Possible Relationship Between Seismicity 

and Warm Intrusive Bodies in the 

Charleston, South Carolina, and 

New Madrid, Missouri, Areas 

U.S. GEOLOGICAL SURVEY BULLETIN 1953 

Prepared in cooperation with the 

U.S. Nuclear Regulatory Commission 
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Possible Relationship Between Seismicity 

and Warm Intrusive Bodies in the 

Charleston, South Carolina, and 

New Madrid, Missouri, Areas 

By LUCY McCARTAN and MARK E. GETTINGS 

Prepared in cooperation with the 

U.S. Nuclear Regulatory Commission 

Positive magnetic, gravity, and thermal 

anomalies in the Charleston and 

New Madrid areas may indicate Tertiary 

midcrustal plutons; regional 

compresssive stress concentrated 

around the plutons may release as 

earthquakes 

U.S. GEOLOGICAL SURVEY BULLETIN 1953 
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, Jr., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

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 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1991 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225 

Library of Congress Cataloging in Publication Data 

McCartan, Lucy. 

Possible relationship between seismicity and warm intrusive bodies in the 

Charleston, South Carolina, and New Madrid, Missouri, areas I by Lucy 

McCartan and Mark E. Gettings ; prepared in cooperation with the U.S. 

Nuclear Regulatory Commission. 

p. em. - (U.S. Geological Survey bulletin ; 1953) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:1953 

1. Earthquakes-South Carolina-Charleston Region. 2. Earthquakes-Missouri-New Madrid Region. 3. Intrusions (Geology)-South CarolinaCharleston Region. 4. Intrusions (Geology)-Missouri-New Madrid 

Region. I. Gettings, M.E. (Mark E.) II. U.S. Nuclear Regulatory Commission. Ill. Title. IV. Series. 

QE75.B9 no. 1953 

[QE535.2.U6] 

557.3 s-dc20 

[551.2'2'09757915] 90-13944 

CIP 
CONTENTS 

Abstract 1 

Introduction 1 

Previous Work 1 

Acknowledgments 2 

Evidence of Tertiary Intrusions Beneath the Charleston Area 2 

Uplift Associated with Coinciding Gravity and Magnetic Anomalies 2 

Thermal Anomalies 6 

Seismic Evidence 6 

Distribution of Earthquake Epicenters in the Charleston Area 6 

Evidence of Tertiary Intrusions Beneath the New Madrid Area 6 

Gravity and Magnetic Anomalies 6 

Thermal Anomalies and Uplifted Tertiary Strata 7 

Distribution of Earthquake Epicenters in the New Madrid Area 10 

Theoretical Considerations 10 

The Hole-in-the-Plate Model 10 

New Version of the Model 11 

Possible Sources of Low to Moderate Positive Heat-Flow Anomalies 13 

Discussion 14 

Uniqueness of the Charleston and New Madrid Areas 14 

Comparison of Charleston, New Madrid, and Yellowstone 14 

Probable Causes of Earthquakes Around Inferred Warm Plutons 14 

Doming in the Charleston Area 15 

Blytheville Arch, New Madrid 15 

Conclusions 16 

References Cited 16 

FIGURES 

1-9. Maps showing: 

1. Location of study areas at Charleston, S.C., and New Madrid, Mo. 1 

2. Residual aeromagnetic anomalies of the Charleston, S.C., area 3 

3. Bouguer gravity anomalies of the Charleston, S.C., area 4 

4. Inferred mafic plutons, heat flow, and seismicity in the Charleston, S.C., 

area 5 

5. Mafic plutons inferred from coincident magnetic and gravity anomalies in the 

New Madrid, Mo., area 7 

6. Residual aeromagnetic anomalies of the New Madrid, Mo., area 8 

7. Bouguer gravity anomalies of the New Madrid, Mo., area 9 

8. Ground-water temperatures and thermal gradients in the New Madrid, Mo., 

area 10 

9. Central part of the New Madrid seismic zone 10 

Contents Ill 
IV Contents 

10. Diagram relating the effects of a midcrustal intrusion in relation to 

seismicity 11 

11. Map and cross sectional view of a hypothetical midcrustal plutonic complex 12 

12. Schematic diagram showing the effect of reducing maximum shear resistance 13 

TABLE 

1. Changes in compressional-wave velocity with temperature 6 
Possible Relationship Between Seismicity and Warm 

Intrusive Bodies in the Charleston, South Carolina, 

and New Madrid, Missouri, Areas 

By Lucy McCartan and Mark E. Gettings 

Abstract 

The largest historical earthquakes in the eastern half 

of the United States occurred in Charleston, S.C., in 1886, 

and New Madrid, Mo., in 1811-12. Comprehensive explanations for these intraplate earthquakes are still lacking, 

and large surface dislocations have not been found in 

association with their estimated epicenters. This report 

focuses on certain geologic and geophysical evidence that 

may explain the occurrence of some of the earthquakes in 

the two areas. This evidence includes uplift of Tertiary 

strata and elevated heat flow in the vicinity of positive 

magnetic and gravity anomalies. We infer this signature to 

indicate midcrustal Tertiary mafic intrusions. Some of the 

hypocenters of historical earthquakes in Charleston and 

New Madrid are located around the inferred bodies. This 

spatial coincidence suggests a causal relationship between 

the earthquakes and intrusions. 

INTRODUCTION 

In 1811 and 1812, three large earthquakes shook the 

eastern two-thirds ofthe United States. The earthquakes have 

been estimated at magnitudes of 8.1 to 8. 3 and were 

centered on New Madrid, Mo. (Johnston and Kanter, 

1990). In 1886, a slightly smaller earthquake occurred near 

Charleston, S.C. (Bollinger and others, 1989); it was also 

felt over most of the eastern half of the United States. 

Although major shocks are less frequent in intraplate 

settings such as the eastern half of the United States than 

they are along a plate margin such as California or Alaska, 

the potential for widespread damage is much greater for 

eastern earthquakes. This risk has prompted the funding of 

large, multidisciplinary studies aimed at discovering the 

causes of such earthquakes. Despite reasonable approaches 

and careful science, no comprehensive, generally accepted 

explanation has been offered for seismicity in either 

Charleston or New Madrid. The present report discusses 

Manuscript approved for publication July 30, 1990. 

GULF OF 

MEXICO 

0 100 200 300 400 MILES I I I I I I I I I 

0 100 200 300 400 KILOMETERS 

ATLANTIC 

OCEAN 

Figure 1. Location of study areas at Charleston, S.C., and 

New Madrid, Mo. 

previously published data from a new perspective and 

summarizes conclusions drawn from new data that support 

an explanation of some of the seismicity in these two areas 

(fig. 1). 

Previous Work 

Many reports have presented data and theories on 

Charleston and New Madrid seismicity. Multichaptered 

volumes edited by Rankin (1977), Gohn (1983), and Hays 

and Gori (1983) on the Charleston area and by McKeown 

and Pakiser (1982) and Gori and Hays (1984) on the New 

Madrid area contain a variety of geologic, geophysical, and 

seismologic information that is the basis of most of the 

conclusions reached in this report. In some cases, the data 

have been reinterpreted here; such changes are noted in the 

Introduction 
text and figure captions. Other papers have presented data 

acquired mainly since publication of the older volumes or 

have discussed new theories or modifications of older ones 

(Costain and Glover, 1982; Daniels and others, 1983; Gohn 

and others, 1978a, b, 1980; Hinze and Braile, 1988; 

McCartan and Architzel, 1988; McCartan and others, in 

press; Phillips, 1988; Ravat and others, 1987). 

Work in both geographic areas has focused mainly on 

finding faults large enough to produce large earthquakes 

(Russ, 1982; Stauder, 1982; Tarr, 1981; Coruh and others, 

1981). Whereas such a structure seems to be lacking in the 

Charleston area, evidence supports the conclusion that a 

large and active fault exists southeast of New Madrid 

(Hamilton and McKeown, 1988). This apparent difference 

between Charleston and New Madrid may be less significant than the similarities, one of which is the recurrence of 

large earthquakes in prehistoric time, inferred from the 

distribution of liquefaction features (Obermeier and others, 

1985, 1987, 1989). That most other areas in the midcontinent and eastern seaboard lack such features again argues 

for an explanation of earthquake generation incorporating 

the major geologic and geophysical characteristics that are 

unique or peculiar to the two areas rather than an explanation that would fit most other areas. 

The earthquake mechanism that best fits this perspective is Kane's (1977) hole-in-the-plate theory of stress 

concentration (see "Theoretical Considerations" and "Discusssion" sections), which falls in the "rheological anomaly" class of earthquake mechanisms (Long and others, 

1986). Kane's idea was considered by Long and Champion 

(1977), Campbell (1978), and McKeown (1978) but eventually fell into disfavor because the "hole" (weak zone) 

could not be accounted for in many cases. 

We maintain that, in Charleston and New Madrid, 

weakening can be attributed to long-term elevated heat 

flow. Although Sass and Ziagos ( 1977) measured heat flow 

over an inferred pluton in Charleston, they failed to recognize the anomalous value because they incorrectly assumed 

a high value for the entire region. Heat-flow data for New 

Madrid were presented by Swanberg and others ( 1982) and 

Douglas Smith (written commun., 1985) and supplemented 

by ground-water temperatures from Swanberg and others 

(1982). A theoretical discussion of the distribution of heat 

in the New Madrid area (Mitchell and others, 1984) was 

helpful in pointing out several possible sources of thermal 

anomalies. The pervasive assumption that no Tertiary 

intrusions exist in the crust beneath New Madrid may have 

been the reason that Mitchell and others ( 1984) did not 

consider magmatic heat to be a cause of low to moderate 

positive heat-flow anomalies there. 

Coinciding magnetic and gravity anomalies in the 

Charleston and New Madrid areas indicate midcrustal mafic 

intrusions (Kane, 1977; Herrmann, 1984; Hildenbrand, 

1985; Hildenbrand and others, 1982), but there is no direct 

evidence of a Tertiary age. Indirect evidence of Tertiary 

intrusive events includes the association of positive thermal 

anomalies (based on data from Mitchell and others, 1984; 

Swanberg and others, 1982; Sass and Ziagos, 1977) and 

uplifted strata (Heron and Johnson, 1966; Ackermann, 

1983; Hamilton and McKeown, 1988) with some of the 

inferred mafic bodies (McCartan and Gettings, 1985). 

Theoretical and observational studies of the seismic 

effects of hot intrusive bodies in the crust are discussed. 

Bollinger and others ( 1985) and Sibson ( 1984) found that 

the shape of the brittle-ductile boundary must be considered 

in any explanation of intermediate-depth earthquakes. Gettings (1988) showed that a midcrustal mafic intrusion could 

alter the shape of the boundary significantly. A practical 

example of the shallowing of the brittle-ductile boundary is 

provided by the Yellowstone rift complex (Anders and 

others, 1989). The crystalline part of that complex is a 

partial analog of the plutons inferred beneath New Madrid 

and Charleston; the distributions of earthquake hypocenters 

are remarkably similar in all three cases. 

Acknowledgments 

This paper has benefited from thoughtful reviews by 

G.P. Bird (University of California at Los Angeles) and 

W.S. Leith, G.S. Gohn, and D.C. Prowell (all of the U.S. 

Geological Survey). The conclusions were drawn solely by 

the authors. This study was partly supported by Nuclear 

Regulatory Commission Agreement #AT( 49-25)-1 000. 

EVIDENCE OF TERTIARY INTRUSIONS 

BENEATH THE CHARLESTON AREA 

Uplift Associated with Coinciding Gravity and 

Magnetic Anomalies 

From magnetic and gravity anomaly data (figs. 2, 3) 

(Popenoe and Zietz, 1977; Daniels and others, 1983; 

Phillips, 1988; J.D. Phillips and R.M. Stewart, written 

commun., 1985), we infer at least five intrusions onshore 

and one offshore (fig. 4). The largest intrusion is beneath 

St. Helena Sound (fig. 4). The geophysical signature 

suggests a highly magnetic rim and a less magnetic center, 

possibly a syenite-cored gabbro. Drill hole logs, seismic 

reflection profiles, and seismic refraction studies reveal 

probable uplifted basement and Mesozoic rocks as well as 

domed Tertiary sediments (cross sections, fig. 4) overlying 

the intrusions. An alternative explanation is that the apparent doming on seismic profiles at this locality is due mostly 

to velocity pull-up, caused by high-velocity limestone-filled 

channels in the upper part of the section (Gohn and Weems, 

1986). Ackermann (1983) inferred faults bounding the 

upwarps in this area, the adjacent lows being filled by 

Mesozoic sediment. Thick marine Oligocene deposits near 

Charleston are replaced by thick marginal marine deposits 

2 Possible Relationship Between Seismicity and Warm Intrusive Bodies in the Charleston, S.C., and New Madrid, Mo., Areas 
\J ~ 

0 5 10 15 20 25 MILES 

I I I I I I I I I I I 

0 5 10 15 20 25 KILOMETERS 

CONTOUR INTERVAL 100 NANOTESLAS 

EXPlANATION 

Magnetic low - Magnetic high 

Figure 2. Residual aeromagnetic anomalies of the Charleston, S.C., area (J.D. Phillips, written commun., 1986; Phillips, 

1988). 

Evidence of Tertiary Intrusions Beneath the Charleston Area 3 
0 

I 

0 

I 

5 

5 

I 

I 

10 

10 

I 

I 

15 

I 

20 

15 

I 

I 

20 

I 

25 KILOMETERS 

CONTOUR INTERVAL 2 MILLIGALS 

EXPlANATION 

Gravity low 

25 MILES 

I 

ATLANTIC 

OCEAN 

- Gravity high 

Figure 3. Bouguer gravity anomalies of the Charleston, S.C., area (J.D. Phillips, written commun., 1985; long and 

Champion, 1977). 

4 Possible Relationship Between Seismicity and Warm Intrusive Bodies in the Charleston, S.C., and New Madrid, Mo., Areas 
41e 

43e 

\ 

\ 

\ 

5 10 15 20 25 MILES I I I I I I I I 1 

10 15 20 25 KILOMETERS 

Figure 4. Inferred mafic plutons, heat flow, and seismicity 

in the Charleston, S.C., area. Mafic plutons beneath 

Coastal Plain sediments inferred from magnetic and gravity data (figs. 2, 3) (Popenoe and Zeitz, 1977; Daniels and 

others, 1983; J.D. Phillips and R.M. Stewart, unpub. data, 

1985). Basement faults and mafic dikes were inferred from 

magnetic and gravity data (no detailed data are east of 80° 

W.); features not shown by Popenoe and Zeitz (1977), 

Daniels and others (1983), or Phillips (1988) are the present 

authors' interpretations. The heat-flow value of 558 

mW/m2 is from the U.S. Geological Survey's Clubhouse 

Crossroads corehole (Sass and Ziagos, 1977); all other 

ATLANTIC 

OCEAN 

EXPLANATION 

Map 

Inferred mafic pluton 

-----Mafic dike 

---- Basement fault or fracture 

---• Shoreline 

45e 

K,J 

Heat-flow value (in mW/m2 ) 

Epicenter of earthquake occurring 

between 1973 and 1979 

(T arr and Rhea, 1983) 

Cross sections 

Oligocene and Eocene limestone 

Mesozoic sedimentary rock, 

diabase, and basalt flows and 

Cenozoic sediment and 

sedimentary rock 

Crystalline basementHas seismic velocities similar to 

exposed Piedmont rocks and may 

be lithologically similar 

Pleistocene and Pliocene deposits 

Miocene deposits 

Oligocene deposits (not limestone) 

Seismic reflectors: 

K, Cretaceous; J, Jurassic 

values are new and are discussed in the text. For all cross 

sections, horizontal distance is enlarged to show detail. 

Cross section A-A' is based on data by Heron and Johnson 

(1966) and new drill logs; dashed lines represent inferred 

bedding planes; vertical exaggeration is extreme. Cross 

section B-B' was constructed by the present authors from 

seismic reflection profiles SC-5 and VT -3B (Hamilton and 

others, 1983; Coruh and others, 1981). Cross sections 

C-C' and D-D' were constructed from seismic refraction 

data (Ackermann, 1983). Plutons are deeper than the 

sections shown but are at least partly within the brittle part 

of the crust (Bollinger and others, 1985). 

Evidence of Tertiary Intrusions Beneath the Charleston Area 5 
south and southwest of St. Helena Sound. The transition 

occurs over the inferred pluton beneath the sound. Postearly Eocene doming and post-Miocene doming are inferred 

in part from Heron and Johnson (1966) and from data 

presented by Gohn and others (1978a, b). 

Thermal Anomalies 

Published mean heat-flow values for the Charleston 

area are about 50 to 54 mW/m2 (milliwatts per square 

meter) (Sass and Ziagos, 1977). A grand average of all 

published heat-flow values for the eastern half of the United 

States, the Gulf of Mexico, and the Canadian part of Lake 

Superior is 45 mW/m2 (McCartan and Architzel, 1988). At 

Charleston, the heat-flow level outside the area of the 

magnetic and gravity anomalies (figs. 2, 3), is about 43 

mW/m2

, essentially the same as that for the eastern part of 

the country. Analogous to the usual definition of a gravity 

or magnetic anomaly, we define a heat-flow anomaly as an 

area having consistent heat-flow values at least 20 percent 

different from the estimated heat flow for the area or 

equivalent differences in thermal gradients or ground-water 

temperatures. 

Nine new values were calculated for the Charleston 

area from the thermal gradients (Costain and Glover, 1982), 

the lithologic logs from each site (mainly from Gohn and 

others, 1978a, b, 1980; G.S. Gohn, written commun., 

1985), and the harmonic averages of thermal conductivity 

measurements that were grouped by lithology and stratigraphic unit and were based on the conductivity measurements from the core at Clubhouse Crossroads (Sass and 

Ziagos, 1977). Although there is no direct evidence of 

Tertiary igneous activity in the Charleston area, dated 

Tertiary volcanic and shallow intrusive rocks are present in 

central Virginia (Fullagar and Bottino, 1969; Gray and 

Gottfried, 1986), an area also characterized by moderate 

heat flow. The wells for which the new values were 

calculated were less than 270 m deep but appear to give 

reasonable values. 

Within the areas of the inferred intrusions (fig. 4), the 

mean heat flow from four sites over inferred plutons is 

about 53 m W/m2

. The background level for the area outside 

the intrusive complex is 43 mW/m2 . Thus, there is a 

positive heat-flow anomaly of about 10 mW/m2 in the areas 

underlain by intrusions. 

Seismic Evidence 

Positive and negative seismic P wave arrival delays 

indicate high- and low-velocity ray paths for 1973-79 

earthquakes (Tarr and others, 1981). Such a pattern would 

be expected in an area containing rocks such as gabbro and 

granite of different densities and elasticities. However, 

compressional waves tend to move more slowly through 

Table 1. Changes in compressional-wave velocity with 

temperature (Volarovich and others, 1974) 

Compressional wave velocity (km/s) 

Rock type1 At 100 oc At 200 oc At 400 oc At 600 oc 

Gabbro ..... 5.88 5.52 4.45 3.72 

Granite 1 ... 5.00 4.25 3.15 1.77 

Granite 2 ... 4.67 4.08 2.95 1.63 

Granite 3 ... 5.25 4.82 3.81 2.28 

'Gabbro and granite are intended to represent the mafic intrusions 

and wall rock, respectively. 

warmer rock than they do through cooler rock of the same 

composition (table 1). Heat flow and temperature are 

elevated in some of the areas inferred to contain mafic 

plutons, and P waves are delayed in the same areas. Had the 

plutons been completely cooled, P-waves moving through 

them would have arrived early rather than late. 

DISTRIBUTION OF EARTHQUAKE EPICENTERS 

IN THE CHARLESTON AREA 

From 1973 to 1979, many small earthquakes occurred 

around one of the inferred Charleston intrusions (fig. 4). 

The two other clusters of epicenters from that period are 

located near possible basement faults, which we infer from 

offsets of magnetic and gravity gradients (figs. 2, 3) (see 

also Phillips, 1988). Other well-constrained locations are 

not published. The 1886 earthquake epicenter is estimated 

to have been located near one of the inferred plutons (fig. 

4). 

EVIDENCE OF TERTIARY INTRUSIONS 

BENEATH THE NEW MADRID AREA 

Gravity and Magnetic Anomalies 

Positive magnetic and gravity anomalies coincide at 

many localities in the four-State area around New Madrid, 

Mo. (figs. 5-7). They are related to Paleozoic, Mesozoic, 

and Cenozoic mafic and ultramafic rocks (Glick, 1982; 

Hildenbrand and others, 1982; Hamilton and Zoback, 1982; 

Hildenbrand, 1985; Ravat and others, 1987). Some of the 

intrusions are exposed outside the area shown in figure 5, 

others have been sampled in drill holes, and a few are 

inferred only from the geophysical anomalies. Our interpretation of the magnetic and gravity data is based on a positive 

anomaly datum slightly lower than the one used by Hildenbrand and others (1982) and by Ravat and others (1987). 

Using this datum results in somewhat larger inferred intrusions and the delineation of an intrusion in the central part 

of the seismic zone that was recognized by Hildenbrand and 

others (1982) but not included in their interpretive map (see 

6 Possible Relationship Between Seismicity and Warm Intrusive Bodies in the Charleston, S.C., and New Madrid, Mo., Areas 
92° 91 ° 90° 89° 88° 38o,---------~.---------~~--------~~--------~ 

NORTH 

20 40 60 

I I I 

I I I I I 

80 

I 

100 MILES 

I 

0 20 40 60 80 100 KILOMETERS 

( 

SOUTH 

EXPLANATION 

Map 

Inferred mafic pluton 

--- Fault, dike, lithologic boundary in basement 

55e 75• Heat-flow values 

>--<A, 8 Seismic profile 

Cross sections 

(/) 

~ ~ 10l 

oJ~==~==~--------------~~-----------------------~ 

-------- Q ~ 

0 Quaternary deposits 

0 Eocene deposits 

~ Paleocene deposits 

0 Cretaceous deposits 

G Paleozoic rocks 

UJ-

~ 

~en ~~ F== Te ~ o 0.5 _______________ TI<pa,.----_____ ~ i ~ 0.7 ~-------------':co..z ____ _ 6 

~ 

Figure 5. Mafic plutons inferred from coincident magnetic 

and gravity anomalies in the New Madrid, Mo., area (figs. 

6, 7) (Hildenbrand and others, 1982; Braile and others, 

1982) (pluton configuration inferred in part by present 

authors). Heat-flow values (in milliwatts per square 

meter): solid circles from Swanberg and others (1982); 

solid squares from Douglas Smith (unpub. data, 1985). 

Basement lineaments have been interpreted from an aeromagnetic map by Hildenbrand and others (1982). The 

also Hildenbrand, 1985). This body, like one beneath St. 

Helena Sound in South Carolina, has a very magnetic rim 

and a less magnetic core and may be thinner or deeper than 

others in the area. We feel justified in choosing a lower 

anomaly datum, because such models are in all cases 

subjective and, more importantly, because this interpretation accommodates most of the geologic, geophysical, and 

seismologic data that were available at the time of the study 

(1986). The lower anomaly datum also closes the 10-km 

gap between the Bloomfield pluton (fig. 5) and the locus of 

epicenters on its southeastern flank (Ravat and others, 

1987). At best, the boundaries of both an intrusion and its 

Blytheville arch is delineated by seismic profiles (Hamilton 

and McKeown, 1988). The upper part of the section shows 

the Lake County uplift (Q, Quaternary) (Russ, 1982); 

vertical scale is in meters. The exaggerated lower section 

shows deeper units generalized from seismic reflection 

profile S-4 (data from Crone and Brockman, 1982; Hamilton and Zoback, 1982; Hamilton and McKeown, 1988). The 

pluton is deeper than the sections shown. 

adjacent envelope of thermally weakened wall rock can be 

located only approximately from the geophysical data, 

because of the inherent ambiguity of potential fields with 

regard to source geometry, density, and magnetization. 

Thermal Anomalies and Uplifted Tertiary Strata 

In New Madrid, as elsewhere, in the absence of direct 

radiometric dating of drill hole samples, relative age_, of 

different parts of a buried intrusive complex can only be 

inferred from heat-flow and uplift data. Some of the 

intrusions in the New Madrid area are spatially associated 

Evidence of Tertiary Intrusions Beneath the New Madrid Area 7 
0 

I 

0 

I 

20 

20 

I I 

40 

40 

I 

I 

60 

I 

80 

60 

I 

I 

80 

I 

100 KILOMETERS 

100 MILES 

I 

CONTOUR INTERVAL 100 NANOTESLAS 

EXPLANATION 

Magnetic low Magnetic high 

Figure 6. Residual aeromagnetic anomalies of the New Madrid, Mo., area (after Hildenbrand and others, 1982). 

with post-Paleozoic uplift and low to moderate positive 

heat-flow anomalies (figs. 5, 8) and may involve more than 

one period of intrusion. Uplift in the area of the Newport 

pluton (fig. 5) (Glick, 1982), which apparently resulted in 

nondeposition or erosion of middle Eocene deposits, may 

have been due to intrusion during the Eocene or later. North 

of Marked Tree, Ark., upwarped Tertiary reflectors coincide with a seismically active part of an intrusion (fig. 5; 

epicenters, fig. 9) (McKeown, 1984). The feature near 

Marked Tree is called the Blytheville arch (fig. 5) and was 

studied by Hamilton and McKeown (1988) (see "Discussion"). The latest doming noted in the New Madrid 

area-that of the Lake County uplift-is probably Quaternary (Russ, 1982). Basement faults inferred from seismic 

profiles may be associated with any of the uplift or doming 

(Hamilton and Zoback, 1982; Hamilton and McKeown, 

1988). The faults, however, also may be adjacent to 

intrusions. 

Intermediate heat-flow values (25, 34, 42 mW/m2

) 

may reflect the cooler (older) parts of a generally warm 

8 Possible Relationship Between Seismicity and Warm Intrusive Bodies in the Charleston, S.C., and New Madrid, Mo., Areas 
0 20 40 60 80 100 MILES 

0 20 40 60 80 100 KILOMETERS 

CONTOUR INTERVAL 5 MILLIGALS 

EXPLANATION 

Magnetic low Magnetic high 

Figure 7. Bouguer gravity anomalies of the New Madrid, Mo., area (after Braile and others, 1982). 

(50-75 mW/m2

) intrusive complex. The only heat-flow 

value well away from the intrusions is very low-14 

mW/m2 southwest of the Newport pluton. However, ground 

water is anomalously warm over several inferred intrusions, 

especially just west of New Madrid, but is generally cooler 

over other areas (fig. 8) (Swanberg and others, 1982). The 

combined gradient and temperature values of all the warm 

areas outside the seismic area are lower than most of those 

in the seismic area; the map area having the highest 

gradients and the hottest wells falls within the seismically 

active area. This pattern suggests that excess heat is 

spatially related to the intrusions and may emanate from 

them (Mitchell and others, 1984). The uplift pattern and 

positive heat-flow anomalies are compatible with the 

presence of late Cenozoic intrusion activity in the New 

Madrid area. 

Evidence of Tertiary Intrusions Beneath the New Madrid Area 9 
MISSOURI 

(J. 

0 20 40 60 80 100 MILES 

I I I I I I I I 1 I I 

0 20 40 60 80 100 KILOMETERS 

EXPLANATION 

D Hot-water wells, high thermal gradients 

D Hottest wells and highest thermal gradients 

• Cool-water wells, low thermal gradients 

(,-1, Inferred mafic plutons 

~::v 

Figure 8. Ground-water temperatures and thermal gradients in the New Madrid, Mo., area (Swanberg and others, 

1982). Most hot-well temperatures and thermal gradients 

over 24 °C/km coincide approximately with plutons in the 

seismically active part of the New Madrid area. Outlines of 

inferred plutons (from fig. 5) are superimposed on this 

map. The combined gradient and temperature values of all 

the warm areas outside the seismic area are lower than 

most of those in the seismic area; the map area having the 

highest gradients and the hottest wells on the map falls 

within the seismically active area. 

DISTRIBUTION OF EARTHQUAKE EPICENTERS 

IN THE NEW MADRID AREA 

Many hypocenters of earthquakes between 1976 and 

1982 (Herrmann, 1984) coincide with the edges of inferred 

intrusions (figs. 5, 9). Focal depths for small earthquakes in 

the area are 4 to 18 km, the median depth being 7 km 

(Stauder, 1982; O'Connell and others, 1982; Russ, 1984). 

We suggest that larger historical earthquakes also originated 

in this depth range, as they probably did in Charleston (Tarr 

and Rhea, 1983). The recent earthquake hypocenters cluster 

closer to the inferred intrusions than to other anomalous 

masses in the crust (Hildenbrand, 1985). 

Most of the recent epicenters are in the generally 

northeast-trending New Madrid fault system of Nuttli and 

0 

I I '• •' 

0 25 

25 

I I 

50 KILOMETERS 

50 MILES 

I 

Figure 9. Central part of the New Madrid seismic zone. 

Outlines of inferred plutons from figure 5 are plotted on 

1976 to 1982 earthquake epicenter map of Herrmann 

(1984). 

Herrmann ( 1984). At least some of the energy released as 

earthquakes in the New Madrid area could be attributed to 

the wall-rock heating mechanism described for Charleston. 

Movement on the fault segments (Nuttli and Herrmann, 

1984), which are generally tangential to the intrusions (fig. 

5) (Hamilton and Zoback, 1982), may reflect episodic 

release of regional stress accumulated along the weakened 

wall-rock boundaries, or local contraction of the cooling 

intrusions and surrounding wall rock, or a combination of 

both processes. 

THEORETICAL CONSIDERATIONS 

The Hole-in-the-Plate Model 

The hole-in-the-plate model was evolved by metallurgical engineers, who observed that compression of a 

metal plate with a hole in it resulted in anomalously high 

stess adjacent to the hole. As applied to geologic situations, 

this model requires a weak zone surrounded by a stronger 

material under compression. 

Kane ( 1977) inferred from geophysical data that 

midcrustal intrusions or intrusive complexes of mafic to 

10 Possible Relationship Between Seismicity and Warm Intrusive Bodies in the Charleston, S.C., and New Madrid, Mo., Areas 
A 

SURFA<;:E .. :: .. . . :. : . ... •••• 0 •• 

8 

• •• 0. 

. . . . .. ·· • • 0 0. 0. 

EXPLANATION 

c 

. .. . . . • • 0. 

. .. .. ·.· 0 ••••• 

• •• 0 

0 •••••••• 

.. ·.· . . .. . . · ... . . ·.· ..... 

• 0 ••• 

: • 0 • 

. · .... 

D Sedimentary rocks 

~ Crystalline rocks 

# Dilation during intrusion or contraction 

during cooling of magma 

[_~: -:,'1 Mafic magma (A) and 

mafic rock (B,C} 

/ Emanation of heat 

• Earthquake hypocenter 

___ Fault-arrow showing direction of movement 

- - - Zone of weakness due to long-term heating 

Figure 10. Effects of a midcrustal intrusion in relation to 

seismicity. Surface to a depth of 2 to 5 km is occupied by 

Mesozoic sedimentary rock, diabase, and basalt flows and 

Cenozoic sediment; pre-Mesozoic metamorphic rock is 

below. Maximum regional compression is horizontal (not 

ultramafic composition underlie the major seismic areas in 

eastern North America. He and other authors (Long and 

Champion, 1977; Campbell, 1978; McKeown, 1978) considered the stress-concentrating property of a hole or inclusion in a brittle plate under uniform stress as a model for 

mafic complexes in a felsic crust. Because a mafic intrusion 

is generally about as strong as the granitic basement, debate 

has focused principally on whether the mafic intrusions are 

sufficiently serpentinized or otherwise altered to cause a 

significant loss of strength. In the case of a relatively weak 

elliptical inclusion having its long axis perpendicular to the 

regional stress field, boundary stress concentration more 

than double the regional stress is possible (Campbell, 

1978). The difficulty with this model has been that, where 

such mafic complexes are exposed, no evidence for ubiquitous alteration or serpentinization has been observed 

(Kane, 1977). Therefore, another cause of weakness must 

be invoked in the subsurface. 

Figure 10 shows some of the processes that accompany intrusion, crystallization, and cooling of a pluton. 

Figures lOA and lOB are shown as background. Figure lOC 

describes the lingering thermal effects of a cooling pluton, 

which are the focus of the present study. 

All intrusions are surrounded by thin, weak zones 

caused by fracturing of wall rock during initial magmatic 

shown), perpendicular to the long horzontal axis of the 

pluton. Sedimentation subsequent to the intrusive event 

is not shown. A, Initial stage, intrusion. 8, Intermediate 

stage (contraction of plutons during crystallization of 

magma.) C, Late stage. 

emplacement (fig. lOA), followed by fracturing of both the 

wall rock and the igneous rock during crystallization of 

magma (fig. lOB). Displacement of country rock would 

result in earthquakes and faulting in both the initial and the 

intermediate phases. The displacement in the second phase 

may be opposite the displacement in the first phase. 

Although intrusion is essentially a compressional event, 

crystallization is tensional because of the 10- to 15-percent 

reduction in volume. The fractured wall-rock zone tends to 

remain weak owing to these processes. In addition, many 

plutons have weak zones caused by hydrous minerals (such 

as serpentine and chlorite), which in turn are caused by 

fluids from the magma. Incompletely cooled intrusions 

make yet another contribution to the weak zone: heated wall 

rock has lower ultimate strength than adjacent cooler wall 

rock (fig. lOC). The thermal effect is more pronounced and 

long lasting around mafic intrusions because the temperature of the intruding magma is invariably higher than that of 

the felsic magmas. 

New Version of the Model 

In this modification of the "hole-in-the-plate" hypothesis, the plate is the North American crustal plate, under 

Theoretical Considerations 11 
A 8 

Surface 

-10 kilometers 

EXPLANATION 

~ Mafic magma or rock 

1::·::·.::· :: :J Zone of stress accumulation and maximum 

potential for earthquakes 

~ Horizontal regional compression 

·-· · · ·-· · · ·- Sharpest thermal gradient 

= = = = = Envelope of thermally weakened rock 

________ ----L.:....L..._ __ .IL-1\CA.ru::-:s::t..-:::-_______ -20 kilometers 

Mantle 

Figure 11. Hypothetical midcrustal plutonic complex. A, 

Vertical section through the crust perpendicular to the 

direction of maximum regional compression. Only plutons that still have elevated temperatures are shown; they 

are inferred to have been intruded in the middle to late 

moderate compression since the Early Cretaceous (Zoback 

and Zoback, 1980), and the holes are the envelopes of 

fractured and thermally weakened wall rock that surround 

the fractured surface of cooling Tertiary intrusions (fig. 11). 

The fact that even warm mafic rock is very strong is not 

important; the regional compressive stress is not transmitted 

to the pluton through the envelope of thermally weakened 

wall rock. The energy concentrated at the boundary 

between weak and strong wall rock long after intrusion is 

derived from ongoing regional compression rather than 

from the dynamic processes that accompanied initial intrusion and crystallization of magma. The greatest accumulation of energy occurs when the long axis of the warm part 

Tertiary. B, Projection of pluton contacts in A onto a 

horizontal plane. The projected contacts are from a depth 

interval of 5 to 10 km. Horizontal regional compression is 

perpendicular to the long axes of the plutons. 

of a pluton is perpendicular to the direction of regional 

compression. This configuration permits the largest planar 

area to fail as stress and strain energy accumulated outside 

the weak zone reaches a critical level. Whereas small 

earthquakes could occur episodically anywhere along a 

weak-strong boundary of any size, a large earthquake 

would require simultaneous failure of an extensive source 

plane. The area of the weak zone, and thus the weak-strong 

boundary, could grow as the thermal aureole propagated 

outward during cooling of the intrusive body. 

One way of explaining the weak -strong boundary is 

to view the relatively thin envelope of wall rock as losing 

strength by transition into the ductile flow field. The loss of 

12 Possible Relationship Between Seismicity and Warm Intrusive Bodies in the Charleston, S.C., and New Madrid, Mo., Areas 
:I: 

t 

w 

Cl 

SHEAR RESISTANCE 

Aanorm 

HEATED ZONE 

INTRUSIVE ZONE 

Figure 12. Effect of reducing maximum shear resistance 

(from 0' norm to O'int) and shallowing the depth (from Znorm 

to zint) to the brittle-ductile transition for failure on a 

planar fracture or fault associated with the zone of heated 

wall rock above a cooling pluton (after Gettings, 1988; see 

also Hand in, 1966). The dashed line represents lowering of 

the maximum shear resistance above a cooling pluton. 

The reduction of shear strength is a result of additional 

weakening caused by long-term heating. "Transition" is 

the brittle-ductile transition before intrusion or away from 

the intrusive zone. 

strength is due to heating by the intrusion (fig. 12) (Gettings, 1988; Sibson, 1984). The boundary between the 

weak hole and the strong plate would then be within the 

wall rock surrounding the intrusive complex. The sharpness 

of the transition from weakened to strong wall rock would 

appear to be an important factor in the model; a very gradual 

transition might not result in sufficient stress concentration. 

In the case of initiation of sliding on existing fractures, 

Gettings ( 1988) demonstrated that the boundary is sharp and 

that the effect due to heat from the intrusion-important for 

times of the order of 10 m. y. for intrusions about 20 km in 

diameter-is the shallowing of the depth to the transition by 

as much as 7 km. The elastic thickness of the plate (and thus 

the strength of the plate) is therefore greatly reduced above 

the intrusive zone. Although figure 12 depicts only the top 

of an intrusive zone, the same weakening effects are 

expected in the third dimension-that is, along the sides of 

an intrusion. Crystallization in an intrusion about 30 km 

across and 3 km thick in the midcrust would be achieved in 

less than 50,000 years, but it would take 20 to 30 m.y. to 

cool to approximately the geotherm (Jaeger, 1968). Thus, 

the weak envelope might be maintained for the first 10 m.y. 

or so by a brittle-ductile transition process, but, later in the 

cooling history, both thermal weakening and hydrothermal 

alteration would become increasingly significant. 

Under continuing horizontal regional compression, 

the weakened zone itself would fail by quasi-plastic flow, 

but stress would accumulate as elastic strain energy outside 

the boundary between weaker and stronger host rocks. The 

shallower depth to the transition in the neighborhood of the 

intrusion relative to the surrounding region may result in 

further concentration of stress and elastic strain energy 

around the intrusion. In some circumstances, this stress may 

exceed elastic limits and suddenly release stored energy in 

the form of an earthquake. 

Another consideration is that the temperature in the 

vicinity of large mafic intrusions can be maintained at a 

high level for 40 m. y. or longer owing to successive 

intrusive events and can thus extend the time period for 

thermally induced reduction of strength (see p. 14 for 

discussion of data from Yellowstone) (Anders and others, 

1989). Mafic magma generally comes from a subcrustal 

source and is likely to preferentially intrude zones of crustal 

weakness, such as preexisting fracture zones. Vertical 

movement of magma generated in the mantle can occur 

within a compressional regime such as eastern North 

America wherever the hydraulic pressure of the magma 

exceeds the horizontal compressive stress within the crust. 

Extension always occurs at 90° to the direction of compression; therefore, if the net result of the injection of magmatic 

heat is to maintain the weakened zone in the crust, subsequent magma is likely to intrude in the same zone, creating 

complexes of multiple intrusions spanning the time interval 

of magma generation. Such complexes are commonly 

observed in the Western United States (Gilluly, 1965) and 

also in central Virginia (Gray and Gottfried, 1986). 

Possible Sources of Low to Moderate Positive 

Heat-Flow Anomalies 

Sources of anomalous heat flow include conduction 

from a mantle hot spot, frictional heating or elastic strain 

release during earthquakes, or convection due to groundwater motion on faults (Hinze and Braile, 1988); radiogenic 

heat from unmetamorphosed granitic intrusions (Costain 

and Glover, 1982) (syenite bodies may also generate 

significant radiogenic heat, but they are too small to 

produce the signatures noted in this study); and thermal 

refraction from basement highs or the intrusion of hot, 

mafic magma at shallow depths (Mitchell and others, 

1984). The hot-spot model yields a longer wavelength for 

the heat-flow anomaly than was observed at Charleston 

(Anders and others, 1989); heat-flow anomalies due to 

frictional heating and stress release are generally smaller 

than the anomalies that we describe (Lachenbruch and Sass, 

Theoretical Considerations 13 
1980); and, although ground-water circulation cannot be 

ruled out, the large size of the vertical conduits that would 

have to be maintained to account for the long-term transfer 

of heat seems unlikely at midcrustal depths. 

Thermal refraction combined with radiogenic heating 

of an uplifted basement block containing a substantial 

granitic component could account for part of the heat-flow 

anomaly but would not fit the gravity and magnetic data. 

Because mafic rocks do not contain appreciable concentrations of radiogenic elements (Heir, 1978; Taylor and 

McLennan, 1981) and because the thermal conductivity of 

mafic rocks (for example, Roy and others, 1981) is nearly 

identical to that of the overlying Coastal Plain rocks and 

sediments (Sass and Ziagos, 1977), thermal refraction from 

a mafic basement block cannot be the cause of the heat-flow 

anomaly. In any case, the calculated refraction anomaly in 

the Charleston area, even for granitic compositions, is only 

about one-third of the observed heat-flow anomaly. 

We postulate that the thermal anomalies may be due 

to mafic intrusions sufficiently recent and shallow that they 

constitute significant heat sources. 

DISCUSSION 

Uniqueness of the Charleston and New Madrid 

Areas 

In the eastern half of the United States, no surface 

faults have been found in the vicinity of earthquakes. The 

most direct and abundant evidence of both historical and 

prehistoric earthquakes larger than about magnitude 5. 5 is 

liquefaction features induced by strong seismic shaking 

(Obermeier and others, 1985). Liquefaction features appear 

to be most abundant in the immediate areas of the major 

historical earthquakes in the eastern half of the country, 

according to recent field studies (Obermeier and others, 

1985, 1987, 1989; Tuttle and Seeber, 1989; Amick and 

others, 1990). This regional distribution of liquefaction 

features strongly suggests that the geologic setting of an 

area such as Charleston, where a major earthquake has 

occurred, is unusual. Contrary to the conclusions of 

Bollinger and others ( 1989), geologic conditions capable of 

producing large earthquakes may be rare or absent in most 

of the eastern half of the United States. 

Examination of geophysical, geologic, and earthquake data for the Charleston and New Madrid areas in light 

of investigations of the brittle-ductile transition layer (Gettings, 1988; Bollinger and others, 1985; Sibson, 1984) and 

of the Yellowstone rift complex (Anders and others, 1989) 

provides the basis for modification of the hole-in-the-plate 

model. A review of part of the data for the eastern half of 

the United States (coinciding magnetic and gravity anomalies and their relationship to large earthquakes) (McCartan 

and others, in press) also suggests that the crust beneath 

Charleston and New Madrid is geologically very unusual. 

Comparison of Charleston, New Madrid, and 

Yellowstone 

Although the earthquake mechanism in the extensional regime of Yellowstone is dissimilar to the one that we 

propose for the compressional regime of Charleston and 

New Madrid, the distributions of heat flow and seismicity 

are similar. In Yellowstone (Anders and others, 1989), an 

elongate intrusive complex that contains magma at one end 

(northeastern) and is completely crystallized at the other is 

associated with seismicity that is concentrated at or beyond 

the edge of the intrusion. Few earthquakes occur directly 

above any part of the intrusive complex, including the 

crystalline part. Earthquakes in Charleston and New Madrid 

also occur mainly at the edge of or beyond the boundaries of 

inferred intrusions. In all three areas, heat flow is highest 

over the intrusions and lower in the surrounding area. 

Apparently, even though the weakest part of the plate is the 

zone above the intrusive complex, the stress concentration 

occurs where effective plate strength changes most rapidly-in this case, along the margins of the intrusive zone. 

Furthermore, seismicity is associated with the oldest 

(6-15 Ma), coolest part of the Yellowstone complex as well 

as with the newer parts. Lingering thermal effects, rather 

than any mechanical processes associated with intrusion, 

are clearly involved with the seismicity at the southwestern 

(oldest) end of the Yellowstone seismic area. The fact that 

the belt of active seismicity is progressively farther from 

older parts of the Yellowstone intrusive complex may be 

due in part to outward propagation of the thermal aureole. 

Because horizontal movement of the crust over a hot 

spot is responsible for the intrusive complex at Yellowstone, the intrusions are arranged linearly. At Charleston 

and New Madrid, what appear to be discrete intrusions may 

in fact be composite bodies that represent multiple events; 

they are probably arranged in complex vertical stacks. 

These intrusions failed to reach the surface because they 

were intruded into crust within a compressional regime. We 

suggest that historical seismicity near part of an inferred 

intrusion in the Charleston area may indicate a relatively 

young intrusive event. Seismicity farther away could be 

associated with the distant, outward-propagating thermal 

aureole of an older intrusive event. 

Probable Causes of Earthquakes Around Inferred 

Warm Plutons 

All the factors discussed in the preceding sections 

suggest that the crust in the vicinity of mafic intrusions is 

weak and remains so throughout the thermal history of the 

complex (figs. 11, 12). It therefore follows that, if the long 

axis (in map view) of the intrusive zone is oriented 

approximately perpendicular to the dominant regional 

stress, significant stress concentrations will occur at the 

14 Possible Relationship Between Seismicity and Warm Intrusive Bodies in the Charleston, S.C., and New Madrid, Mo., Areas 
boundaries of the weakened zone and will constitute a 

source of sufficient differential stress to cause seismicity. If 

the magma supply is too meager to provide enough heat to 

weaken the country rock, or if the configuration of the 

thermally weakened zone is not favorable relative to the 

regional stress field, an intrusive complex will not have 

related seismicity. Too little magma or an unfavorably 

oriented weak zone may account for aseismic areas having 

positive gravity, magnetic, and thermal anomalies. 

Seismic activity associated with an intrusion may 

parallel the history of intrusion and cooling of that intrusion 

(fig. lOA-C). We envision an initial seismic period accompanying intrusion of magma and thermal expansion and 

displacement of country rock (fig. lOA), an intermediate 

period of seismicity resulting from release of stress related 

to crystallization of the magma and contraction during 

cooling (fig. lOB), and the final (and probably the longest) 

phase, in which episodic seismic activity occurs as a result 

of the release of regional compressive stress concentrated 

adjacent to the weakened wall-rock envelopes (fig. lOC). 

The cycle would be repeated for each major intrusive event. 

We infer that the largest historical earthquakes at Charleston 

and New Madrid may be associated with the extensive, thin 

zone of weakness caused by the outward-propagating thermal aureole around an intrusive complex. Many earthquakes in the Charleston and New Madrid areas are not 

immediately adjacent to the inferred plutons (figs. 4, 9). If 

these earthquakes are related to the thermal weakening 

mechanism that we propose, then the intrusive bodies 

constituting the heat sources must have been intruded in the 

20- to 40-Ma range. Alternatively, the earthquakes may be 

related to small faults that move in response to the cooling 

and contraction of the intrusions and surrounding rock (as at 

Yellowstone) (Anders and others, 1989). Movement on 

small faults, however, would not explain the generation of 

major earthquakes. 

Because accumulation and release of stress around an 

intrusive complex are likely to be episodic and spatially 

complicated, some parts of an intrusion's boundaries may 

be active while others are dormant at any particular time. 

For the same reason, inferred intrusions marked by low to 

moderate positive heat-flow anomalies and late Cenozoic 

doming may not be associated with historical seismicity. 

Doming in the Charleston Area 

Local uplift of units between the basement and the 

upper Eocene datum southwest of Charleston is masked by 

the steep regional dip produced by progressive, basincentered downwarp. The uplift becomes more apparent 

when pre-Oligocene contacts are compared with a preOligocene datum rather than with modern sea level. The 

deep drill-hole data alone are too sparse to permit assignment of the post-Eocene uplift to the site of a particular 

intrusion. However, the doming is compatible with postEocene intrusion, and the doming discovered near the 

surface (cross section A-A', fig. 4) indicates that the uplift 

of deeper units is probably mostly over the largest intrusion 

at St. Helena Sound (fig. 4). The tops of the inferred 

intrusions lie below the level of the cross sections but are at 

least partly within the brittle part of the crust (Bollinger and 

others, 1985). 

Two other mechanisms that might account for small 

domes-erosion and differential compaction-do not 

account for the geologic data in the Charleston area. Sea 

level has risen above the domed area several times since the 

Eocene (McCartan and others, 1990). Sedimentation and 

erosion accompanying sea-level rise tend to bevel off a 

dome and fill in a depression. 

Blytheville Arch, New Madrid 

Evidence from New Madrid is similar in many 

respects to evidence from Charleston: geophysically 

inferred midcrustal mafic plutons coincide with low to 

moderate positive heat-flow anomalies and late Tertiary 

uplift. Some small earthquake hypocenters are clustered 

around the edges of some inferred plutons, and the hypocenters estimated for the three main 1811-12 earthquakes 

were near one of the plutons. However, whereas no major 

fault has been discovered in the Charleston area, seismic 

evidence suggests at least one major fault within the New 

Madrid seismic zone. This fault, which is associated with 

the Blytheville arch (Hamilton and McKeown, 1988), may 

have the surface area necessary to generate a major earthquake. The presence of the fault does not eliminate the 

possibility that warm plutons in the crust beneath the New 

Madrid area are directly or indirectly related to historical 

seismicity in the area. 

The Blytheville arch (Hamilton and McKeown, 1988) 

is partly bounded on the southeast by an inferred pluton, 

and the northern end of the arch appears to terminate in the 

vicinity of a second inferred pluton (fig. 5). Discontinuous 

reflectors in profiles through the two boundaries are attributed to faults by Hamilton and McKeown (1988). In view of 

the possible presence of warm intrusive material in the 

vicinity of the eastern half of the northern profile (profile 

A-A' of Hamilton and McKeown (1988); A, fig. 5), an 

alternative explanation for the disruptions is possible. The 

western half of profile A, which contains only small patches 

of reflectors below 1 s, may indicate an Early Cretaceous or 

post-Paleozoic intrusion. This part of the intrusive complex 

would be entirely cooled to the geotherm. A later intrusion, 

below 3. 3 s, may account for both the uplift of Tertiary beds 

in the eastern half of profile A and the low to moderate 

positive heat flow (66 wM/m2

, fig. 5). Seismicity in this 

area may be related to long-term thermal weakening of wall 

rock. 

Uplift and disrupted reflectors in the northern half of 

profile B cannot be attributed to mafic intrusions, because 

Discussion 15 
the inferred intrusion is beneath the southern half of the 

profile. If the age of the reflectors is correctly identified, 

then the intrusion there must be pre-Cretaceous. However, 

it is likely that a zone of weakness, or at least a zone of 

significant inhomogeneity, exists along the boundary 

between the inferred pluton and the country rock. This zone 

may be an important factor controlling the position of the 

Blytheville arch. 

CONCLUSIONS 

We conclude that the 10- to 20-Ma intrusive phases of 

mafic igneous complexes in the Charleston and New 

Madrid areas are the cause of some domed Tertiary rocks 

and that these rocks are spatially associated with low to 

moderate positive heat-flow anomalies. Some of the large 

historical earthquakes and clusters of smaller earthquakes in 

the two areas may have occurred as a result of sudden 

release of regional compressive stress concentrated along 

the boundaries of thermally weakened wall-rock zones 

around the Tertiary intrusions. Although quantitative modeling would be required to differentiate between the causes 

of large-magnitude and smaller earthquakes, we infer that 

their causes are similar, because both sizes of earthquakes 

occur in the vicinity of the intrusive complexes in the 

Charleston area. 

Seismicity in the New Madrid area may be related to 

crustal inhomogeneities adjacent to pre-Tertiary (cool) 

intrusions and to weak wall-rock envelopes caused by 

long-term thermal anomalies emanating from mid-Tertiary 

or younger (warm) intrusions. Faulting may be involved in 

both cases. 

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18 Possible Relationship Between Seismicity and Warm Intrusive Bodies in the Charleston, S.C., and New Madrid, Mo., Areas 


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