junction, as implied in figure 11.

The abrupt change in the trend of the Reelfoot rift 

(northeast) to the trend of the Rough Creek graben (east) 

may be due to:

1. A preferred strain direction—Rifting may have propagated northeastward and then followed existing 

east-west structures in western Kentucky that represented a path of less resistance. A change in stress 

from dilatation to shear would, for example, cause a 

trend change.

2. An obstacle—The competent, homogeneous, 

batholithic rocks deep beneath the eastern flank of 

the Ozark uplift may have represented a crustal 

obstacle, diverting rift propagation. 

Bends or flexures in rifts, without a third or fourth arm 

(i.e., triple and quadruple junctions) are common, particularly where extensional and shear stress combine. Such 

bends occur in, for example, the Baikal rift (Logatchev and 

Zorin, 1987), White Nile rift (Browne and others, 1984), 

Rhinegraben (lilies and Greiner, 1978), Red Sea rift 

(Cochran, 1983), and Rio Grande rift (Cordell, 1978). Thomas (1993) suggests that the east-striking Rough Creek graben is a transfer zone connecting the Reelfoot rift to another 

extensional structure in eastern Kentucky (the Rome 

trough).

SOUTH-CENTRAL MAGNETIC LINEAMENT (SCML)

Prominent northwest-trending magnetic and gravity 

anomalies cross the region where the grabens join along 

mapped and inferred faults (Lidiak and Zietz, 1976; Lidiak 

and others, 1985; Hildenbrand, 1985b; Hildenbrand and others, in press). Of particular interest is the south-central magnetic lineament (SCML) (Hildenbrand and others, 1983; 

Ravat, 1984; Hildenbrand, 1985b), which reflects a geologic 

feature that extends from eastern Tennessee to Missouri (fig. 

11). The crustal model (fig. 10) indicates that an intermediate rock such as granodiorite or quartz diorite (density of 

about 2.75 g/cm3 and susceptibility of about 0.05 SI) produced the linear anomaly pattern of the SCML. These intrusions seem to be laterally differentiated (i.e., their cores are 

more magnetic).

The age of the source of the SCML can be speculated 

upon by using geophysical data. Some north-trending 

magnetic anomalies associated with Keweenawan structures 

in eastern Tennessee cross the SCML, but other anomalies 

end abruptly at the SCML. Seismic lines (Heigold and 

Kolata, 1993) that cross the region of magnetic anomaly T 

(fig. 2) also indicate horizontal layering in the Phanerozoic 

sediments with no indication of massive invasion of magma,
E20 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

except possibly for dipping horizons in the Precambrian. 

These observations prompted Hildenbrand and others (in 

press) to propose that the source of the SGML developed 

during or prior to Keweenawan time (roughly 1.1 Ga). The 

SGML possibly delineates a Precambrian basement feature 

that developed along a northwest-trending structure of the 

old (1.6-Ga) metamorphic basement, similar to that underlying Missouri. The exceptional linearity of the SGML suggests that the source is a shear zone. The age of the igneous 

intrusions along the SGML may be younger than 

Keweenawan. Heigold and Kolata (1993) observed that the 

SGML lies along the projected trend of the Early Proterozoic 

Central Plains orogen and thus may represent a convergent 

margin of this orogen. Of importance here is that the SGML 

appears to represent a Precambrian feature.

This Precambrian feature does not appear to be structurally related to rifting. At the juncture of the grabens, the 

SGML reflects an igneous event that emplaced large volumes of high-susceptibility rocks along a roughly 40-kmwide, northwest-trending zone (figs. 2 and 10). Where the 

Reelfoot graben bends east, this zone is linear (Hildenbrand, 

1985b) and thus appears structurally unaffected by deformation during rifting or rift reactivation. This structural decoupling is more apparent when one observes that the SGML 

trends linearly from the St. Louis area southeast to Tennessee. Thus the SGML probably has no structural relationship 

to the Rough Creek and Reelfoot grabens. Because these grabens are younger than the proposed age of the source of the 

SGML (Keweenawan or older), the linearity of the SGML 

severely restricts the amount of lateral movement along the 

axial and border faults of the grabens.

PADUCAH GRAVITY LINEAMENT (PGL)

Hildenbrand and others (in press) defined the Paducah 

gravity lineament (PGL, fig. 3) as a broad, northwest-trending gravity high that crosses the bend in the Reelfoot graben 

as it joins with the Rough Creek graben (fig. 11). Many of 

the individual gravity peaks within the PGL coincide with 

isolated magnetic highs. The presence of both magnetic and 

very dense sources (fig. 10) suggests that the area of the PGL 

is heavily intruded by mafic rock. Four prominent gravity 

gradients (fig. 3) bound highs of the PGL. These gradients 

probably represent strike-slip and dip-slip faults. Evidence 

for strike-slip motion is based largely on a noticeable southeastward deflection between faults a and b (fig. 1) in the 

southeast margin of the Reelfoot graben by about 12 km 

(Hildenbrand and others, 1982). The inferred Precambrian 

fault, along which the Ste. Genevieve fault developed, may 

extend into Kentucky either along lineament b (Hildenbrand 

and others, 1982) or lineament a (Lidiak and Zietz, 1976). 

The St. Genevieve fault zone is mainly a high-angle reverse 

fault dipping to the northeast. Hildenbrand and others (1982) 

proposed that this faulted region represented a transition

from a rift structure to a block-faulted region. The four interpreted faults may have developed along northwest-trending 

features related to the old (1.6-Ga) metamorphic basement, 

similar to that underlying Missouri.

Of particular interest is that anomalies P and Q (fig. 3) 

have sharp corners on their southeastern edges. Hildenbrand 

and others (1982, in press) suggested that the source of 

anomaly P is an intrusion emplaced along axial faults of the 

Reelfoot graben (see intrusion located on the Kentucky-Tennessee border, figs. 1 and 11). We propose that the intrusions 

associated with anomalies P and Q were emplaced along 

both the Reelfoot axial faults and the proposed northwesttrending faults related to the old (1.6-Ga) metamorphic basement. The sharp corners of anomalies P and Q indicate that 

the associated edges of the intrusions are structurally controlled by the intersecting northwest-trending Precambrian 

faults and the early Paleozoic axial faults of the Reelfoot graben (fig. 11). The age of the intrusions should therefore be 

early Paleozoic or younger.

One could argue that nearby igneous bodies and structures indicate a late Paleozoic or Cretaceous age for these 

intrusions. Moreover, to the northwest, Early to Middle 

Devonian igneous rocks lie along the trend of the PGL near 

Avon, Mo. (fig. 1). Without additional data, it is difficult to 

speculate on the age(s) of the intrusions that produce the 

PGL. Intrusions of different ages may be present. Probable 

ages based on those of nearby igneous rocks and structures 

include Cambrian (syn-rift), Devonian, late Paleozoic, and 

(or) Cretaceous.

Does the PGL represent a third rift arm? The lateral 

extent of PGL (about 80 km beyond the northwest margin of 

the Reelfoot graben) is considerably smaller than the length 

of either the Reelfoot rift or Rough Creek graben. If a third 

rift arm is expressed by the PGL, it has a lateral extent 

uncharacteristic of the other arms. There are also no data to 

support the existence of an associated graben or rift-related 

sedimentary rocks along the PGL (Kolata and Nelson, 1991). 

An alternative interpretation for the structures related to the 

PGL is a fault zone that accommodated strain at the juncture 

of the Reelfoot rift and Rough Creek graben.

The calculated density (2.8 g/cm3 ) and susceptibility 

(0.035 SI) of the intrusion along the PGL (fig. 10) suggest a 

mafic source rock, such as gabbro or diorite. Many of the 

steep gradients and small widths of large-amplitude gravity 

anomalies along the PGL (fig. 10) clearly require that these 

mafic intrusions lie at depths shallower than 6 km (fig. 8Q. 

Most of the anomalous mass producing the prominent 

gravity highs in the study area, therefore, resides in the upper 

crust. An anomalously high density mass at the base of the 

crust may be present, as previously proposed by Cordell 

(1977), but its gravity effect is considerably lower in amplitude and longer in wavelength than that produced by the 

shallow mafic intrusions.

Lying between the PGL and SGML is a magnetic and 

gravity low. In the crustal model (fig. 10) source rocks are
GEOPHYSICAL SETTING OF THE REELFOOT RIFT, NEW MADRID SEISMIC ZONE E21

assumed to have negligible magnetic properties to a 15-km 

depth. This assumption was based on the intensity of the 

magnetic low, which is uncharacteristic in the magnetic field 

of the Midcontinent. A reduction of magnetic properties by 

hydrothermal alteration is unlikely due to the large region 

involved (figs. 2 and 6). We propose that the geophysical 

low reflects a Precambrian terrane with little or no magnetic 

minerals (a basin?).

FAULT ZONES

Many of the fault zones, such as the Ste. Genevieve 

fault system, correlate well with geophysical features. The 

northeast edge of the PGL parallels the Ste. Genevieve fault 

zone and is inferred to represent an ancestral fault zone that 

extends into Kentucky (Lidiak and Zietz, 1976; Hildenbrand 

and others, 1977,1982). Another example is the west-trending Rough Creek fault system that, in the region intruded by 

igneous rocks related to the SGML, appears to terminate or 

bend southwest as the Lusk Creek fault zone. The SGML follows the Cottage Grove fault system northeast into Missouri. 

On the other hand, the numerous northeast-trending faults in 

the eastern part of the study area have no apparent expression 

on the gravity and magnetic anomaly maps. Hildenbrand and 

Keller (1983) suggested that fault displacements here are too 

small to be detected by regional potential-field methods.

SEISMICITY

The present-day seismicity pattern (figs. 13-15) suggests several linear active zones (Andrews and others, 1985; 

Himes and others, 1988). Three principal linear zones are (1) 

the northeast-trending zone from Marked Tree, Ark., to 

Caruthersville, Mo., (2) the north-northwest-trending zone 

from Dyersburg, Tenn., to New Madrid, Mo., and (3) the 

north-northeast-trending zone near New Madrid to Charleston, Mo. These zones are referred to here as the MTC, DNM, 

and NMC seismic zones, respectively (figs. 13-15). Secondary, active seismic zones, such as the east-west-trending 

zone between the Bloomfield and Maiden plutons (anomalies M and L, respectively) and the one north of the Covington pluton (anomaly I), are also apparent. Two of the three 

devastating 1811-12 earthquakes series occurred near 

Marked Tree, Ark., and New Madrid, Mo. (Nuttli, 1982).

Recent precise locations of hypocenters and new studies of focal mechanisms (Andrews and others, 1985; Himes 

and others, 1988; Chiu and others, 1992) have advanced our 

knowledge on the nature of faulting along the three principal 

active seismic zones. Along the MTC seismic zone, earthquakes generally occur at depths between 4 and 14 km. It is 

worthy to note that foci along the southwestern part of MTC 

not only display a greater degree of randomness with depth 

but also are dispersed more laterally compared to the tight 

distribution pattern along the northeastern part of the zone.

This change in distribution pattern occurs near Blytheville, 

Ark. Focal mechanisms indicate that the MTC zone is characterized by right-lateral strike-slip motion on a northeasttrending, nearly vertical plane.

The DNM zone, where earthquake frequency is the 

greatest, is characterized by southwestward-dipping fault 

zones (33° to 52°). These dips cause, in part, the apparent 

wide lateral dispersion of earthquakes in map view. Focal 

mechanisms indicate a heterogeneous mix of thrust, strikeslip, and normal faulting (Chiu and others, 1992).

Along the NMC zone, foci occur as deep as 17 km 

along a well-defined, near-vertical plane. Right-lateral 

strike-slip motion is again prominent.

To better understand the distribution of earthquakes, a 

map (fig. 15) showing the logarithm of the number of occurrences was compiled using earthquakes of magnitude 1.5 or 

greater. Earthquakes are numerous northwest of the Reelfoot 

graben but seldom occur southeast of it. The three primary 

seismic zones are evident, but distinct secondary zones are 

also present, such as the one north of the Covington pluton 

(I, fig. 13). For a distance of 50 km northeast of New Madrid, 

earthquakes appear to follow the northwest margin of the 

Reelfoot graben. Farther north, earthquakes occur along the 

northwest trend of the Paducah gravity lineament (fig. 15). 

Braile and others (1986) also suggested a northwest-trending 

seismic zone in this region, but their zone is much broader. 

Two other seismic zones associated with the quadruple rift 

junction proposed by Braile and others (1982, 1986) extend 

about 200 km northeast into Indiana and east in Kentucky. 

These two zones are not apparent on figure 15.

Guinness and others (1982) pointed out that many of 

the epicenters of the New Madrid seismic zone are within the 

region defined by the intersection of the Missouri gravity 

low and the Reelfoot graben. The correspondence of seismicity and this intersection is remarkable, considering that 

the only active section along the 400-km-long Reelfoot rift 

axis occurs within this 100-km-wide intersection zone. Thus, 

there seems to be an intimate relationship between earthquake occurrence and the intersection zone of the Missouri 

batholith and the Reelfoot graben (herein called the structures' intersection zone or simply the intersection zone). For 

example, southwest of the intersection zone (i.e., near the 

intersection of the axis of the Reelfoot graben and the southwest margin of the Missouri batholith), earthquake frequency along the MTC seismic zone is considerably less 

than that within the intersection zone. Outside the intersection zone to the northeast and within the graben, historic 

earthquakes are rare. It should be noted that the NMC and 

MTC seismic zones cross the lines representing the margins 

of the Missouri batholith shown in figures 13 and 14. These 

margins may have, however, substantial widths, such as the 

estimated width of 18 km in the model shown in figure 9. 

Due to this uncertainty in the width of the margins, it is 

difficult to assess whether the seismic zones cross the margins or terminate within a broad margin.
E22 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE 

90°

36°

-400 -100 

0 25 50

200 400 NANOTESLAS 

75 100 KILOMETERS

Figure 13. Reduced-to-pole magnetic anomaly map of the New Madrid seismic zone with earthquake epicenters and boundaries 

of major geophysical features. Open triangles denote epicenters detected by the Missouri Valley regional seismic network from 

1975 to 1991. Letters B, I to S, and V represent anomalies discussed in text. MTC, DNM, and NMC denote seismic zones. Dashed 

lines are the approximate boundaries of the igneous complexes within the graben, expressed as anomalies O and N.
GEOPHYSICAL SETTING OF THE REELFOOT RIFT, NEW MADRID SEISMIC ZONE E23 

90° 88°

36°

-25 -15 -5 5 15 MILLIGALS 

0 25 50 75 100 KILOMETERS

Figure 14. Complete Bouguer gravity map of the New Madrid seismic zone with earthquake epicenters and boundaries of 

major geophysical features. Open triangles denote earthquake epicenters detected by the Missouri Valley regional seismic network from 1975 to 1991. Letters B, I to S, and V represent anomalies discussed in text. MTC, DNM, and NMC denote seismic 

zones. PGL represents the Paducah gravity lineament. Dashed lines are the approximate boundaries of the igneous complexes 

within the graben, expressed as anomalies O and N.
E24 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

38C

36°

Rough' ^

X^reek 

^ graben

EXPLANATION 

Logarithm of number of earthquakes

0.4

50

1.2 1.6 2.0

100 150 KILOMETERS

Figure 15. Earthquake occurrence map. Gray shades represent the logarithm of the number of earthquakes in 10x10- 

km cells. Because cells overlap by half their width, cells are spaced 5 km apart. Only earthquakes of magnitude 1.5 or 

greater were considered. The cities of Marked Tree, Blytheville, Caruthersville, Dyersburg, New Madrid, and Charleston 

are located, respectively, by MT, BL, CA, DY, NM, and CH. MTC, DNM, and NMC denote seismic zones. Dashed line 

represents the approximate boundary of the igneous bodies expressed as the Paducah gravity lineament (PGL).
GEOPHYSICAL SETTING OF THE REELFOOT RIFT, NEW MADRID SEISMIC ZONE E25

Another striking correlation is the widening of seismic 

zones in the regions'underlain by igneous intrusions. Earthquakes between intrusive complexes N and O (dashed lines, 

fig. 13) occur in a tight cluster. Within the intrusive complexes, however, earthquakes disperse and zones become 

much broader. Hildenbrand and others (1982, 1992) characterized these complexes as several individual plutons, dikes, 

sills, and plugs that may extend to substantial depths. The 

small intrusion northeast of the main part of intrusive complex N resists deformation as earthquakes concentrate at its 

northeastern edge and few earthquakes occur within its 

boundary.

The Covington and Bloomfield plutons (anomalies I 

and M) have earthquake concentrations near their edges. In 

particular, seismicity near the Covington pluton indicates 

that the southeast margin of the Reelfoot graben is active. 

The Bloomfield pluton also appears to influence the distribution of stress: Earthquakes occur near its edges but avoid the 

interior of this intrusion. Correlations between stress accumulations and large igneous intrusions in the Eastern United 

States have been previously discussed by Kane (1977) and 

McKeown (1978). Ravat and others (1987) pointed out, 

however, that because earthquakes occur at a distance 

(10-15 km) from the edge of the Bloomfield pluton, stresses 

may be related to a zone of weakness marginal to the thermally hardened aureole of the pluton.

DISCUSSION

Why do earthquakes occur in the northern Mississippi 

Embayment region? Clearly, along the early Paleozoic rift 

axis, strain (different from that which formed the rift) is presently being relieved (Hildenbrand and others, 1977; Kane 

and others, 1981). The near east-west regional stress field, 

possibly due to lithospheric drag related to plate interactions 

(Zoback and Zoback, 1980) or ridge push force (Zoback and 

Zoback, 1989), is optimally oriented to reactivate northeastand northwest-trending faults. Many other major crustal 

flaws in the Midcontinent (e.g., Oklahoma aulacogen, Midcontinent rift system), however, also have favorable orientations for reactivation but are not presently active. Hamilton 

and McKeown (1988) asked if additional special factors, 

such as high pore pressure, plutons, structure interaction, or 

unusual petrology contribute to the crustal weakness in the 

Mississippi Embayment region, which manifests itself as the 

most seismically active seismic zone east of the Rocky 

Mountains. We consider these additional factors below, 

except for pore pressure, which McKeown and Diehl (this 

volume) address.

Another factor not considered in detail here is the possible relationship between heat sources and seismicity. 

McCartan and Gettings (1991) postulated that low to moderate positive heat-flow anomalies (Swanberg and others, 

1982) in the vicinity of New Madrid seismic zone are due to

young (20- to 10-Ma), shallow mafic rocks. They suggested 

that seismicity is related to crustal inhomogeneities formed 

at the boundary between competent, cool pre-Tertiary intrusions and weak, warm mid-Tertiary or younger intrusions. 

On the other hand, Swanberg and others (1982) explain the 

positive heat-flow anomalies as possibly due to convection 

from ground water ascending along active faults. Because 

the absolute heat flow values (1.3-1.6 heat-flow units) are 

not significantly above the Midcontinent mean (1.2 heatflow units), the source of the thermal anomalies is difficult 

to evaluate.

Historic earthquakes are concentrated within the zone 

as defined by the intersection of the Missouri batholith and 

the Reelfoot graben. The 400-km-long Reelfoot rift axis is 

only active within the intersection zone. This intersection 

zone also geographically correlates with the late PaleozoicMesozoic Pascola arch and an early Paleozoic bulge in the 

anomalous crust underlying the Reelfoot rift at the crustmantle boundary (Mooney and others, 1983; Hildenbrand, 

1985a). The relation of these features to present-day seismicity suggests that the intersection zone lies within a crust that 

is unstable or susceptible to tectonic forces. Modeling (fig. 

9) indicates that the 15-km-thick granitic block at the intersection zone is less dense and may be less competent than 

that of the amphibolite-grade metamorphic terrane along its 

flank within the graben. Does the intersection zone represent 

a weak zone that tends to focus earthquake activity?

Because the MTC and DNM seismic zones widen over 

intrusive complexes O and N (dashed lines, fig. 13), the individual intrusions in these complexes probably represent 

inhomogeneities in a relatively homogeneous granitic crust. 

Stress may concentrate around the periphery of these individual intrusions, in much the same manner as stress concentrates near knots in wood under stress (see Campbell, 1978). 

Thus seismic zones necessarily widen in these heavily 

intruded regions. Moreover, seismicity studies to characterize fault zones need to incorporate these dense, mafic intrusions in their velocity models.

The geographic correspondence of the DNM seismic 

zone and intrusive complex O (figs. 13 and 14) is worthy of 

additional discussion. The boundary of the DNM seismic 

zone closely coincides with the edge of the intrusive complex (dashed line, fig. 13). Its northwestern terminus correlates both with the edge of the intrusions and a corner of the 

intersection zone. Do stress accumulations at the peripheries 

of individual intrusions of complex O and the presence of the 

northeast margin of the Missouri batholith both contribute to 

the abrupt change in trend in seismicity from northeast 

(along MTC) to north-northwest (along DNM)?

In our proposed model for the cause of earthquakes, the 

axial-fault zone of the Reelfoot graben represents a major 

crustal flaw along which present-day strain is being relieved. 

The northeast-trending MTC seismic zone reflects the 

associated strain release. The upper crustal region defined by 

the intersection of the Missouri batholith and Reelfoot
E26 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

graben is weak and focuses earthquake activity. However, 

strain release abruptly changes trend to form the DNM seismic zone by following a path of less resistance along intrusive complex O. Apparently this trend change is due, in part, 

to the localizing of stress at the periphery of intrusions and, 

in part, to the difficulty of brittle fracture propagating northeastward into a more competent, metamorphic terrane. Near 

the juncture of the northwest margin of the graben and the 

northeast margin of the Missouri batholith, the DNM seismic 

zone changes into three seismic zones that trend east, northnortheast, and northeast (figs. 13 and 1.4). This trend change 

in seismicity may be controlled by the presence of the 

Bloomfield pluton, which represents a dense, 35-km-wide 

homogeneous mass in the upper crust. The resulting preferred directions of strain release parallel the southern and 

eastern edges of the pluton. Northeast of New Madrid, a few 

earthquakes appear to align northeastward along the defined 

northwest boundary of the graben. Thus, in our model, the 

linear seismic zones change trend to follow paths of less 

resistance or avoid more competent structures.

The 10- to 15-km-wide buffer zone between the Bloomfield pluton and adjacent seismic zones, however, requires 

additional physical-property data to properly characterize it. 

The buffer zone has been interpreted as a thermally hardened 

aureole (Ravat and others, 1987) or a region with magmacemented faults. Such a zone is significantly wider than the 

width of aureole zones related to large intrusions. Moreover, 

because we propose that the age of the pluton is syn-rift or 

older, the unusual physical properties of this buffer zone 

would likely lessen or disappear with time from continual 

exposure to moderate temperatures and pressures, particularly at the deeper parts of the fault zones (as deep as 16 km). 

Although the origin of the buffer zone is uncertain, the close 

parallelism of the seismic zones and pluton edges suggests 

an intimate relation.

Is there geophysical evidence for a continuous, northeast-trending structure linking the New Madrid seismic zone 

and the active Wabash Valley fault zone (fig. 1)? Close 

inspection of the potential-field anomaly maps does not 

reveal such a structure. To the contrary, the PGL and SGML 

correspond in direction to old northwest-trending crustal 

structures in the intervening region. Earthquakes (fig. 15) 

here seem to occur along faults and intrusions related to the 

PGL. Immediately northeast of the PGL, earthquakes are 

rare.

The northwest-trending features related to SGML and 

PGL cross the Reelfoot graben with no substantial lateral 

offsets. A similar observation was made above for the margins of the Missouri batholith. If true, these linear features 

have severely limited the amount of lateral movement along 

the axial faults of the Reelfoot graben since the formation of 

these features. Thus, the accumulative, right-lateral offset 

along the MTC seismic zone is small (< 10 km).

The above discussions address possible relations 

between interpreted local structures and earthquake activity 

but do not address the characteristics of the New Madrid 

seismic zone that distinguish it as the most active in Eastern 

United States. The Reelfoot rift, similar to the Oklahoma 

aulacogen and Midcontinent rift system, represents major 

crustal flaws that are favorably oriented with the regional 

stress field for strike-slip faulting. Unlike these other rifts, 

the Reelfoot rift has had several periods of reactivation. During late Paleozoic and into Mesozoic time, the northern Mississippi Embayment region experienced uplift and erosion 

accompanied by intrusive activity in Permian time. A major 

igneous event in the Cretaceous resulted in the emplacement 

of numerous plutons along the graben's margins and southern terminus. Today, earthquakes occur along its margins 

and axis. The Reelfoot rift is apparently more susceptible to 

tectonic forces or reactivation than other Midcontinent eratonic rifts.

The existence of the interpreted weak zone at the intersection of the Missouri batholith and the Reelfoot graben 

may be a contributing factor to the occurrence of earthquakes. Increased pore pressure (McKeown and Diehl, this 

volume) may also be a contributing factor. We suspect that 

further studies will show that several factors combine to distinguish the New Madrid seismic zone as the most active in 

Eastern United States.

SUMMARY AND CONCLUSIONS

Magnetic and gravity data in the northern Mississippi 

Embayment and surrounding region provide a geologic picture of the subsurface that indicates a long and complex tectonic and magmatic history. To explain the marked change 

in magnetic trends flanking the Reelfoot graben (figs. 5 and 

7), Hildenbrand (1985a) proposed that a Precambrian (about 

1.5 Ga) shear zone represented a zone of weakness along 

which early Paleozoic rifting took place. Prior to rifting (1.5 

to 1.3 Ga), magmatic activity formed the St. Francois granite-rhyolite terrane and the proposed southeast-trending Missouri batholith. Subsequent to the formation of this batholith, 

the Bloomfield pluton was emplaced along the batholith's 

northeast margin. The age of the pluton may be syn-rift 

(about 0.55 Ga). There is, however, no direct evidence for 

major Cambrian volcanism associated with rifting. Magnetic 

and gravity highs (figs. 2 and 3), lying north of the Arkansas 

transform fault in eastern Arkansas (about lat 35°N.), may 

reflect the presence of massive igneous bodies emplaced 

within the graben during rifting (Martin F. Kane, oral commun., 1991). On the other hand, these potential-field highs 

may represent Cretaceous intrusions, similar to those southwest of the Arkansas transform fault and along margins of 

the Reelfoot graben. Two intrusive complexes along the graben's axis (O and N, fig. 2) and a northwest-trending
GEOPHYSICAL SETTING OF THE REELFOOT RIFT, NEW MADRID SEISMIC ZONE E27

intrusive zone in southern Illinois and western Kentucky (the 

Paducah gravity lineament) may be syn-rift, late Paleozoic, 

or Cretaceous in age. The large plutons and intrusive complexes along the margins of the Reelfoot graben were 

emplaced after rift formation in Ordovician-Cretaceous 

time. The northern Mississippi Embayment region is apparently susceptible to tectonic forces or reactivation. Today 

this region is the most seismically active in the Eastern 

United States.

We propose that the Reelfoot rift extends from eastcentral Arkansas to western Kentucky. In Arkansas, the rift 

is abruptly terminated at the Arkansas transform fault. 

Although it once may have extended farther southwest, 

thrusting and sedimentation associated with the Ouachita 

orogeny obliterated any structures related to the rift To the 

north, we propose that the rift bends eastward to merge with 

the Rough Creek graben.

Several potential-field features appear to focus earthquake activity. On a regional scale, the intersection of the 

Reelfoot graben and the Missouri batholith appears to form 

a weak zone susceptible to seismic activity. The proposed 

granitic weak zone may contrast with more competent, 

flanking metamorphic terranes. In our proposed model for 

the origin of the New Madrid seismic zone, earthquakes are 

concentrated along the axis of the graben due partly to this 

weak zone and partly to a major crustal flaw along the axis. 

Local changes in the trend of seismic zones are related to 

obstacles that divert strain along paths of less resistance. The 

change in trend from northeast (along the graben's axis) to 

north-northwest may be associated with both stress concentrations around the peripheries of intrusions along a northnorthwest-trending intrusive zone and with the nearby northeast margin of the weak zone. The Bloomfield pluton diverts 

the seismic zones along its southern and eastern edges and 

along the northwest margin of the Reelfoot graben. Farther 

north, faults and intrusive zones related to the Paducah gravity lineament seem to influence the release of seismic 

energy. Thus, our model suggests that linear seismic zones 

change trend in response to preferred directions of strain 

release related to intrusions and older faults.

Interpretation of regional magnetic and gravity anomalies has led to identification of subsurface structures important to our understanding of the control or release of seismic 

energy. Recent potential-field studies that characterize the 

New Madrid seismic zone are directed toward the collection 

and interpretation of high-resolution gravity data (Langenheim, this volume) and magnetic data (Hildenbrand and others, 1992). These promising data have mapped many shallow 

(<1 km) sources that parallel seismic zones. Future detailed 

analysis employing these data may lead to stress models that 

accommodate the fault patterns interpreted from the potential-field grain. In particular, these studies may provide physical-property information important to the understanding of 

the cause of fracturing in one particular area but not in 

nearby regions.

ACKNOWLEDGMENTS

The study has greatly benefited from reviews or discussions with Lindrith Cordell, Anthony Crone, Andrew 

Griscom, Robert Jachens, Arch Johnston, Martin Kane, 

Vicki Langenheim, and Walter Mooney. In conversation, 

Martin Kane strongly argued that the source of the Missouri 

gravity low was a batholith; quantitative analyses in our 

study have lead to the same conclusion.

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Published in the Central Region, Denver, Colorado 

Manuscript approved for publication November 9,1993 

Edited by Richard W. Scott, Jr. 

Graphics prepared by Gayle Dumonceaux 

Photocomposition by Shelly Fields

A U.S. QOWn»MENT PRINTING OFFICE: 1995-673-046/86079