Structure of the Reelfoot-Rough Creek Rift System,

Fluorspar Area Fault Complex, and Hicks Dome,

Southern Illinois and Western Kentucky—

New Constraints from Regional

Seismic Reflection Data

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1538-Q

MISSOURI

- ^^*- » ~: ^

ARKANSAS
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 from 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 (chapter E in this series).
Structure of the Reelfoot-Rough Creek Rift System, 

Fluorspar Area Fault Complex, and Hicks Dome, 

Southern Illinois and Western Kentucky— 

New Constraints from Regional 

Seismic Reflection Data

By Christopher J. Potter, Martin B. Goldhaber, Paul C. Heigold, 

and James A. Drahovzal

INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

Edited by Kaye M. Shedlock and Arch C. Johnston

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1538-Q

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

Structure of the Reelfoot-Rough Creek rift system, Fluorspar area fault complex, and Hicks 

dome, southern Illinois and western Kentucky : New constraints from regional seismic 

reflection data / by Christopher J. Potter....[et al.].

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

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

Includes bibliographical references.

Supt.ofDocs.no. : I 19.16: 1538Q

1. Geology, Structural—Illinois. 2. Geology, Structural—Kentucky. 3. Geology, 

Stratigraphic—Cambrian. 4. Seismic reflection method. I. Potter, Christopher J. 

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

QE535.2.UGI59 1995 vol. Q 

[QE627.5.13] 

551.2'2'09778985—dc20

[551.8'09773] 95-3057

CIP
CONTENTS

Abstract........................................................................................................................... Ql

Introduction.................................................................................................................... 2

Geologic Setting............................................................................................................. 2

Data Acquisition and Processing.................................................................................... 4

Correlation of Reflections with Regional Stratigraphy.................................................. 4

Velocity Assumptions..................................................................................................... 5

Interpretation of the Seismic Profile............................................................................... 6

Stephens Well and Vicinity.................................................................................... 6

Tabb Fault System.................................................................................................. 6

Fluorspar Area Fault Complex............................................................................... 7

Ohio River-Rock Creek Graben Area.................................................................... 10

Discussion....................................................................................................................... 13

Reelfoot-Rough Creek Rift.................................................................................... 13

Fluorspar Area Fault Complex............................................................................... 16

Hicks Dome............................................................................................................ 16

Conclusions.................................................................................................................... 17

References Cited............................................................................................................. 18

PLATE

[Plate is in pocket] 

1. Interpretive line drawing of seismic reflection profile, southern Illinois and western Kentucky

FIGURES

1. Regional geologic setting and location of seismic line, southern Illinois and western Kentucky................................ Q3

2. Seismic reflection data near the Tabb fault zone, western Kentucky............................................................................ 5

3. Detail, seismic reflection data in the Tabb fault zone, western Kentucky .................................................................... 7

4. Seismic reflection data, Griffith Bluff graben, western Kentucky................................................................................ 8

5. Seismic reflection data, Mexico graben, western Kentucky......................................................................................... 9

6. Seismic reflection data, Hicks dome, southern Illinois................................................................................................. 11

7. Map showing regional variation in Cambrian rift structure, U.S. Midcontinent.......................................................... 15

III

STRUCTURE OF THE REELFOOT-ROUGH CREEK RIFT SYSTEM, 

FLUORSPAR AREA FAULT COMPLEX, AND HICKS DOME, 

SOUTHERN ILLINOIS AND WESTERN KENTUCKYNEW CONSTRAINTS FROM REGIONAL 

SEISMIC REFLECTION DATA

By Christopher J. Potter, 1 Martin B. Goldhaber,2 Paul C. Heigold,3 

and James A. Drahovzal4

ABSTRACT

Because the New Madrid seismic zone coincides spatially with a segment of the Cambrian Reelfoot rift (RFR), 

regional studies of subsurface Cambrian rift structures may 

provide important insights into the structural fabric of the 

New Madrid seismic zone. High-quality seismic reflection 

data examined in this study show that, north of the New 

Madrid seismic zone within the major dogleg bend in the rift 

system in southern Illinois and western Kentucky, the geometry of the Cambrian rift system is that of a half-graben that 

thickens to the southeast. This contrasts with the northwardthickening half-graben observed to the east, in the Rough 

Creek graben in Kentucky, and with the more symmetric 

graben to the south, in the seismogenic part of the RFR.

The 83-km segment of seismic reflection data examined 

here is oriented northwest-southeast within the major bend in 

the Cambrian rift, in southeastern Illinois and western Kentucky. The margins of the 110-km-wide rift (as determined 

by aeromagnetic data) lie beyond the ends of the seismic line. 

The data show that, near Hicks dome (southeastern Illinois), 

Lower and Middle Cambrian syn-rift sedimentary rocks 

occupy about 0.4 s two-way travel time on the seismic reflection section (corresponding to a thickness of about 1.1 km). 

This stratigraphic interval thickens to the southeast within 

the rift. The syn-rift sedimentary sequence occupies about

1 U.S. Geological Survey, Mail Stop 939, P.O. Box 25046, Denver 

Federal Center, Denver, CO 80225.

2 U.S. Geological Survey, Mail Stop 973, P.O. Box 25046, Denver 

Federal Center, Denver, CO 80225.

3 Illinois State Geological Survey, Champaign, IL 61820 [now at Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439].

4 Kentucky Geological Survey, 228 Mining and Mineral Resources, 

University of Kentucky, Lexington, KY 40506.

0.76 s (2.1 km) near the Ohio River and is thickest against 

basement-cutting normal faults in the area of intersection of 

the Tabb and Pennyrile fault systems in western Kentucky, 

where it occupies 1.13 s (3.1 km). Near the southeast margin 

of the rift, the Cambrian syn-rift sequence thins markedly to 

the southeast across steep basement-cutting normal faults in 

the Tabb-Pennyrile zone; these northwest-dipping faults 

appear to have been the dominant faults in this part of the 

Cambrian rift. Post-Mississippian reverse displacements are 

also evident in the Tabb-Pennyrile fault zone.

Comparison of these seismic reflection data with other 

published reflection data in the Cambrian rift system reveals 

several major along-strike changes in the cross-sectional 

geometry of the rift, similar to other rift systems. We propose 

that the localization of New Madrid seismicity within a discrete segment of the Reelfoot rift is inherited from Cambrian 

rift structure; the zone of seismicity appears bounded on the 

north and south by Cambrian accommodation zones that 

linked segments with differing rift geometry.

The seismic data also provide new information on the 

late Paleozoic development of Hicks dome (southeastern 

Illinois) and the surrounding Fluorspar area fault complex 

(FAFC) in southeastern Illinois and western Kentucky. Near 

Hicks dome (a center of carbonatitic intrusions), strong Mississippian reflections are relatively continuous, whereas the 

Cambrian-Ordovician section is greatly disrupted. This disruption is probably related to the combination of intrusive 

and explosive processes that formed the dome. The entire 

section of Paleozoic rocks and the top of the Precambrian 

basement are domed, demonstrating that the uplift originated 

in the basement rather than within the sedimentary section. 

Southeast of Hicks dome in western Kentucky, a series of 

grabens and horsts in the FAFC document a late Paleozoic

Ql
Q2 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

reactivation of the Cambrian rift. Beneath two of the FAFC 

grabens, the bounding faults meet within the Knox Group 

and do not continue to depth. This requires that extension 

was locally accommodated laterally within the Knox. Other 

normal faults in the FAFC clearly offset the top of Precambrian basement.

The broad FAFC disappears to the southwest beneath 

Cretaceous and Tertiary sedimentary rocks of the Mississippi Embayment; the strike of these faults is collinear with 

the Reelfoot rift and the principal zone of New Madrid seismicity to the southwest. It is not known how far the FAFC 

continues beneath the Cretaceous and Tertiary cover in the 

embayment. It is possible that the FAFC terminates northeast of the well-defined cluster of New Madrid earthquake 

epicenters and that strain transferred from the New Madrid 

seismic zone is distributed in the much wider FAFC so that 

no individual fault accumulates sufficient stain energy for a 

major earthquake within the FAFC. This may help explain 

the abrupt northward decrease in seismicity in the northernmost Mississippi Embayment.

INTRODUCTION

Recurring historic seismicity in the New Madrid and 

Wabash Valley seismic zones has focused attention on the 

tectonic history of the midcontinent region. These seismic 

zones lie in a structurally complex part of the North American Midcontinent. A Cambrian rift complex comprised two 

major structures, the Reelfoot rift and Rough Creek graben, 

which intersect to form a "dogleg" map pattern centered 

roughly on the junction of the Mississippi and Ohio rivers 

(fig. 1). Late Paleozoic reverse, normal, and strike-slip faulting and Mesozoic normal faulting affected the region, probably as an intracratonic response to Alleghanian-Ouachita 

orogenesis and subsequent continental breakup. In many 

cases, these later movements occurred along older Cambrian 

rift structures (Nelson and Lumm, 1984; Nelson, 1991). 

Because the well-established zones of modern seismicity 

occur within the Cambrian Reelfoot rift and near its northwest margin, it is important to understand the regional patterns of Cambrian extensional structures and the subsequent 

history of faulting in and near the rift complex. The vast 

majority of New Madrid earthquakes occur within the Precambrian basement; only a small percentage have shallow 

focal depths within the Paleozoic sedimentary rocks (Chiu 

and others, 1992). Thus, comparison of the style of faulting 

within the Paleozoic section with that in the Precambrian 

basement is another important goal of structural studies in 

the rift complex.

This study is based on interpretation of an 82.8-km segment of a northwest-southeast-trending dynamite-source 

seismic reflection profile that lies along the axis of the major 

bend in the Cambrian rift system in southern Illinois and 

western Kentucky (fig. 1). This survey produced high-quality

data that effectively image the regional structure and stratigraphy. The area has historically been nearly aseismic, and it 

lies in a transitional area between the well-defined New 

Madrid seismic zone and a more diffuse area of seismicity in 

southern Illinois and Indiana. The study area contains major 

structures that are critical to understanding the regional tectonic context of this transition in seismicity.

Acknowledgments.—We are grateful to James W. Baxter of the Illinois State Geological Survey for his contributions to our understanding of the geology of Hicks dome and 

environs. J.P. Fagin of the Illinois State Geological Survey 

prepared two synthetic seismograms, which provided crucial 

constraints for seismic interpretation. M.C. Noger of the 

Kentucky Geological Survey provided valuable insights that 

improved our discussion of the western Kentucky part of the 

seismic profile. Cliff Taylor of the U.S. Geological Survey 

contributed to the early stages of geologic interpretation of 

the seismic line. Formal and informal collaborations between 

the U.S. Geological Survey and the Illinois Basin Consortium were instrumental in this study. The study was jointly 

supported by the U.S. Geological Survey's Evolution of Sedimentary Basins Program and Paducah CUSMAP (Conterminous United States Mineral Assessment Program) Project, 

as well as the Earthquake Hazards Reduction Program.

GEOLOGIC SETTING

The seismic reflection profile crosses numerous faults 

that originated during Cambrian rifting (fig. 1). These 

include the Herod fault (an extension of the Lusk Creek fault 

zone) near the northwest end of the profile in southern Illinois, the Tabb and Pennyrile fault systems near the southeast 

end of the profile in western Kentucky, and several unnamed 

faults. The north-trending Shawneetown fault system, also 

crossed by the seismic line in southern Illinois (fig. 1) did not 

originate in Cambrian time (as seen in the data in this study), 

although it merges into the east-trending Rough Creek fault, 

which was clearly a major Cambrian rift-bounding fault. In 

western Kentucky, the Rough Creek fault system defines the 

northern boundary of the Cambrian Rough Creek graben 

(Nelson, 1991; Bertagne and Leising, 1991), and the Pennyrile fault system (in the broad sense used by Lumm and 

others, 1991) forms the graben's southern boundary. (Lumm 

and others' use of "Pennyrile fault system" includes the 

"unnamed faults to the south" referred to as part of the rift 

boundary by Nelson, 1991.) East of the study area, displacement along the northern boundary greatly exceeds that along 

the southern boundary (Bertagne and Leising, 1991). Within 

the study area, Nelson (1991) has identified the Lusk Creek 

fault zone as the northwest boundary of the Reelfoot rift near 

its intersection with the Rough Creek graben; Hildenbrand 

and others (1992, in press) present potential-field data that 

suggests that this rift boundary is about 30 km to the northwest of the Lusk Creek fault zone. In this paper, we present
STRUCTURE OF REELFOOT-ROUGH CREEK RIFTS FROM SEISMIC REFLECTION DATA Q3

87°

INDIANA

ILLINOIS

J^ f

r^J

37°

100 KILOMETERS

Figure 1. Map of the southern Illinois, southern Indiana, western Kentucky and adjacent States showing location of the seismic line and 

geologic features near the bend in the Reelfoot-Rough Creek rift system. Bold dashed lines are the boundaries of the Cambrian rift system 

based on the depth to magnetic basement (Hildenbrand and others, 1992). Light stipple is the northern part of the New Madrid seismic zone. 

Heavy solid lines are faults. Lusk Cr. FZ, Lusk Creek fault zone; HF, Herod fault; SF, Shawneetown fault; HD, Hicks dome; FAFC, Fluorspar area fault complex (shown schematically only—the number of faults is too large to illustrate at this scale); RCG, Rock Creek graben; 

TF, Tabb fault system; TA, Tolu arch.

evidence that the Lusk Creek and related faults were active 

during Cambrian rifting but probably did not form the rift 

margin. The seismic data also demonstrate that the Cambrian 

rift sequence thickens to the southeast and that the greatest 

Cambrian displacement occurred along faults at the southeastern edge of the rift (Tabb and Pennyrile fault systems) 

within this major bend in the rift system. This contrasts with 

the structural pattern to the east in Kentucky, where the Cambrian rift sequence is thickest along the northern rift margin, 

and contrasts with the more symmetric graben to the south in 

the Reelfoot rift.

The seismic profile also crosses the Fluorspar area 

fault complex (FAFC) and Hicks dome (fig. 1), major structures that originated in late Paleozoic time. The FAFC is a 

complex zone of mineralized northeast-trending faults in

southern Illinois and western Kentucky that offset rocks as 

young as mid-Pennsylvanian (Nelson, 1991). Normal faults 

are most abundant, and the FAFC also includes reverse and 

strike-slip faults. Fluorspar area faults commonly displace 

Permian alkalic dikes that are part of a regional system of 

dikes that radiate from Hicks dome in southern Illinois 

along a northwest-trending axis (Nelson, 1991). A component of the fluorspar mineralization along the faults appears 

related to the Permian magmatism (Plumlee and Goldhaber, 

1992; Plumlee and others, in press); thus, Fluorspar area 

faulting must have begun prior to the end of hydrothermal 

activity that accompanied this Permian intrusive event.

Hicks dome is an asymmetric northwest-southeast 

elongate structure that lies at the northwestern terminus of 

the northwest-trending Tolu arch. Devonian through
Q4 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

Pennsylvanian strata crop out around the dome, which is 

steepest on its northeast flank. Hicks dome is thought to 

have an igneous origin (Trace, 1974; Heyl, 1983; Nelson, 

1991) because igneous dikes, pipes, and several stock-like 

bodies of peridotite, lamprophyre, and carbonatite breccia 

(Bradbury and Baxter, 1992) radiate from the dome. Snee 

and Hayes (1992) report a Permian age (272 Ma) for an 

intrusion in this suite. In addition to the carbonatite breccia, 

in which fragments of igneous rocks and Paleozoic sedimentary rocks are entrained in an altered carbonate matrix, shatter breccias and vent breccias made up of fragments of 

Paleozoic country rock crop out near Hicks dome (Bradbury 

and Baxter, 1992). The dome lies on the flank of a major 

positive magnetic anomaly that strongly suggests the presence of an intrusion at depth (McGinnis and Bradbury, 

1964). This inferred intrusion is not centered beneath Hicks 

dome; however, the dome does not appear to overlie a local 

laccolithic body. The prevailing model calls on a "cryptovolcanic" origin related to igneous vapors or ground water 

flashing to steam within the Paleozoic section (Brown and 

others, 1954; Nelson, 1991). The seismic reflection 

data discussed herein demonstrate that the entire Phanerozoic section and top of Precambrian basement are domed. 

We examine the origin of Hicks dome and the FAFC in the 

context of seismic reflection patterns and the overall tectonic 

evolution of the region.

DATA ACQUISITION AND 

PROCESSING

The seismic data discussed in this paper were collected 

as part of a regional exploration effort undertaken in the 

southern Illinois Basin by a private corporation. An 83-km 

segment of a northwest-southeast-oriented seismic reflection 

profile (fig. 1) through southern Illinois and western Kentucky was purchased by the U.S. Geological Survey with 

limited publication rights. The data were collected in 1989 

using a dynamite source, 2-ms sample rate, and a 120-channel geophone spread. Geophone and source arrays were 

spaced at 46-m (150-ft) intervals, producing a stacking fold 

of 60. The recording time was 8 seconds; 5 seconds of data 

were processed. Processing (performed by a private contractor) included surface-consistent deconvolution, refraction 

static corrections, velocity analysis, automatic surface-consistent static corrections, wave equation migration, and 

noise-reduction filtering. The data acquisition and processing techniques employed in this survey were standard oilindustry techniques that provide a level of resolution that is 

appropriate for investigating the regional geologic framework. They are distinctly different from high-resolution, 

shallow, seismic reflection surveys carried out in some other 

U.S. Geological Survey investigations of the New Madrid 

seismic zone (e.g., Luzietti and others, 1992; Schweig and 

others, 1992; Sexton and others, 1992), which image smaller 

areas and shallower depths in greater detail.

The data were not reprocessed after purchase by the U.S. 

Geological Survey. Because the authors did not participate in 

processing of the seismic data, care was taken to avoid overinterpreting geologic structure and stratigraphy. In general, 

seismic reflection data commonly contain artifacts of dataacquisition problems and signal-processing algorithms that 

can produce offset reflections that do not correspond to geologic breaks. Other processing artifacts can produce "reflection" events that are more continuous than the actual 

reflectors. In interpreting the seismic data, we examined the 

pre-migration and pre-noise-reduction stacked sections, as 

well as the final migrated sections. This allowed us to evaluate the effects of the post-stack processing and helped ensure 

that our interpretations of strata and structures were not based 

on processing artifacts. Specific familiarity with the static 

corrections, filters, and other processing steps would be necessary for interpretation of very small scale structural and 

stratigraphic effects. For this reason, the interpretations in 

this paper focus on major structures and stratigraphic trends.

CORRELATION OF REFLECTIONS 

WITH REGIONAL STRATIGRAPHY

Mississippian (and locally Pennsylvanian) rocks are 

exposed at the surface along the seismic line. In most areas, 

reflections are clearly identifiable down to the top of Precambrian basement; therefore, reflections on the seismic line 

generally correspond to the Cambrian through Mississippian 

stratigraphic section. Five segments of the regional seismic 

data are shown in figures 2-6; these are keyed to reflection 

patterns along the entire profile in plate 1. Major reflections 

have been identified based on synthetic seismograms produced from sonic logs in two deep exploration wells (Texas 

Pacific Streich 1 well, Pope Co., Illinois, less than 1 km 

northwest of the Shawneetown fault, and the Sun 1 Stephens 

well, Caldwell Co., Kentucky, about 13 km northwest of the 

southeast end of the profile) and by correlation with other 

published seismic reflection profiles from the area (Bertagne 

and Leising, 1991; Heigold and Oltz 1991). Reflection patterns are reasonably consistent, with the exception of structurally complex zones in the vicinity of Hicks dome, Rock 

Creek graben, and the Tabb and Pennyrile fault systems, 

where it is difficult to trace Paleozoic reflections (plate 1; 

figs. 2, 6). Where such complexities are absent (figs. 2, 4, 

and 5), the following major reflections or reflection patterns 

are mappable (generally corresponding to boundaries 

between clastic and carbonate strata, as summarized elsewhere in the Illinois Basin by Heigold and Oltz, 1991): (1) a 

reflective Devonian and Mississippian sequence that generally occupies the upper 0.2-0.3 s (two-way time) across the 

seismic line, with a strong reflection at the top of the Middle 

Devonian to Lower Mississippian New Albany Shale; (2) the 

boundary between the New Albany and the underlying thick
STRUCTURE OF REELFOOT-ROUGH CREEK RIFTS FROM SEISMIC REFLECTION DATA Q5

NORTHWEST

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Stephens

well

Tabb 

fault system SOUTHEAST 'D

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4 KILOMETERS

Figure 2. Segment of seismic reflection data, Caldwell Co., Kentucky, in the vicinity of the Tabb fault system. DMna, top of Middle Devonian to Lower Mississippian New Albany Shale; Og, top of Upper Ordovician Galena Group; €Ok, top of the Knox Group; €ec, top of 

Upper Cambrian Eau Claire Formation; p€, top of Precambrian basement. Sun 1 Stephens well (Caldwell Co., Kentucky) drilled to a total 

depth of 3,952 m (12,965 ft) and bottomed in the Eau Claire. A, B, C, and D denote faults that are discussed in the text.

carbonate section in the Silurian and Devonian Hunton 

Group; (3) the top of the Upper Ordovician Galena Group 

carbonates (Shaver, 1985); (4) the boundary between the 

Middle Ordovician St. Peter Sandstone and the underlying 

Upper Cambrian and Lower Ordovician dolomite of the 

Knox Group; and (5) the Upper Cambrian Eau Claire Formation. The tops of the New Albany, Galena, Knox, and Eau 

Claire are generally traceable, but there are a few places 

(even outside of the structurally complex zones noted above) 

where the reflections lose continuity and interpretation of the 

seismic reflection section becomes difficult. Although the 

Knox is generally less reflective than the rest of the Paleozoic section, there is a strong, persistent reflective horizon 

about 0.2 s below the top of the Knox across most of the seismic profile, and in some parts of the profile (Crittenden 

County, Kentucky) there are many strong reflections in the 

Knox. The pre-Knox Cambrian clastic section (consisting of 

the Upper Cambrian Eau Claire Formation, the Upper Cambrian Mount Simon Sandstone, and an unnamed Lower and 

Middle Cambrian shale and sandstone rift sequence) (Sargent, 1991) contains several very strong continuous reflections and is, in general, much more reflective than is the 

Knox throughout most of the region. The top of Precambrian 

basement is marked in places by a strong reflection;

elsewhere it is picked at or near the base of an acoustically 

transparent interval that generally corresponds to the basal 

part of the rift-related clastic section. The study area is south 

of the widespread Precambrian reflective sequence that 

underlies the center of the Illinois Basin (Pratt and others, 

1989, 1992). The Precambrian in this area does not contain 

well-organized, regionally continuous reflections or 

sequences of reflections.

VELOCITY ASSUMPTIONS

In the interpretations that follow, we use velocity 

assumptions based on sonic logs from two deep wells along 

the seismic line (Texas Pacific Streich 1, Pope Co., Illinois; 

Sun Stephens 1, Caldwell Co., Kentucky) to convert twoway-travel-time intervals to thicknesses and to estimate true 

dips for structures. For interpretive purposes, it was most 

useful to establish representative velocities for the Paleozoic 

section in three parts: (1) the pre-Knox Cambrian clastic 

sequence (5.55 km/s or 18,200 ft/s); (2) the Knox Group 

(dominantly carbonates) (6.47 km/s or 21,227 ft/s); (3) the 

post-Knox Upper Ordovician through Mississippian carbonate and clastic strata (5.35 km/s or 17,552 ft/s). The Knox
Q6 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

and post-Knox velocities are averages of the velocities measured in the Streich and Stephens wells; the pre-Knox velocity is based on the Streich well only because the sonic log for 

the Stephens well passed through only 110 m (361 ft) of this 

sequence. The velocities used here are somewhat faster than 

those reported by Heigold and Oltz (1991) for the Union Oil 

Company 1 Cisne Community well (Wayne Co., Illinois).

INTERPRETATION OF THE 

SEISMIC PROFILE

Because both ends of the seismic profile include numerous structural complexities and stratigraphic questions, the 

discussion of the data begins with a well-constrained, 

straightforward area 10-15 km from the southeast end of the 

line (plate 1; fig. 2). We then trace reflections from this relatively simple area into more complex areas; first, to the 

faulted southeast end of the line and then to the Fluorspar 

area and Hicks dome to the northwest. The discussion 

emphasizes interpretations of fault patterns.

STEPHENS WELL AND VICINITY

About 13 km from the southeast end of the line, reflections can be tied to a synthetic seismogram generated from a 

sonic log in the Sun 1 Stephens well (Caldwell Co., Kentucky), which was drilled to a total depth of 3,952 m (12,965 

ft) and bottomed in the Cambrian Eau Claire. In this area, the 

Cambrian through Mississippian reflections can be traced 

laterally with reasonable confidence, with a few exceptions 

(fig 2). Strong continuous reflections occur in the Mississippian, as well as at tops of the New Albany, Galena, Knox, 

and Eau Claire (fig. 2). The reflection at the top of the Knox 

loses definition to the east. The top of Precambrian basement 

is marked by a strong reflection that dips gently to the southeast. The depth to basement beneath the Stephens well is 

about 6,400 m (21,000 ft). (This figure is calculated using the 

position of the Eau Claire top at 3,490 m (11,239 ft) in the 

Stephens well and the 1.09 s (two-way time) occupied by the 

pre-Knox Cambrian clastic interval on the seismic section. 

We assumed an interval velocity of 5.55 km/s (18,200 ft/s).) 

The Stephens well is near the deepest part of the basin along 

this line of section. The greatest two-way travel time to Precambrian basement (2.3 s) along the seismic line occurs 1.4 

km to the southeast. The corresponding depth to basement is 

about 6,600m (21,600 ft).

TABB FAULT SYSTEM

In a 15-km segment of the seismic line to the northwest 

of the Tabb fault system (TFS), the entire pre-Knox section 

defines a southeast-thickening wedge. Within this wedge 

there is a gentle angular unconformity at about 1.7 s beneath

which reflectors dip more steeply to the southeast than those 

above the unconformity (plate 1). The pre-Knox Cambrian 

clastic sequence thickens to the southeast to a point where the 

top of Precambrian basement, and reflectors below the angular unconformity, are clearly offset by northwest-side-down 

normal faults in two locations (A, B on plate 1 and fig. 2) 

northwest of the TFS. The interval beneath the unconformity 

thins abruptly across fault A and, to a lesser degree, across 

fault B; thus, we interpret A and B as Cambrian normal faults, 

and the unconformity is interpreted as the upper boundary on 

a sequence of strata that accumulated in an early phase of rifting during which faults A and B were active. We do not interpret significant reactivation to have occurred along fault A 

because it does not offset the upper part of the Cambrian clastic section or the Eau Claire reflector.

Above the faulted basement along fault B (plate 1; fig. 

2), the Cambrian through Mississippian reflectors are offset 

in a normal sense along a northwest-dipping zone that 

projects to the surface location of the TFS. We interpret the 

TFS as a post-Mississippian normal fault that has reactivated 

a basement-cutting Cambrian normal fault (B). Another 

post-Mississippian normal fault that strikes obliquely to the 

TFS intersects the seismic line 1 km to the southeast of the 

TFS. This fault has clearly offset post-Eau Claire reflections 

in a northwest-side-down normal sense and appears to merge 

with the TFS near the top of the Eau Claire.

Southeast of the TFS, strong reflections within the Mississippian section are offset by two oppositely dipping 

reverse faults (C, D, plate 1; figs. 2, 3). Although the preDevonian part of the section southeast of the TFS is very difficult to interpret, it appears that these two reverse faults meet 

at about 0.8 s (two-way travel time) within the Knox and isolate a rotated keystone block in which Ordovician through 

Mississippian reflectors have a northwest component of dip. 

Faults C and D can be correlated with mapped faults (Trace 

and Kehn, 1968; Rogers and Trace, 1976) that strike northeasterly and east-west, respectively, oblique to northwest 

trend of the TFS. As discussed below, fault D (the north-dipping reverse fault) reactivates a Cambrian normal fault.

The poor data quality immediately southeast of the TFS 

makes it difficult to confidently correlate pre-Galena reflections at the extreme southeast end of the seismic line with 

those to the northwest of the TFS. The Galena and New 

Albany can be correlated through the fault-bounded blocks 

southeast of the TFS at consistent stratigraphic positions. 

The top of the Knox is difficult to identify at the southeastern 

end of the line—its position was picked on the basis of stratigraphic position with respect to other reflections. At the 

southeastern end of the line, the Eau Claire was picked on the 

basis of reflection character; the top of the Eau Claire (at 

about 1.1 s) is near the top of the particularly distinctive set 

of high-amplitude reflections that occur at this level across 

much of the seismic line. The top of Precambrian basement 

was picked at 1.8 s, corresponding to the base of a 0.3-s-long 

interval of semi-continuous strong reflections below which 

the rocks are essentially unreflective.
STRUCTURE OF REELFOOT-ROUGH CREEK RIFTS FROM SEISMIC REFLECTION DATA Q7 

NORTHWEST SOUTHEAST

0 -i

1 -1

2 KILOMETERS

j

Figure 3. Detail of part of the data shown in figure 2 showing faulting southeast of the Tabb fault system. DMna, top of Middle Devonian to Lower Mississippian New Albany Shale; Og, top of Upper Ordovician Galena Group;-COk, top of the Knox Group. C and 

D denote faults that are discussed in the text.

Correlation of reflections at the extreme southeast end 

of the profile with faulted reflection segments immediately 

to the northwest, and with reflections northwest of the TFS, 

shows that Eau Claire and younger reflectors are offset in a 

reverse sense along fault D (plate 1; figs. 2, 3). At lower 

stratigraphic levels, this reverse displacement is replaced by 

northwest-side-down normal displacement, evidenced by 

normal displacement of the top of Precambrian basement, 

and by abrupt northwestward thickening of the pre-Knox 

Cambrian clastic sequence. Thus, one can interpret fault D as 

a Cambrian normal fault that was reactivated by post-Mississippian north-side-up reverse faulting. The latter movement 

had a dip-slip component of about 350 m.

Strike-slip faults commonly exhibit opposite senses of 

apparent dip-slip displacement at different stratigraphic levels (Harding, 1983); thus, it is possible to interpret fault D as 

a post-Mississippian strike-slip or oblique-slip fault. Several 

geologists interpreted dominant dip-slip displacement on 

late Paleozoic and younger faults in this region and found a 

lack of evidence for significant strike-slip displacement 

(Trace, 1974; Hook, 1974; Trace and Amos, 1984). Heyl and 

Brock (1961) suggested that strike-slip displacements in the 

region were significant. The map pattern along fault D 

(which corresponds to an east-west-striking fault zone

mapped at the surface near the southeast end of the seismic 

line) (Trace and Kehn, 1968; Rogers and Trace, 1976) could 

have been produced by several kilometers of late Paleozoic 

or younger strike-slip displacement or by several hundred 

meters of dip-slip displacement. Regardless of the kinematic 

interpretation for late Paleozoic faulting here, there must 

have been a major Cambrian normal-fault zone that encompassed faults A, B, and D and other unnamed faults, as evidenced by the 1,200 m of thickening of the pre-Knox section 

across this zone.

FLUORSPAR AREA FAULT COMPLEX

Between the TFS and the Ohio River, there is a general 

northwestward thinning of the pre-Knox Cambrian clastic 

section and, to a lesser extent, of the Cambrian-Ordovician 

Knox Group. (This regional trend is only locally interrupted 

by structures such as fault E (plate 1), a northwest-side-down, 

minor Cambrian normal fault in Crittenden Co., Kentucky, 

just northwest of the younger Moore Hill graben.) Numerous 

FAFC normal faults cut the Paleozoic section (plate 1); these 

faults are post-Pennsylvanian structures that do not appear to 

have had an earlier history. The FAFC normal faults
Q8 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

NORTHWEST

Commodore 

fault

Griffith Bluff graben

SOUTHEAST

0 -i

1 -

3 -J

SJWMeW.'v'i • •TM""' K7s^~; «M»«s-.*«a>i <*, f.»'5Bsrw.

r^s^^^££5f£^^^^

Figure 4. Segment of seismic reflection data across the Griffith Bluff graben, Crittenden Co., 

Kentucky. Og, top of Upper Ordovician Galena Group; COk, top of the Knox Group; Cec, top 

of Upper Cambrian Eau Claire Formation; pC, top of Precambrian basement.

generally define a series of grabens. Some graben-bounding 

faults clearly cut basement. Beneath other grabens, bounding 

faults lose displacement downward in the Paleozoic section, 

requiring lateral accommodation of deformation. Geologic 

relations (e.g., lack of significant lateral offset of older dikes) 

indicate that there were not major strike-slip components of 

displacement in the FAFC (Trace and Amos, 1984).

Beneath both the Shady Grove and the Griffith Bluff 

grabens (plate 1), one of the bounding faults cuts basement. 

In both of these grabens, the bounding faults have dips of 

about 50°-70° (after correcting for seismic velocities) and 

the opposing normal faults intersect within the Knox. 

Beneath this level of intersection in each of the two cases, 

displacement is accommodated along a southeast-dipping 

normal fault that offsets the top of Precambrian basement. 

Figure 4 shows reflection data from the Griffith Bluff graben, beneath which the bounding faults splay upward in Silurian and younger strata forming subsidiary fault blocks 

within the larger graben. Figure 4 also shows the southeastdipping Commodore fault offsetting basement. The seismic 

data show an apparent decrease in fault dip within Knox and 

deeper strata for the "master" faults beneath the Griffith

Bluff and Shady Grove grabens. In the case of the Griffith 

Bluff graben, this is an effect of "velocity pull-up" produced 

by fast velocities in Knox carbonates; the true fault geometry 

is planar, after correcting for the variation of velocity with 

depth. In the case of the Shady Grove graben, the true fault 

geometry does flatten (using velocity assumptions stated 

previously for depth conversion) by about 15°.

Examples of grabens that are confined to the Paleozoic 

section include the Mexico and Moore Hill grabens (plate 1; 

fig. 5). In both cases, the Knox and younger reflectors are 

clearly offset along normal faults with 60°-75° dips, 

whereas the Eau Claire and older reflectors are not. Beneath 

the Mexico graben, the reflections between the base of the 

Knox and base of the Cambrian are somewhat disrupted, but 

not clearly offset. Conceivably, this disruption could be an 

expression of strike-slip faulting at depth—in such an interpretation, the Mexico graben would be a transtensional 

structure. Beneath the Moore Hill graben, the Eau Claire and 

lower reflections are absolutely continuous and do not 

appear affected at all by the faulting that cuts younger strata.

Immediately southeast of the Shady Grove graben, a 

narrow horst block (F, plate 1) is apparent. It is bounded on 

the southeast by a southeast-dipping normal fault (mapped as
STRUCTURE OF REELFOOT-ROUGH CREEK RIFTS FROM SEISMIC REFLECTION DATA Q9

NORTHWEST SOUTHEAST

Mexico graben

0 -H

2 KILOMETERS

Figure 5. Segment of seismic reflection data across the Mexico graben, Caldwell Co., Kentucky. DMna, 

top of Middle Devonian to Lower Mississippian New Albany Shale; Og, top of Upper Ordovician Galena 

Group; COk, top of the Knox Group; Cec, top of Upper Cambrian Eau Claire Formation.

a northwest-dipping normal fault by Trace and Palmer, 

1971). This fault is also the northwest boundary on an 

unnamed, 1.5-km-wide graben immediately to the southeast. 

The faults bounding this graben cut upper Knox and younger 

strata, but deeper reflectors continue without discernible offset beneath these faults.

The structural style of the Moore Hill and Mexico grabens, and of the unnamed graben just described, strongly 

suggests that the extension accomplished by the shallow grabens in the upper and middle Paleozoic part of the section is

accommodated laterally along a detachment horizon in the 

Knox. Because there are only about 100 m of extension associated with any of these grabens, the lateral displacement 

required along this detachment horizon would be of the same 

order, and this displacement could be transferred to basement-cutting faults such as those noted above.

Because displacement along both the Moore Hill and 

Mexico grabens increases to the southwest, it is possible 

that the bounding faults pass into basement-cutting structures along strike to the southwest of the seismic line. For
Q10 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

example, about 10 km southwest of its intersection with the 

seismic line, the southeastern bounding fault on the Mexico 

graben merges with the Tabb fault, which clearly cuts basement as discussed previously. The Claylick fault system, 

which bounds the southeast side of the Moore Hill graben, 

has 105-120 m (350-400 ft) of stratigraphic throw where it 

intersects the seismic line; the stratigraphic throw increases 

to about 150 m (500 ft) near the Lucile mine (about 2 km 

along strike to the southwest) and to 150-185 m (500-600 

ft) in the vicinity of the Mineral Ridge mine complex (about 

20 km southwest of the seismic line's intersection with the 

fault) (Trace and Amos, 1984). The presence of significant 

(basement-derived?) mineralization to the southwest, in 

conjunction with the increase in fault displacement, suggests that basement may become involved in faulting southwest of the seismic line.

In the horst block between the Griffith Bluff graben and 

the Moore Hill graben, there is broad arching of Paleozoic 

reflectors—this corresponds to a gentle anticline delineated 

by structural contours on the geologic map of the western 

Kentucky Fluorspar district (Trace and Amos, 1984). This 

broad arching appears to represent local flexure in response 

to the impingement of the two bounding normal-fault systems within this southward-tapering horst (geologic map 

pattern shown in Trace and Amos, 1984). About 1.5 km 

southeast of the Griffith Bluff graben, the line crosses the 2- 

m-wide Howard Stout lamprophyre dike (Trace and Amos, 

1984), which is too narrow to have expression on the seismic 

data. Paleozoic reflectors pass uninterrupted beneath the 

dike, demonstrating that the dike is not connected to a sizable 

intrusive body at depth in the Paleozoic section.

Northwest of the Griffith Bluff graben, near the axis of 

the Tolu arch (between the Commodore fault and the Ohio 

River), the seismic line crosses several faults, each of which 

has at most a few meters of displacement. These faults have 

no expression on the seismic data.

OHIO RIVER-ROCK CREEK GRABEN AREA

The character of the reflection data changes dramatically across a 2.75-km gap in the data at the Ohio River (plate 

1). On the southeast side of the river, in Kentucky, continuous 

reflections can be identified throughout the Paleozoic section 

from the near-surface Mississippian strata to the top of Precambrian basement at about 1.9 s (two-way time). On the 

northwest side of the river, in Illinois, reflections beneath the 

Upper Devonian are discontinuous, and very few coherent 

reflection segments occur below 1.4 s (two-way time). Even 

though problems with geophone coupling along this part of 

the line may have contributed to the discontinuity of reflections (P. Scaturro, Seismic Specialists, Inc., oral cornmun., 

1991), the closely spaced faulting and associated structural 

complexities in the Rock Creek graben-Hicks dome area 

(fig. 1) were certainly an important factor in the degradation

of reflections. There may be other significant geologic 

changes across this zone, as suggested below in the discussion of the Cambrian section and basement reflections.

The north west-trending Rock Creek graben is about 3.2 

km wide where it is crossed by the seismic line in Hardin 

Co., Illinois (fig. 1). This is an intensely faulted area, and the 

graben boundaries consist of complex fault zones. Both 

bounding fault zones have several hundred meters of displacement on them. Some of the faults (including both normal and reverse faults) can be traced through the Devonian 

and Mississippian section on the seismic data (plate 1). Some 

small-displacement, mapped faults are not resolvable. 

Although complex structure in the graben makes it very difficult to correlate individual reflectors across this area, some 

major structures are resolvable throughout the Paleozoic section beneath the graben (plate 1).

To the west, the Devonian and Mississippian section (as 

a whole) can be traced with relative ease across Hicks dome 

and across the faults and folds northwest of Hicks dome 

(plate 1, fig. 6), although there are places where individual 

reflections within this part of the section are discontinuous. 

The underlying Paleozoic section is quite well defined where 

the seismic line crosses the southwest flank of Hicks dome 

and in the area northwest of there where these reflections tie 

to a synthetic seismogram produced from a sonic log in the 

Texas Pacific Streich 1 well (plate 1), drilled to a total depth 

of 4.55 km (14,942 ft) into the Cambrian Mount Simon 

Sandstone. The pre-Devonian section is very difficult to 

trace through the projected axis of Hicks dome and the area 

between Hicks dome and the Ohio River because reflections 

are discontinuous. Our interpretations of the stratigraphy in 

this area (plate 1, fig. 6) rely to a large degree on plausible 

ties to the well-constrained parts of the seismic section to the 

southeast and northwest and also on some aspects of the 

character of the reflections in this zone.

Correlation of lower Paleozoic and basement reflections across the Ohio River is somewhat speculative. The 

pre-Knox Cambrian clastic section occupies about 0.75 s 

(two-way time) on the seismic section immediately southeast of the Ohio River, and a strong, continuous, two-cycle 

reflection (X) at or near the base of this section occurs at 

1.80-1.85 s (plate 1). On the northwest side of the river 

(plate 1), a prominent reflection at about 1.8 s can be traced 

for about 1.5 km to the northwest; we interpret this as the 

continuation of reflector X. A band of strong but discontinuous reflections (Y, plate 1) at about 1.4 s is traceable (with 

some minor offsets) for 6.5 km northwest of the Ohio River. 

We correlate this band of reflections with a strong, continuous set of reflections that occurs in the Cambrian clastic 

sequence at about the same two-way travel time on the 

southeast side of the river. We interpret the top of the Eau 

Claire at the top of a semi-continuous, relatively prominent, 

three-cycle reflection that is traceable for 2.8 km on the 

northwest side of the river, at 1.1-1.2 s. It is present at about 

the same two-way travel time on the southeast side of the
STRUCTURE OF REELFOOT-ROUGH CREEK RIFTS FROM SEISMIC REFLECTION DATA Qll

NORTHWEST SOUTHEAST

0 -i

I 2

o

3 -1

Hicks dome

Figure 6. Segment of seismic reflection data across the southwest flank of Hicks dome, Pope Co. and Hardin Co., Illinois. Msg, 

top of Upper Mississippian Ste. Genevieve Limestone; COk, top of the Knox Group; Cec, top of Upper Cambrian Eau Claire Formation; pC basement, top of Precambrian basement. SF, Shawneetown fault; HF, Herod fault.

river, where it marks the top of a prominent reflective zone 

(the typical seismic signature of the Eau Claire). The Galena 

on the northwest side of the Ohio River corresponds to a 

faint, continuous reflection that occurs at the same two-way 

travel time as the Galena reflection of the southeast side of 

the river; the top of the Knox was inferred on the basis of 

stratigraphic position with respect to other reflections and on 

the tie to the Knox on the southeast side of the river.

On the northwest side of the Ohio River, reflection Y 

(plate 1), within the Cambrian at 1.4 s, is broken by small 

offsets along its 6.5-km span, but there appear to be larger 

offsets in the deeper (near-basement) reflection patterns. 

Variations at deeper levels across this area are produced by: 

(1) a major Early Cambrian structure (masked by poor data 

quality) at the Ohio River or within a few kilometers to the 

northwest, or (2) degradation in data quality related to acquisition problems and structural complexity. In spite of serious 

problems with data quality here, we interpret a southeastside-down Cambrian normal fault (G, plate 1) about 1.5 km 

northwest of the Ohio River data gap. Correlations with

basement picks northwest of here (see below) suggest a step 

of about O.ls (two-way time, corresponding to about 215 m) 

on the top of Precambrian basement. Fault G offsets reflection Y and reflection Z (within the Knox—plate 1) with a 

reverse sense of displacement (two-way time offset of 0.04 

s, or about 120 m on reflection Y and about 0.08 s or 245 m 

on reflection Z). Displacement on other key horizons, such 

as the tops of the Eau Claire, Knox, and Galena, are not 

known because these tops do not correspond to strong reflections in this area of poor data quality. Fault G projects to the 

surface into a zone of intense faulting along the southeast 

margin of the Rock Creek graben and probably splays 

upward into several strands within the Devonian and Mississippian section, but its specific configuration within these 

upper strata is not clear from the seismic data.

From the above observations, we interpret the southeast boundary of the Rock Creek "graben" as a post-Mississippian (Permian?) reverse fault zone (G, plate 1) that 

experienced about 245 m of displacement where crossed 

by the seismic line. This reverse fault reactivated a
Q12 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

Cambrian normal fault. The lesser reverse displacement on 

reflector Y (within the pre-Knox Cambrian strata) relative 

to that on reflector Z (within the Knox) suggests that some 

of the Cambrian normal faulting was younger than reflector Y (presumably Middle Cambrian). Our interpretation 

suggests a total Cambrian normal displacement of about 

500 m. Alternatively, the differing amounts and conflicting 

senses of displacement at different stratigraphic levels 

along this fault may indicate a post-Mississippian strikeslip displacement history.

Bouguer gravity data show a distinct break of 4 mGal in 

the area just northwest of the Ohio River, consistent with the 

idea that there is a significant Cambrian structure there. 

More negative values occur toward the southeast, compatible with a downward stepping of basement to the southeast 

and consequent infilling of clastic sediments (T. Hildenbrand and R. Kucks, written commun., 1990). A gravity 

anomaly of this magnitude could have been produced by 

Cambrian thickening and basement offset on the order of 

several hundred meters, as shown by the gravity modeling of 

the Penny rile fault system (Lumm and others, 1991).

About 6 km northwest of the Ohio River data gap, 

reflection Z (plate 1) is offset (down to the east) by about 

0.07 s (215 m) across a broad fault zone (H) that underlies 

the northwestern margin of the Rock Creek graben. This 

fault zone appears to be nearly 1 km wide within the Knox. 

We interpret a broadening of the zone, at shallower levels, 

into the several-kilometer-wide zone of faults mapped along 

the northwest margin of the graben (Baxter and Desborough, 

1965). Along fault H below the Knox, reflection Y is disrupted but is not offset more than a few milliseconds across 

this fault zone; we speculate that the post-Mississippian eastside-down displacement across this fault reversed early Paleozoic west-side-down displacement here in order to produce 

this pattern or that the displacement on the eastern boundary 

of the Rock Creek graben (fault H) was largely accommodated laterally within the lower Knox and Eau Claire. One 

possibility is that the Rock Creek graben formed in response 

to Hicks dome-related magmatism and explosive processes 

that affected the lowermost Paleozoic section—this would 

explain the faulting of reflectors within the Knox, above preKnox reflectors that are disrupted but not offset. This idea is 

discussed in a later section of this paper.

About 7 km northwest of the Ohio River data gap, just 

west of fault H, reflection Y (plate 1) is displaced by about 

0.06 s (two-way travel time; corresponding to about 200 m 

of throw) along a gently east-dipping normal fault (J, plate 

1). Fault J displaces basement and may continue to shallower 

depths, as suggested by possible normal drag on reflectors 

within the Knox (plate 1). The average dip on fault J from the 

Knox to the top of the basement is about 27°, assuming an 

average velocity of 5.8 km/s for the interval between the top 

of the basement and the middle of the Knox. The fault cannot 

be traced above the Knox due to the poor data quality in the 

Hicks dome area, but it may be the deep continuation of a 

normal fault seen in the reflective Devonian-Mississippian 

section (plate 1).

Directly west of fault J (plate 1), the lowermost Cambrian interval is reflective, and an intermittent reflection at 

1.63 s marks the top of Precambrian basement. Beneath this 

horizon, the basement is characterized by short segments of 

reflections that do not define continuous stratigraphic or 

structural patterns. East of fault J, strong reflections are not 

present at the top of basement or in the lowermost part of the 

Cambrian. Instead, there is a subtle change in reflection character from an opaque, nonreflective zone in the lowermost 

Cambrian to short, disorganized reflections in the basement.

Reflections from the Cambrian and Ordovician are 

very disrupted within a 3-km zone that coincides with the 

projection of the crest of Hicks dome (plate 1, fig. 6). One 

cannot trace any individual reflections through this zone. 

The general reflective character of the Cambrian clastic 

sequence is preserved, but short discontinuous reflections 

are present, in contrast to the well-defined continuous 

reflections within this interval to the west and to the somewhat continuous reflections (discussed above) between 

Hicks dome and the Ohio River. Short, disrupted, reflection 

segments also occur in the Knox, in contrast to the adjacent 

continuous reflections seen on the west flank of Hicks 

dome. The top of Precambrian basement is not readily discernible beneath this 3-km zone. Reflections in the basement beneath this zone are also disrupted; coherent westdipping basement reflections at 2-3 s beneath the west flank 

of Hicks dome terminate and do not continue beneath the 

zone where Paleozoic reflections are disrupted.

In a west-dipping panel on the heavily faulted west flank 

of Hicks dome (plate 1, fig. 6), all Paleozoic reflections 

(lower Cambrian through Pennsylvanian) have an apparent 

dip of 13°-15° in the plane of the seismic line, using velocity 

assumptions stated previously. Dips of the near-surface 

reflections correspond to dips mapped at the surface, and 

velocity analyses demonstrate no strong lateral velocity gradients within the Paleozoic section; thus, we interpret the dipping reflections to represent the true geologic structure of the 

Paleozoic strata. Because near-basement Cambrian reflectors 

are domed, the top of the basement must also be involved in 

doming. Displacements on most of the mapped faults in this 

area are too small to be resolved on the seismic data.

The west edge of the west-dipping panel corresponds to 

the Herod fault, which is the northward continuation of the 

regionally important Lusk Creek fault system. The seismic 

data (plate 1, fig. 6) demonstrate that Cambrian through 

Pennsylvanian strata are displaced in a normal sense across 

the Herod fault. This post-Pennsylvanian normal fault has 

produced about 230 m (0.1 s) of displacement of uppermost 

Knox and younger strata. The fault appears to bifurcate 

downward within the Knox; the westerly strand continues at 

a steep (70°-75°) dip to depth. The easterly strand has less 

dip. There also appears to have been some eastward thickening of the Cambrian clastic section (by about 0.1 s or 280 m) 

across this easterly strand, suggesting that it was active during Cambrian rifting.
STRUCTURE OF REELFOOT-ROUGH CREEK RIFTS FROM SEISMIC REFLECTION DATA Q13

To the west, the seismic line crosses the steeply eastdipping Shawneetown fault (plate 1, fig. 6), which strikes 

north-northeast. Knox and younger reflectors are offset by 

about 0.03 s in a reverse sense. The fault appears to bifurcate 

downward, and it is difficult to resolve any net displacement 

of Eau Claire and older reflectors, but the position of the 

fault zone can be easily identified on the basis of disrupted 

reflections throughout the Paleozoic section. The Cambrian 

clastic section retains a constant thickness across the fault 

zone, demonstrating that the Shawneetown fault in this place 

was not active during Cambrian rifting. Geologic map patterns (Baxter and others, 1967) along this part of the Shawneetown fault require either: (a) major late Paleozoic reverse 

faulting followed by younger normal faulting (Nelson, 

1991), or (b) a single phase of post-Pennsylvanian strike- or 

oblique-slip faulting. The seismic data demonstrate that 

cumulative displacements on this fault zone produced little 

net dip-slip displacement. Nelson (1991) also noted that this 

part of the Shawneetown fault was not a Cambrian rift 

boundary and that post-Cambrian movements have produced 

small net dip-slip.

Northwest of the Shawneetown fault, the seismic line 

crosses the McCormick anticline (plate 1), a northeast-trending asymmetric structure that is steeper on the northwest 

limb. Folding affects strata from the Mississippian (exposed 

at the surface) down through the Knox. Nelson (1991) interpreted this folding to be driven by west-directed horizontal 

movement along a decollement in the Knox. The seismic data 

show that the folding is cored by a basement-cutting(?) blind 

thrust fault (K, plate 1). This fault cuts the Eau Claire 2 km 

northwest of the position where the Shawneetown fault cuts 

the Eau Claire (plate 1) and dips easterly at about 30°. Both 

the Shawneetown fault and fault K almost certainly penetrate 

the top of the basement, although the seismic data do not 

explicitly show this because the net offset is small and there 

is not a strong reflection marking the top of basement here.

About 1.5 km northwest of the west flank of the McCormick anticline (plate 1), there is an east-dipping fault (L) that 

produced minor reverse movement at the Eau Claire level 

(and the top of basement?) and at the top of the Knox; this 

fault appears to have transferred displacement into a gentle 

monocline in the upper Paleozoic. The interpretation of fault 

L in the Upper Ordovician through Devonian interval is 

problematic, however, because there is normal offset of 

these strata in contrast to the reverse sense of offset above 

and below this interval. Such contradictory dip-slip displacements are indicative of strike-slip displacement on this fault, 

although these observations do not provide information on 

the sense of strike-slip displacement (Harding, 1983). At the 

extreme northwest end of the seismic line, folding and displacement of the Upper Ordovician reflectors suggest the 

presence of another reverse or thrust fault (M) that may splay 

from fault L (plate 1).

Within the Cambrian clastic sequence to the west of 

fault L, there are apparent offsets of reflections; this may 

have been a zone of southeast-side-down normal faulting 

near the northwest flank of the Cambrian rift. Hildenbrand 

and others (1992, in press) place the rift boundary about 9 km 

to the northwest, based principally on their interpretation of 

the depth to magnetic basement and the coincidence of positive magnetic and gravity anomalies in Johnson Co., Illinois.

DISCUSSION

The study area has experienced only diffuse historic 

and modern seismicity, yet it shares many of the geologic 

characteristics of the most active part of the New Madrid 

seismic zone, 140 km to the southwest. A comparison of 

structural patterns in the southeast Illinois-western Kentucky Fluorspar area (this study) with those in the seismically active New Madrid area should provide some 

perspective on the geologic controls on New Madrid seismicity. The following section provides such comparisons 

along with a discussion of significant geologic relations in 

the vicinity of the seismic line.

REELFOOT-ROUGH CREEK RIFT

The segment of seismic data examined in this paper lies 

entirely within the Cambrian rift, which is about 110 km 

wide in this "bend area" (fig. 1) (Hildenbrand and others, 

1992, in press). The northwest end of the seismic line is 

about 5 km southeast of the northwest rift boundary, and the 

southeast end of the seismic line is about 25 km northwest of 

the southeast rift boundary as defined by depth to magnetic 

basement (Hildenbrand and others, 1992, in press). Because 

the data cover a distance of 83 km across the strike of the rift 

in the "bend area," we feel that the main aspects of the rift 

structure are imaged. At the northwest end of the line (in the 

vicinity of the McCormick anticline), the pre-Knox Cambrian clastic sequence is about 1.1 km thick (corresponding 

to a 0.4-s two-way-travel-time interval). This sequence 

thickens to the southeast as the basement steps down across 

several Cambrian normal faults and achieves its maximum 

thickness of 3.1 km (1.13 s) in a southeast-thickening wedge 

that terminates against the Tabb-Pennyrile fault zone. Southeast of this fault zone, at the southeast end of the line, the preKnox Cambrian clastic section is about 1.9 km thick (0.7 s). 

In the Dupont 1 Fee well, situated in Davidson Co., Tennessee (about 100 km southeast of the Cambrian rift), the preKnox Cambrian clastic sequence is 0.2 km (737 ft) thick. In 

the Union Oil Company 1 Cisne Community well, situated 

in the central Illinois Basin outside of the Cambrian rift (in 

Wayne Co., Illinois, 80 km north of the Rough Creek fault 

system), the pre-Knox Cambrian clastic sequence is 0.4 km 

(1,312 ft) thick. If we assume that normal thicknesses for this
Q14 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

sequence outside of the rift are 0.2-0.4 km, the true rift 

boundaries must lie beyond the limits of the seismic line, as 

suggested by Hildenbrand and others (1992, in press). It is 

reasonable to expect that Hildenbrand's magnetically 

defined rift boundaries correspond to rift-bounding normalfault zones. The presence of 1.9 km of pre-Knox Cambrian 

clastic rocks at the southeast end of the seismic line implies 

that an additional 1.5-1.7 km of thinning of this sequence 

occurs between there and the true rift boundary, 25 km to the 

southeast. The 1.2 km of Cambrian displacement across the 

Tabb-Pennyrile fault zone is by far the most significant riftrelated faulting seen on this seismic section. It is likely that 

another Cambrian normal-fault zone with similar offset (or 

several zones with as much as 1.7 km combined offset) 

occurs to the southeast of the seismic line. Candidate fault 

zones include the "central branch" and "southern branch" of 

the Pennyrile fault system (5 and 25 km to the southeast of 

the seismic line, respectively) as defined by Lumm and others (1991). Gravity data and modeling discussed by Lumm 

and others (1991) strongly suggest that the "southern 

branch" of the Pennyrile fault system is the southern edge of 

the Rough Creek graben near its intersection with the Reelfoot rift and that the Cambrian clastic sequence thickens 

northward by about 1 km, within 10 km (map distance) north 

of this rift edge. The available geologic and geophysical data 

support the idea that a broad Pennyrile fault system (more 

than 30 km across strike), rather than a more localized fault 

zone, formed the southern margin of the Rough Creek graben in Cambrian time.

The major Cambrian fault zone inferred from the seismic data occurs beneath the area where the Tabb and Pennyrile fault systems intersect at the surface. These two fault 

systems (mapped on the basis of post-Pennsylvanian displacements) differ in strike by about 45° (fig. 1). The seismic 

data provide no information on the strike of the Cambrian 

fault zone that is imaged here. The gravity modeling of 

Lumm and others (1991) suggests that, about 20 km due 

south of the southeast end of the seismic line (east of Lake 

Barkley), the rift margin swings from an east-northeast 

(Rough Creek graben) orientation toward the northeasterly 

orientation of the Reelfoot rift. This overall pattern seems 

inconsistent with significant Cambrian normal displacement 

along the northwesterly Tabb trend, and we favor the eastnortheast (Pennyrile) trend for the major Cambrian fault 

zone that we interpreted from the seismic data.

The overall geometry and genesis of the Reelfoot rift, 

Rough Creek graben and Rome trough (a northeasterly 

trending Cambrian basin that extends from eastern Kentucky to western Pennsylvania in the subsurface) have been 

briefly addressed by several authors. Thomas (1991) considered these zones of anomalously thick Cambrian strata 

to be linked segments of a "failed" intracratonic rift system 

of Early and Middle Cambrian age that was contemporaneous and compatible with a northwest-trending AlabamaOklahoma transform that coupled Ouachita rifting with

mid-Iapetus spreading. Thomas' (1991) view of the Reelfoot, Rough Creek, and Rome basins was based on their 

map-view geometry and on available paleontological control on rift fill. While conceding that the proposed 

Rome-Rough Creek connection in eastern Kentucky is not 

well understood, Thomas (1991) suggested that the Rough 

Creek graben was an "oblique transfer zone" between the 

Reelfoot and Rome rift segments. The geometry is essentially that of a right-stepping rift offset as defined by Nelson and others (1992), who stress that an oblique rift offset 

segment (the Rough Creek graben, in this case) would be a 

zone of complex strain and that bounding faults (Cambrian 

Rough Creek and Pennyrile fault systems, in this case) 

would have undergone oblique displacement with a significant strike-slip component. Chandler and others (1989) 

illustrated a higher angle rift offset zone in the southern 

Minnesota part of the Proterozoic Midcontinent rift system, 

and rift offsets have been documented in many other rift 

systems worldwide (Nelson and others, 1992). Walker and 

others (1991) attributed the development of the Rome 

Trough-Rough Creek graben system to extensional failure 

resulting from subsidence and flexure of the passive continental margin to the east. Drahovzal and others (1992) suggested that the Early and Middle Cambrian Rome trough 

and Rough Creek graben were separated by a stable block 

in central Kentucky that corresponds to the pre-Grenville 

East Continent rift basin. Goetz and others (1992) postulated that the Rough Creek graben and Rome trough originated in the Precambrian, before the initiation of the 

Reelfoot rift, and that the Reelfoot rift became linked with 

these earlier structures in Cambrian time, but Goetz and 

others (1992) provided no data to substantiate this idea.

The seismic line discussed here clearly records a major 

transition in rift polarity within the major bend in the Cambrian rift system; such transfer zones between oppositely 

dipping half-grabens are common in rift systems (Rosendahl 

and others, 1986; Morley and others, 1990). Other major 

transfer zones, in which regional extensional strain is conserved between zones of differing fault configuration, can be 

inferred in the rift system. The seismically active part of the 

Reelfoot rift appears to have had a different Cambrian geometry than that of adjacent segments, as discussed below. In 

considering structural controls on seismicity, one should 

consider (a) the effects of the major change in strike of the 

rift system, (b) the effect of the variations in cross-sectional 

geometry of the rift, and (c) mechanical effects of lateral 

variations in crustal rheology. These are evaluated below.

In examining old fault zones as planes of weakness in 

the modern stress field, the dogleg-shaped Cambrian rift system includes a wide range of fault orientations that may (or 

may not) be exploited for strain release in the modern stress 

field. The Reelfoot rift is controlled by steeply dipping, 

northeast-striking faults that originated as Cambrian normal 

faults and have, in many cases, been reactivated as late Paleozoic reverse faults. A northeast-trending linear cluster of
STRUCTURE OF REELFOOT-ROUGH CREEK RIFTS FROM SEISMIC REFLECTION DATA Q15

New Madrid epicenters coincides with the central part of the 

Reelfoot rift, but northeast-trending Cambrian and younger 

faults continue to the southwest and northeast of the seismic 

zone (for example, the Lusk Creek-Herod fault system of the 

study area). The present-day maximum compressive stress is 

oriented about N. 60° E. (Zoback, 1992); the steeply dipping 

fault system, trending N. 30° E., would be susceptible to 

right-lateral strike-slip reactivation in such as stress field. 

Andrews and others (1985) relocated epicenters and analyzed fault-plane solutions for microearthquakes in the New 

Madrid seismic zone; they found that the dominant northeasterly trend of the cluster of epicenters is associated with 

right-slip in a discrete zone along the rift axis (now the 

Blytheville arch), south of the Pascola arch and along the 

northwest margin of the rift for a short segment north of the 

Pascola arch (fig. 7). Chiu and others (1992) found that the 

central cluster of New Madrid hypocenters in the vicinity of 

the Lake County uplift define two planes, dipping moderately to the southwest, and they infer that these have undergone reverse movement consistent with the modern stress 

field. It is not clear why these clusters of seismicity are confined to one 150-km zone along the 350-km-long Reelfoot 

rift. The Rough Creek graben contains east-striking faults, 

such as the Rough Creek system, east-northeast-striking 

faults, such as the Pennyrile system, and northeast-striking 

faults of the Fluorspar area fault complex that are essentially 

on trend with faults of the Reelfoot rift. The east-northeaststriking faults would have low resolved shear stress because 

they are nearly parallel to a principal stress axis. The eastand northeast-striking faults would appear to be candidates 

for strike-slip seismic activity if fault orientation were the 

controlling parameter. That is apparently not the case. The 

FAFC is strongly faulted at all scales and may be too weak 

to support appreciable strain energy.

The regional variation in the overall structure of the 

Reelfoot-Rough Creek rift system, obvious from comparison of the present study with other seismic reflection studies 

in the rift system, may contribute significantly to the localization of New Madrid seismicity. This study of the Cambrian rift establishes the presence of a strongly southeasttilted graben in the "dogleg bend" area, in contrast with the 

strongly north tilted asymmetric graben in the main part of 

the Rough Creek graben to the east. The major transfer zone 

that must exist between these areas of opposing rift polarity 

is one of several transfer zones inferred in the rift system. To 

the south, in the area of maximum New Madrid seismicity, 

the seismic reflection data across the Blytheville arch (Howe 

and Thompson, 1984; Howe, 1984) clearly show that there 

was an axial Cambrian rift zone that contained a locally 

thickened section (a medial graben). This axial zone underwent later reverse faulting that is obvious in a seismic reflection section from northeast Arkansas (Howe, 1984), and this 

zone (the seismogenic part of the Blytheville arch) contains 

the most complex structure in this part of the rift. In contrast, 

profiles to the south (PI from McKeown and others, 1990;

Figure 7. Generalized diagram showing the along-strike variation of structural patterns within the Reelfoot-Rough Creek rift 

system. The New Madrid seismic zone is shown in dark stipple; 

the Reelfoot-Rough Creek rift system is shown in a lighter stipple. BA, Blytheville arch; PA, Pascola arch. Horizontally ruled 

pattern denotes parts of the rift system characterized by major 

half-grabens with alternating directions of polarity. Diagonally 

ruled pattern denotes the part of the Reelfoot rift that is characterized by a more symmetric geometry, with an axial maximum of 

Cambrian rift-related sedimentary rocks ("axial thick"). See text 

for further discussion.

COCORP data, Nelson and Zhang, 1991) show that, near the 

southern end of the well-defined line of epicenters, the rift 

consists of simple half-grabens that appear to change polarity along the rift axis with no pronounced axial thickening. 

The Blytheville arch at that latitude is a structurally simple 

gentle antiform. The along-strike transitions between fulland half-graben structural styles, although insufficiently 

imaged in studies of this rift system, are almost certainly 

examples of major transfer zones, as described in the East 

African and other rift systems by Morley and others (1990). 

These inferred Cambrian transfer zones closely coincide 

with geographic limits of the most active part of the New 

Madrid seismic zone, as defined by tightly clustered epicenters of the northern Mississippi Embayment (fig. 7).

Howe (1984) and McKeown and others (1990), among 

others, have emphasized that New Madrid seismicity is 

localized in a weak zone corresponding to the structurally 

complex Blytheville arch. The body of published seismic 

data from the Cambrian rift system, including the present 

study, indicates that this seismically active area has a geometry that is unlike other parts of the rift system. We hypothesize that the downdropped, structurally complex axial rift 

zone is the weakest part of the northeast-oriented rift system, 

and it fails most readily in response to intraplate stresses. The 

well-defined line of rift-axis seismicity dies out southeast of 

Blytheville, Ark., because of the along-strike change in character from structurally complex "axial thick" to a gentle antiform; this transition is a consequence of the preexisting
Q16 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE 

Cambrian transfer zone that separated the axial graben from 

the half-graben style to the south. Seismicity declines dramatically north of the Pascola arch, probably because the 

axial rift zone gave way to the strongly asymmetric graben 

seen in the present study (fig. 7). (Seismicity within the Pascola arch, and along the northwest rift margin north of the 

Pascola arch, represents a dissipation of accumulated strain 

at the north end of the axial zone.) In the present study, we 

identified aseismic northeasterly striking faults that originated as Cambrian normal faults in the Hicks dome area and 

the Fluorspar area of western Kentucky; however, they are 

not associated with a discrete axial graben, so their aseismic 

nature fits the regional pattern. 

Rheological contrasts in the basement may also affect 

seismicity. The possible mechanical effects of pluton 

boundaries, and of thermal metamorphism surrounding plutons, were addressed by Ravat and others ( 1987). A longlived northwest-southeast crustal boundary that crosses our 

study area was discussed by Heigold and Kolata (1993); 

this broad boundary crosses the nearly aseismic part of 

southern Illinois and western Kentucky and lies between 

the New Madrid and Wabash Valley seismic zones. As 

Heigold and Kolata (1993) suggest, the quite different characteristics of these two seismic zones may be related to the 

fact that they lie in different basement provinces that have 

different mechanical properties. 

FLUORSPAR AREA FAULT COMPLEX 

The nature of the Fluorspar area fault complex may provide another possible explanation for the abrupt northward 

decline in seismicity in the northernmost Mississippi 

Embayment. The Fluorspar area faults, which are probably 

not older than Permian, follow the same trend as the seismogenic Blytheville arch but occupy a zone that is several 

times broader. The Fluorspar area faults disappear to the 

southwest beneath Cretaceous and Tertiary sedimentary 

rocks of the Mississippi Embayment, and their extent 

beneath the embayment is unknown. It is possible that a significant amount of "New Madrid" strain is transferred from 

the well-defined axial zone of the Reelfoot rift to the covered 

southwestward extension of the FAFC, where it is distributed throughout a much broader zone-sufficient strain 

energy would not accumulate on any single fault in the 

FAFC to produce a large-magnitude earthquake. This postulated transition, northeast of the Pascola arch, may be governed by either a northeasterly termination of the axial zone 

of the Cambrian Reelfoot rift, or by the southwesterly limit 

of the Permian and younger F AFC, or by both. 

HICKS DOME 

Although Hicks dome is aseismic, it has many characteristics in common with the seismically active Pascola arch. 

Like the Pascola arch, Hicks dome is a late Paleozoic structural high forming the northern terminus of an elongate arch 

(Tolu arch) within the Reelfoot rift (compare McKeown and 

others, 1990, and Kolata and Nelson, 1991). A well drilled 

near the crest of the Pascola arch (the USGS New Madrid 

test well) contained extremely high concentrations of Be, 

Nb, Th, and La in Paleozoic non-carbonate strata (Goldhaber 

and others, 1993); these authors interpreted these elements to 

be metasomatically introduced. This group of elements is 

characteristic of alkalic igneous processes (e.g., Mariano, 

1989), similar to the igneous activity at Hicks dome. 

Because of these geologic and geochemical similarities 

between Hicks dome and the Pascola arch (although Hicks 

dome is considerably smaller), it is instructive to examine 

the subsurface characteristics of Hicks dome. 

The seismic data provide new information on the origin 

of Hicks dome and on the nature of subsurface rocks (and 

processes) in the vicinity of Hicks dome. It is clear from the 

data that the entire Paleozoic section is domed. Thus, contrary to one popular hypothesis (Brown and others, 1954; 

Nelson, 1991), the doming originated in the basement, rather 

than being produced by explosive processes within the Paleozoic section. Hicks dome is situated on the southwest flank 

of a large, northwest-trending positive magnetic anomaly, 

the maximum intensity of which occurs near Karbers Ridge, 

9 km northeast of the center of the dome (McGinnis and 

Bradbury, 1964). McGinnis and Bradbury (1964) interpreted 

the source of this anomaly to be a mafic intrusive body at 

depths of 3.35 km (11,000 ft) and greater, which would place 

the top of the intrusion in lower Paleozoic strata or Precambrian basement. Although there are no major potential-field 

anomalies centered on Hicks dome, it is clearly an intrusive 

center and is likely a product of explosive magmatism fed by 

the Karbers Ridge intrusion. Previous workers (e.g., Heyl, 

1983; Baxter and others, 1989; Bradbury and Baxter, 1992) 

concluded that mantle-derived volatiles were involved in the 

genesis of Hicks dome and its associated mineralization, 

based in part on the presence of intrusive breccias (including 

carbonatitic breccias) and an "alkalic suite" of elements. As 

these workers suggested, brecciation at Hicks dome may be 

explained by an explosive release of igneous volatiles; if so, 

this must have occurred in the Precambrian basement 

beneath the dome. The doming was probably not produced 

by a single large intrusion near the top of the basement, 

unless this intrusion was pervasively altered during the proposed explosive processes and related fluid flow to erase a 

"typical" potential-field signature of an intrusive body. 

The disruption of lower Paleozoic and intra-basement 

reflections in a 3-km-wide zone, beneath the projection of 

the crest of the dome, and the discontinuous nature of 

reflections between Hicks dome and the Ohio River may be 

the signature of: ( 1) myriad small alkalic and carbonatitic 

intrusions ge~etically related to those at the surface near 

Hicks dome, (2) anomalous physical properties due to alteration of the original lithologies by processes such as 
STRUCTURE OF REELFOOT-ROUGH CREEK RIFTS FROM SEISMIC REFLECTION DATA Q17 

silicification, (3) brecciation and fracturing of these layers, 

perhaps during the explosive event that has been invoked to 

explain Hicks dome, (4) closely spaced minor faults, as 

mapped at the surface in this area (too closely spaced to 

show on quadrangle maps, according to Heyl, 1983), or (5) 

a combination of these. Goldhaber and others ( 1992) and 

Plumlee and others (in press) presented fluid-inclusion and 

geochemical evidence that igneous activity at Hicks dome 

(and presumably the much larger buried Karbers Ridge 

intrusion) was a heat source during Permian Fluorspar area 

mineralization and that hydrofluoric acid-rich fluids related 

to Hicks dome magmatism were a significant component of 

the mineralizing fluids in the Fluorspar area. We hypothesize that these strongly acidic fluids, released during Hicks 

dome magmatism, pervasively altered parts of the Paleozoic section and that this is one of the principal causes of 

the discontinuous nature of the reflections beneath and 

southeast of Hicks dome. This postulated flow appears to 

have been highly directional because the southwest flank of 

Hicks dome has not been affected similarly. 

The term "cryptovolcanic," which has repeatedly been 

applied to Hicks dome, implies that subaerial explosive 

activity occurred at Hicks dome. Devonian and Mississippian reflections across Hicks dome and the Rock Creek graben have a quite coherent signature (although it is difficult to 

trace them continuously because of complex faulting) that 

argues against the "cryptovolcanic" interpretation for the 

origin of Hicks dome, unless all volcanic vents were outside 

the plane of the seismic section. 

Reflection patterns beneath the northwest boundary of 

the Rock Creek graben (discussed in a previous section of this 

report) may shed light on the role of Hicks dome magmatism 

in the structural evolution of the region. The obvious fault 

offset of reflections in the Knox is not seen in the underlying 

pre-Knox reflections, although the latter are disrupted. Perhaps faulting of Knox and younger layers occurred in 

response to pervasive intrusion and brecciation of deeper 

strata. The latter processes, even though they did not produce 

discrete offsets of individual layers, would have produced a 

bulk dilatation of the lower Knox, pre-Knox, and uppermost 

basement(?) rocks in the area between Hicks dome and the 

Ohio River. Overlying strata were faulted in response to this. 

In some areas (presumably over the top of the intruded and 

brecciated zone), overlying faults were extensional, as along 

the northwest boundary of the Rock Creek graben. One might 

expect the lateral boundary of the intruded and brecciated 

zone to be characterized by shortening, as seen in the reverse 

faulting identified along the southeast boundary of the Rock 

Creek graben. Thus, the Rock Creek graben may have originated in Permian time as a result of Hicks-dome-related 

magmatism and pervasive brecciation. 

CONCLUSIONS 

1. Seismic reflection data demonstrate a major alongstrike transition in the geometry of the Cambrian 

Reelfoot-Rough Creek rift system in the vicinity of 

the dogleg bend that connects the RFR and the Rough 

Creek graben. The highly asymmetric, southeast-dipping graben in the bend area (southern Illinois and 

western Kentucky), controlled by a major northwestdipping Cambrian normal fault zone near the southeastern edge of the rift, contrasts with the north-dipping half-graben to the east in Kentucky, controlled 

by the south-dipping Rough Creek fault system. 

2. The overall style of rifting changes to the south of the 

bend area from one of highly asymmetric grabens and 

half-grabens (Rough Creek graben and bend area) to 

one characterized by a structurally complex zone of 

axial thickening (main part of the RFR). 

3. The localization of historic New Madrid seismicity as 

a discrete zone within the RFR may be inherited from 

Cambrian rift structure-the northern and southern 

ends of the strongly seismogenic zone appear to correlate with major transfer zones that accommodate 

along-strike changes in Cambrian rift geometry. 

4. The seismic data demonstrate that the top of Precambrian basement is domed beneath Hicks dome, in Hardin Co., Illinois, and that reflectors in the preDevonian section near the dome are pervasively disrupted. We interpret the doming to have occurred as a 

result of a combination of intrusive and explosive 

processes in the basement during Permian alkalic 

magmatism, and we interpret the disruption of lower 

Paleozoic reflectors as the product of alteration by 

acidic fluid flow and intense faulting. Both the acidic 

fluids and the faulting appear to be genetically related 

to alkalic magmatism. There are several geologic similarities between the aseismic Hicks dome and the 

seismogenic Pascola arch. 

5. Fluorspar area Permian and younger normal faulting 

that reactivated the RFR is well-imaged on the seismic data; some grabens are bounded by faults that 

cannot be traced below the Knox and must be accommodated laterally within the Knox, whereas other 

faults clearly offset basement. 

6. The broad FAFC may absorb strain from the narrow 

New Madrid seismic zone; this strain may be distributed aseismically in the FAFC. This would help 

explain the nearly aseismic nature of the northernmost Mississippi Embayment north of the New 

Madrid seismic zone. 
Q18 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

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Published in the Central Region, Denver, Colorado 

Manuscript approved for publication November 10, 1994 

Edited by Richard W. Scott, Jr. 

Photocomposition and graphics preparation by Carol Quesenberry

U.S. GOVERNMEM1 PRINTING OFFICE: 1995-673-046/86129