Reelfoot Rift: Reactivated Precursor to the Mississippi Embayment

?' PA™CK ERVIN I Department of Geology, Northern Illinois University, DeKalb, Illinois 60115 L. D. McGINNIS '

ABSTRACT American Rift System (Midcontinent Grav- sive basinal development in Cambrian- ity High) (Ocola and Meyer, 1973), and Ordovician time (Schwalb, 1969). This Tectonic elements involved in the forma- epeirogenic uplifts (Burke and Wilson, basin, which Schwalb named the Reelfoot tion of the Mississippi embayment, as in- 1972; Faure, 1971, 1972, as reported by basin, was later transected by the gradual ferred from geophysical and geological in- Menard, 1973). Burke and Dewey (1973) uplift of the Pascola arch connecting the formation, originated in late Precambrian proposed that former plume-generated tri- Ozark uplift and the Nashville dome (Wil- time with continental rifting (the Reelfoot ple junctions lie along the present continen- son and Stearns, 1963). The center of basi- rift) and intrusion of high-density magma tal boundaries. In their model, active nal deposition concurrently shifted north- into the crust. Isostatic subsidence in early spreading occurred along two arms parallel ward from western Tennessee, reaching Paleozoic time formed the Reelfoot basin, to the continental edge, while the third arm southern Illinois by Silurian time and merg- approximately coincident with the modern often projected perpendicularly into the ing with the southward-shifting deposi- embayment, in which several kilometers of continent as a rift that failed to reach the tional center of the Eastern Interior basin. sediment were deposited. Closing of the spreading stage. On the basis of this The Pascola arch was subsequently de- proto—Atlantic Ocean and subduction in hypothesis, they suggested that the Missis- pressed and buried beneath the late the southern Appalachian Mountains in sippi embayment is the failed arm of a late Mesozoic and Cenozoic sediments that middle to late Paleozoic time were accom- Paleozoic triple junction that was located were deposited in the subsiding embay- panied by uplift and widespread erosion in near Jackson, Mississippi, and that was as- ment. the midcontinent. A period of rift reactiva- sociated with the opening of the Gulf of tion and intrusion in late Mesozoic time, in Mexico. We propose that the genesis of the REGIONAL GEOPHYSICAL SETTING association with rapid subsidence in the Mississippi embayment can be interpreted Gulf of Mexico, prompted renewed isosta- within the context of plate-tectonic theory Burke and Dewey's (1973) designation of tic subsidence within the embayment, form- but that its development is more ancient the Mississippi embayment as a failed rift ing the elongate depositional trough ob- and complex than suggested by Burke and was based on regional geology; however, served today. Continued seismicity and Dewey. this classification is also supported by positive free-air gravity anomalies indicate geophysical data. that isostatic adjustment continues to the REGIONAL GEOLOGIC SETTING present time. Key words: continental and Gravity failed rift, anomalous mantle. The Mississippi embayment is a re- entrant into the North American craton In the southeastern United States, areas INTRODUCTION from the south, delimited geographically by of positive Bouguer anomaly are found the maximum updip extent of post- along the Piedmont and within the embay- With the recent national concern about Paleozoic sedimentary rocks (Fig. 1). It is a ment (Fig. 2). The positive areas in the em- the possibility of predicting and preventing broad, spoon-shaped trough lying between bayment occur along a linear trend gener- major earthquakes, attention has focused the Ozark uplift to the west and the Nash- ally centered over the axis, which lies a few on the New Madrid earthquake zone, ville dome to the east and opening south- kilometers to the west of the Mississippi which is approximately coincident with the ward into the southerly-dipping beds of the River (Caplan, 1954) and extends north- axis of the Mississippi embayment. Studies Gulf Coastal Plain. Paleozoic strata are un- ward into the Illinois basin (Ervin and of this region are particularly important be- conformably overlain by Cretaceous and McGinnis, 1974). Superimposed on these cause an earthquake with the same intensity Tertiary unconsolidated to poorly consoli- linear anomalies are several approximately as the 1811 to 1812 sequence would cause dated sediments that thicken southward circular anomalies, 30 to 100 km in diame- extensive damage and loss of life in the from the feather edge in southern Illinois to ter, with amplitudes of 30 to 40 mgal. Phe- midcontinent. This paper presents a 1 km near Memphis, Tennessee (Stearns lan (1969) studied two of these and found hypothesis for the evolution of the embay- and Wilson, 1972). For the purposes of this them to be plutons extending through much ment that may serve as a guide for these paper, we will assume that the southern of the crust and having densities in the investigations. limit of the embayment is marked by a line range of 3.1 to 3.3 g/cm3. Plate- and block-tectonic theories have extending from the southern Appalachians provided a new basis for the interpretation to the southeastern Ouachita Mountains. Seismology of intraplate processes, such as formation Although sediment deposition indicates of the African rifts (Sowerbutts, 1969; that subsidence of the present trough began Although located within the otherwise Griffiths and others, 1971; Maasha and in Cretaceous time, subsurface information stable craton, the embayment is an area of Molnar, 1972), intracratonic basins obtained from drilling clearly shows that moderate seismicity. The majority of the (McGinnis, 1970), the Central North the embayment was also the site of exten- earthquakes, including the great New Ma-

Geological Society of America Bulletin, v. 86, p. 1287-1295, 5 figs., September 1975, Doc. no. 50913.

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drid sequence of 1811 to 1812, have occurred in a zone along its axis (Stearns and Wilson, 1972). A single reversed refraction profile fur- nishes the only deep control on the crustal structure underlying the embayment (McCamy and Meyer, 1966). The profile (Rl) is located along its western edge between Little Rock, Arkansas, and Cape Girardeau, Missouri (Fig. 1). The interpreted crustal section near Batesville, Ar- kansas, is shown in Figure 3. A reversed refraction profile between St. Joseph and Hannibal, Missouri (R2 in Fig. 1), shows the crustal structure in an area of normal craton. The interpreted structure at Hannibal (Stewart, 1968) is included in Figure 3 for comparison between normal and anomalous crustal sections. The seismic crustal structure of the embayment has the following three anomalous characteristics: (1) the presence of an anomalous velocity layer (7.4 km/sec) at the crust-mantle interface, (2) the great depth of the mantle (8.1-km/sec layer), and (3) the shallowness of the 6.5-km/sec crustal layer. A velocity of 7.4 km/sec is intermediate between normal continental crust and mantle velocities, suggesting that this is a zone of transitional composition. If the anomalous layer is removed from the embayment profile and adjustment made for the isostatic depression of the mantle surface, the two velocity structure sections in Figure 3 are identical, suggesting that the overlying crust was upwarped by emplacement of the 7.4- km/sec layer.

CRUSTAL MODEL FROM GEOPHYSICS AND GEOLOGY

The above data were used as constraints in the construction of a crustal model along a gravity profile (G, Fig. 1) perpendicular to the embayment axis, using Woollard and Joesting's Bouguer anomaly map (1964). The profile extends from Yellville, Arkan- Figure 1. Location map. Light stippled pattern: Cretaceous and younger sediments. Rl: refraction sas, on the west to Scottsboro, Alabama, on profile by McCamy and Meyer, 1966. R2: refraction profile by Stewart, 1968. G: gravity profile in the east (Fig. 1). Because the embayment is Figure 4. AX: probable rift axis. (Adapted from Cohee and others, 1962.) an essentially linear feature, it can be ap- proximated by a two-dimensional model contour map of Stearns and Wilson (1972). is between Ste. Genevieve, Missouri, and that extends to infinity in the direction Away from the axis of the embayment, the Hercules, Missouri (Stewart, 1968), about parallel to its axis. Therefore, the gravity base of the 6.2- and 6.5-km/sec layers were 50 km north of Yellville. This profile was field to be fitted by the model is that com- constrained to approach the "standard" ignored because the data were poor and ponent of the observed field that is theoreti- crustal section of Stewart (1968). No at- would not reverse. In a study of P-wave cally invariant in the axial direction. This tempt was made to constrain the base of the spectra, Kurita (1973) also encountered regional component was estimated from 6.1-km/sec layer, because other refraction low-quality data in this area. Lateral in- both the original map and the profile by profiles in the midcontinent (Healy and homogeneities in the crust under the Ozark visual smoothing (Fig. 4). Computations Warren, 1969) suggest that this interface is uplift appear to be the cause of the data were made using the two-dimensional Tal- not laterally continuous. The structure also degradation. wani algorithm (Talwani and others, was required to conform to the seismic The second profile is between Moulton, 1959a). model (McCamy and Meyer, 1966) at the Alabama, and Tullahoma, Tennessee (War- The interpreted crustal model and its cal- intersection of the refraction profile and the ren, 1968). This profile intersects the cross culated field are shown in Figure 4. Several cross section at the western edge of the em- section about 100 km west of Scottsboro, additional conditions were imposed on the bayment. with the southern shotpoint lying on a model. The depth to the base of the Cre- Two additional refraction profiles in the broad, 40-mgal positive gravity anomaly taceous rocks was interpolated from the area were not strictly honored. One of these approximately 60 km south of the intersec-

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Figure 2. Positive Bouguer anomalies superposed on regional tectonic features, suggesting concentration of high-density masses within the Mississippi embayment.

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tion point (Fig. 2). The gravity anomaly is Ramberg, 1972) and suggests that a failed- dence had progressed sufficiently that sedi- probably caused by a large mafic intrusion rift interpretation of the Mississippi em- ments began to accumulate over the former with an associated high seismic velocity bayment is compatible with the available uplift (Fig. 5B). Isostatic adjustment con- that would produce an erroneously deep in- geophysical data. tinued into Ordovician time, producing the terface if the profile were solved using a deep Reelfoot basin that was approxi- planar layer equation. The computed depth TECTONIC INTERPRETATION mately coincident with the present embay- to mantle under the southern shotpoint of ment, but which extended into southeastern 50+ km suggests that this may have oc- The interpreted sequence of tectonic Illinois and western Kentucky and Tennes- curred. Therefore, the interfaces were ad- events culminating in the formation of the see (Schwalb, 1969). Total Paleozoic sedi- justed upward as required by the gravity present embayment is presented below and ment accumulation exceeded 4.25 km data. is illustrated by the diagrammatic cross sec- (Bond and others, 1971). Lateral displace- Layer densities for the model were de- tions in Figure 5. ment of mantle material from below the rift duced from several sources (Table 1). War- initiated the elevation of the adjacent Ozark ren and others (1966) computed a density Precambrian and Nashville domes in Ordovician time. of 2.30 g/cm3 for the surficial Upper Cre- taceous and Tertiary deposits in Missis- In late Precambrian time, the area com- Middle Paleozoic to sippi. This value was used for the equival- prising the Mississippi embayment and the Middle Mesozoic ent sediments of layer 1 of the model. Den- Illinois basin underwent epeirogenic uplift sities for the high-velocity mantle layers due to emplacement of "anomalous," At the end of early Paleozoic time, a (model layers 6 and 7) were determined low-density mantle material at the base of widespread, intermittent, gradual uplift from the Nafe and Drake velocity-density the crust (Fig. 5A), undoubtedly accom- commenced (Fig. 5C) that lasted through curve (Talwani and others, 1959b). panied by local intrusions into the crust. the end of the Paleozoic Era (Caplan, 1954; Crustal densities were computed from This uplift, which is here termed the "Reel- Ham and Wilson, 1967; Tikrity, 1968). In the relationship determined by Ocola and foot rift," was part of the widespread rift- the region of the rift, this tectonism pro- Meyer (1973) for the Central North Ameri- ing activity that affected the North Ameri- duced continued elevation of the Ozark up- can Rift System. Where seismic velocities can continental masses between 1.3 and 1.1 lift and the Nashville dome. The uplift and could be correlated with specific rocks, they b.y. B.P. (Burke and Dewey, 1973). Cessa- dome were connected by a broad arch found that intermediate velocities (4.7 to tion of igneous activity in the flanking St. (named the Ozark arch by Tikrity, 1968) 6.9 km/sec) are associated with higher den- Francois Mountains, following intrusion of consisting of the Pascola arch (Grohskopf, sities than are predicted by the Nafe and basic dikes and sills into the older granite 1955) and the Clifton saddle (Stearns and Drake relationship. This association is in and felsite about 1.1 b.y. B.P. (Tikrity, Marcher, 1962). The arch transects the agreement with the known data in the em- 1968) probably marks the end of the period northern end of the embayment, implying bayment, where the velocity for the lower of active uplift. that the tectonic forces creating it were and middle Paleozoic rocks (model layer 2) The model in Figure 4 implies that the neither related to nor generally controlled is 4.7 km/sec (McCamy and Meyer, 1966). total uplift was on the order of several by the earlier rift. The average density for these rocks, both kilometers, but examination of modern As the arch was elevated, the center of within the embayment (Thoenen and analogues suggests that the average relief deposition shifted northward from the others, 1945) and in the Illinois basin was probably 1 to 2 km (Khan and Reelfoot basin until it reached the site of the (Heigold, 1970), is approximately the pre- Mansfield, 1971; Le Bas, 1971; Ramberg, present Illinois basin in Devonian time 3 dicted 2.60 g/cm . 1972; Wong and Von Herzen, 1974). Be- (Bond and others, 1971). Tensional forces The model in Figure 4 is similar to the cause of the brittleness of the upper crust, due to the uplift were sufficient to cause crustal structure of rifts (Ansorge and an uplift of this magnitude may be expected crustal failure between the Ozark others, 1970; Khan and Mansfield, 1971; to produce an axial rift or graben. Subse- uplift—Ozark arch and the Illinois basin, quent erosion and later tectonic activity producing the Ste. Genevieve fault zone. SEISMIC CRUSTAL STRUCTURE have obliterated any surficial evidence of Tikrity (1968) reported fault movement as the original graben. However, axial faults early as Devonian time, with as much as 0.9 STEWART McCAMY & MEYER in the embayment have remained active km total displacement, downthrown to the DEPTH since Precambrian time (Caplan, 1954; Phe- north. Decoupling of the uplifted arch and (KM) 5.0 -0- lan, 1969) and are probably the cause of the the Illinois basin allowed the basin to con- 4.7 6.1 axial graben structure in the Paleozoic tinue isostatic subsidence due to the high- 6.2 rocks reported by Bond and others (1971). density material emplaced during the earlier -10- 6.2 Near the end of Precambrian time, ero- rifting activity and to subsequent sedimen- sion was actively leveling the surface of the tary loading (McGinnis, 1970), with the -20- 6.5 rift. Although most detrital deposits derived basin eventually reaching a depth of over 4 from the uplift have been destroyed by later km. 6.6 tectonism and erosion, they are preserved in The late Paleozoic (Sutton, 1971) Cot- -30- Kansas and western Missouri as a large belt tage Grove—Shawneetown—Rough Creek of arkosic sandstone and shale (Ham and fault system, which trends east through the -40- 7.4 Wilson, 1967) that has an approximate age center of the basin, may also represent de- 8.0 of 1.2 b.y. (Denison, 1966). coupling, but the history of fault movement 8.1 and its relationship to other 38th-parallel 50- Early Paleozoic lineaments is a matter of some controversy BATESVILLE, ARK. HANNIBAL, MO. (Clark and Royds, 1948; Snyder and Ger- VELOCITIES IN KM/SEC Dissipation of the rifting forces would demann, 1965; Sutton, 1971; Heyl, 1972). Figure 3. Normal and anomalous cratonic also allow the large mass of anomalous Deformation of the uplift's northern flank crustal sections interpreted from seismic data mantle material in the lower density crust was accompanied by explosive igneous ac- (after Stewart, 1968; McCamy and Meyer, to initiate isostatic subsidence of the up- tivity (Snyder and Gerdemann, 1965) and 1966). lifted area. By early Paleozoic time, subsi- by intrusion by basic dikes and sills in Late

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Pennsylvanian and Permian time (Zartman 1969) until early Late Cretaceous time Late Mesozoic to Present and others, 1967). (Stearns and Marcher, 1962), with erosion Schwalb (1971) reported an unnamed removing as much as 4.6 km of strata By early Late Cretaceous time, subsi- east-southeast—trending Paleozoic fault (Marcher and Stearns, 1962). dence of the Gulf Coastal Plain was lying on the south side of the arch and ap- The early and middle Mesozoic Era (Fig. sufficient to generate tensional forces proximately passing through the junction 5C) was a time of emergence and relative within the Ouachita front that crosses the of the Missouri, Tennessee, and Arkansas quiescence throughout the midcontinent southern end of the embayment, producing state borders. Displacement is up to the (Moore, 1970). Paleozoic rocks from the zones of weakness intruded by nepheline north as much as 1.2 km (Bond and others, Mississippi Valley to the Appalachians syenite (Kidwell, 1951). Formation of the 1971), representing a partial decoupling were extensively eroded, with the detritus modern embayment began concurrently by along the uplift's southern flank. being deposited westward in the Great "reactivation" of the ancient rift (Fig. 5D). The Pascola arch remained emergent Plains (Moore, 1970) and southward into This reactivation apparently was initiated from Early Pennsylvanian time (Schwalb, the subsiding Gulf of Mexico. by the tectonism of the Gulf Coastal Plain because embayment subsidence began in the south and proceeded north and west Anomaly A Regional Bouguer (Caplan, 1954). The original axis of the + Observed Bouguer embayment was located some distance to (taken from Contour the east and subsequently migrated west- 20- Intersections) ward (Cushing and others, 1964) because of downwarping along an earlier zone of weakness (Caplan, 1954). Caplan con- 0 cluded that the present embayment is the E§> result of readjustment of the Earth's crust along zones of weakness established in early Paleozoic or Precambrian time. The Black Warrior basin probably focused the initial subsidence eastward of the more re- sistant Ouachita front. As downwarping progressed, the increasing stress overcame the resistance of the front, initiating intrusion of the nepheline syenite and allowing the embayment deformation axis to shift westward to approximately coincide with Regional Bouguer the present seismic zone. + Computed Reactivation of a rift is commonly indi- cated by alkaline igneous activity (Burke and Dewey, 1973). This is also true in the embayment, where the earliest stage of subsidence was accompanied by the Creta- ceous emplacement of syenite and 1am- prophyre (Moody, 1949; Kidwell, 1951; Grohskopf, 1955; Bond and others, 1971) in southeastern Missouri and western Ten- nessee (Fig. 1). These intrusions are located along the embayment axis and at the intersection of the axis and the Pascola arch, areas that were the focus of considerable stress. Most of the embayment subsidence occurred in Eocene time (Stearns and Marcher, 1962), but there are no reports of igneous activity after Cretaceous time. Stearns and Wilson (1972) believe that the embayment has been stable since its emergence during the widespread uplift of the continent in Neogene time (Durham and Murray, 1967). However, geophysical studies indicate that relative subsidence has continued to the present time (Fig. 5E), and an additional adjustment of approximately 120 m is required to reach isostatic equilibrium (McGinnis, 1963, 1970).

RELATIONSHIP TO ADJACENT TECTONIC PROVINCES Figure 4. Computed crustal profile from Yellville, Arkansas, to Scottsboro, Alabama. The pillow- shaped body of anomalous mantle material (layer 6) is characteristic of continental rifts. Velocity and Continental rifting was a widespread density values for layers 1 through 6 are given in Table 1. phenomenon within the American plate in

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late Precambrian time. The Central North Yungul (1971) argued that growth of the gesting emplacement in association with a American Rift System developed to the Gulf basin since at least early Mesozoic compressive force from the southeast north and west of the Reelfoot rift (Ocola time may have been a tensional response to (McGinnis and Bradbury, 1964) that was and Meyer, 1973). The proto-Atlantic the separation of the North and South probably caused by the closing of the Ocean began to form to its east, and the American plates. Divergence of the plates proto-Atlantic. This activity was apparently initial rift that was to create the Gulf of was caused by sea-floor spreading in the unrelated to the rift. Mexico crossed its southern extremity. North Atlantic. During the early opening The only known intrusions that can be Although the relationship between the phase, the South American plate was joined directly associated with rifting activity are Reelfoot rift and the tectonism of adjacent to the African plate. As North America and the Late Cretaceous syenite and nepheline regions is not well defined, the simulta- Africa were driven apart, South America syenite of Tennessee (Moody, 1949; Kid- neous rifting activity in the immediately was rotated away from North America, well, 1951) and the lamprophyre of Mis- adjacent Gulf of Mexico (Keller and Cebull, opening the Gulf of Mexico-Caribbean re- souri (Kidwell, 1951; Grohskopf, 1955). 1973) strongly suggests that the two tec- gion (Le Pichon and Fox, 1971). Emplacement of these coincides with the in- tonic events were not independent. We The onset of subsidence of the Gulf itial subsidence stage of the modern em- propose that a late Precambrian, plume- Coastal Plain in Mesozoic time, which ul- bayment, which implies moderate reactiva- generated triple junction occurred near the timately initiated downwarping of the em- tion of the rift. A rejuvenated rift origin is intersection of the embayment axis and the bayment, is a direct result of the tensional also supported by comparison to the lithol- Gulf Coastal Plain, similar to the model stress associated with Yungul's (1971) ogy of other rifts, such as the Oslo Graben, suggested by Burke and Dewey (1973) for mechanism of opening. Additional features where the plutonic rocks are 65 percent late Paleozoic time. Separation occurred on attributable to the tensional regime are syenite, including 1.5 percent nepheline the two arms paralleling the Gulf of Mex- early Mesozoic diabase intrusive rocks syenite (Turner and Verhoogen, 1960), and ico, whereas the Reelfoot rift became the along the Ouachita front (Kidwell, 1951) the Callander Bay complex of the "failed" arm when active spreading began. and a series of grabens bounded by flexure Ottawa—St. Lawrence graben system, Direct evidence of this triple junction has faults (Durham and Murray, 1967) that which includes both lamprophyre and been destroyed by the subsequent Ouachita parallel the Ouachita trend but lie to the nepheline syenite (Ferguson and Currie, orogeny. south of it (Walthal and Walper, 1967). 1971). By early Paleozoic time, an Atlantic-type From the time of the Ouachita orogeny The syenite of southwestern Tennessee continental margin existed south and east until the Cretaceous Period, sediment de- requires closer examination. Moody (1949) of North America (Keller and Cebull, position was limited to the Gulf basin south and Kidwell (1951) reported the occurrence 1973). Subsidence of the Reelfoot basin of the orogenic front. Upper Cretaceous of "normal" syenite and nepheline syenite, during this hiatus in nearby tectonic activity breaching of the front during subsidence of respectively, in this area. Bond and others implies dominance of isostatic forces in the the embayment was accompanied by a sec- (1971) further described these as weath- absence of significant external stresses. This ond, but alkalic, intrusive episode within ered, which implies shallow emplacement, was followed by the development of sub- the front (Kidwell, 1951), concurrent with and suggested that their distribution is fault duction zones parallel to the margins and similar intrusions within the upper embay- controlled. These intrusions are coincident either underthrusting of the North Ameri- ment. with one of the high-amplitude gravity can plate (Bird and Dewey, 1970; Hatcher, anomalies modeled by Phelan (1969). This 1972; Keller and Cebull, 1973) or, along PETROLOGY OF EMBAYMENT anomaly is transected by profile G and is the eastern plate margin, underthrusting of IGNEOUS ROCKS designated as Anomaly A in Figure 4. Phe- the African plate (Slaymaker and Watkins, lan interpreted the anomaly source to be a 1974). As noted above, the craton began a Data on the petrology of igneous rocks pluton extending through the crust to gradual intermittent uplift that continued related to rift development are sparse. within about 8 km of the surface and hav- 3 throughout Paleozoic time. The period of Within the Mississippi embayment and the ing a density of 3.1 to 3.3 g/cm . This den- uplift culminated in the upwelling of an Illinois basin, igneous rocks emplaced dur- sity is much higher than the average 2.52 3 "orogenic welt" as the Ouachita orogeny in ing the initial Precambrian rifting phase g/cm found for syenite near Little Rock, Carboniferous time (Keller and Cebull, have not been encountered. This can be at- Arkansas (Thoenen and others, 1945), and 1973). tributed to subsequent erosion or burial of the depth to top is considerably greater Closing of the proto-Atlantic started in much of the area. The only record of intru- than the reported depth of 839 m to the north in Late Devonian time and moved sive activity at this time is the emplacement nepheline syenite (Kidwell, 1951; Illinois southward during middle to late Paleozoic of diabase dikes and sills in the adjacent St. Geological Survey, 1971) in the Pure Oil time. Uplift of the Ozark dome-Ozark Francois Mountains (Tikrity, 1968). No. 1 Robroy-McGregor well. arch—Nashville dome area was undoubt- The next occurrence of igneous activity The difference between the observed and edly related to this general tectonism. was the intrusion of kimberlite dikes and Although the initial rifting that preceded diatremes near Avon, Missouri, also west of TABLE 1. VELOCITY-DENSITY RELATIONSHIP USED formation of the Gulf of Mexico occurred the embayment (Kidwell, 1947; Watson, FOR CONSTRUCTING CRUSTAL MODEL in late Precambrian time, followed by active 1967). Fracturing associated with the Ste. 3 spreading and subduction in Paleozoic Genevieve fault apparently provided the Velocity (km/sec) Density (g/cm ) Model Layer time, the time of maximum extension and channel-way for the intrusions in Early to — 2.30* 1 formation of the Caribbean was the Middle' Devonian time (Zartman and 4.7 2.60+ 2 Mesozoic Era (Le Pichon and Fox, 1971). others, 1967). 6.1 2.87t 3 6.2 4 The apparent change in the rate of opening The first recorded igneous intrusions 2.89t 5 may reflect a significant alteration in the 6.5 2.97t within the rifted area are Late Pennsylva- 7.4 3.175 6 driving mechanism. The early stage was nian and Permian (Zartman and others, 8.1 3.301 7 caused by a thermally driven sea-floor 1967) kimberlite dikes and diatremes in the spreading system that culminated in the Illinois-Kentucky fluorspar district (Wat- *Warren and others, 1966. Ouachita orogeny (Keller and Cebull, tOcola and Meyer, 1973. son, 1967). The dikes are usually restricted §Na£e and Drake (Talwani and others, 1959b). 1973) and the cessation of active spreading. to the northwest-trending fractures, sug-

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A. Late Precambrian: rifting B. Early Paleozoic: subsidence C. Middle Paleozoic- Middle Mesozoic: gradual uplift

rift reactivation and intrusion subsidence

Figure 5. Schematic cross sections of the Reel- foot rift through geologic time. Black bodies are mafic intrusions. A. Emplacement of anomalous mantle. B. Isostatic subsidence and trough formation. C. Pascola arch elevated across trough. D. Intrusion of additional high-density masses into the upper crust. E. Formation of present Mississippi embayment.

computed densities and depth suggests that contemporaneous with the early emplace- currences of intrusive rocks indicate that only the top of the intrusion is syenite and ment of the anomalous mantle layer. Rift most or all of the large Cretaceous plutons that most of the pluton consists of a denser reactivation caused partial melting of the were emplaced south of lat 36.3°N. Seismic- mafic rock. From studies of the Callander plutons and moderate igneous activity in ity within this region, covering most of the Bay complex, Ferguson and Currie (1971) Cretaceous time. Partial melting of the plu- embayment, is controlled primarily by proposed a mechanism that accounts for tons would also imply some melting of the block tectonics. Isostatic adjustment is this configuration. They suggested that an enclosing crustal rock, which has not been being achieved by movement of the linear ultrabasic magma may split into various observed. However, if a granitic dike or sill plutons or of lithospheric blocks as coher- immiscible fractions as it cools, producing were encountered in the Cretaceous rocks, ent units, with normal faulting along the both lamprophyre and syenite, as well as it would probably be termed Precambrian nearly vertical contacts between the blocks, some other common rift rocks, from the basement, and drilling would be terminated as has been found in the Illinois basin same parent magma. Currie (1972) further without attempting to penetrate to deeper (McGinnis and Ervin, 1974). suggested that anatexis of pre-existing al- horizons. kalic rock is probably required for the for- We believe that an alternate interpreta- SUMMARY AND CONCLUSIONS mation of large syenite bodies. tion is more probable. Renewed rifting of Examination of the literature (M. G. the embayment in Cretaceous time was as- Geophysical and geological data indicate Mudrey, Jr., 1974, oral commun.) shows sociated with partial melting of the mantle that the Mississippi embayment is the site that the available chemical and mineralogi- or Precambrian anomalous mantle and em- of a former continental rift, herein called cal data are insufficient for accurate placement of the large shallow plutons and the Reelfoot rift. The arching of the crust definition of the genetic constraints on the additional deeper intrusions. Subsequent over a thick linear body of anomalous origin of the igneous embayment rocks. subsidence of the embayment can then be mantle, as interpreted from geophysical However, the rock types and textures sug- attributed to the resultant increase in data, suggests that several kilometers of up- gest derivation from mantle sources and crustal mass. lift and erosion occurred. The absence of rapid emplacement along deep-seated frac- The second interpretation also explains such a volume of sediments originating tures. This implies a rifting or crustal exten- the differences in earthquake focal from this source area in post-Precambrian sion mechanism, but with varying degrees mechanism solutions, which show a dom- time argues that the rifting must have oc- of partial melting in the source region, and inantly east-west compressive stress system curred during Precambrian time. Although would seem to argue against a magma- in the embayment north of lat 36.3°N and most Precambrian deposits in the adjacent supplied thermal plume in the embayment an east-west tensional system to the south regions have been destroyed by later ero- area during the Cretaceous igneous activity. (Street and others, 1974), indicating differ- sion and tectonism, a wide belt of ent modes of subsidence. This suggests that 1.2-b.y.-old arkosic sediment in western Tectonic Significance of the movement in the northern tip of the em- Missouri and Kansas suggests a probable Petrologic Data bayment is controlled by the inward- age for the rift. This date is also supported dipping faults formed during the major rift- by the widespread rifting activity that oc- There are thus two possible sequences of ing phase in Precambrian time. curred in North America about 1.2 b.y. events. The first is that the plutons are all Both the gravity data and the known oc- B.P., by the concurrent opening of the Gulf

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of Mexico, by the simultaneous intrusion of of the model we have proposed, the data and Arkansas [Ph.D. dissert.]: Austin, Univ. mafic rocks into the adjacent Ozark uplift presently available are sufficient to argue Texas, 328 p. [abs. in Dissertation ab- area, and by the formation of a deep, early that the overall concept of a rift origin for stracts, 1967, v. 27, p. 2409-B], Paleozoic basin approximately coincident the embayment is valid. Durham, C. O., Jr., and Murray, G. E., 1967, with the rift axis. Tectonism of Atlantic and Gulf Coastal province: Am. Jour. Sei., v. 265, p. Following failure of the rift as the conti- ACKNOWLEDGMENTS 428-441. nental arm of a triple junction, isostatic Ervin, C. P., and McGinnis, L. D., 1974, Crustal forces caused subsidence of the rift to form We appreciate the assistance of J. Kohs- change from craton to Mississippi embay- the Reelfoot basin, which was several mann. Some of the ideas presented in this ment determined from gravity fields [abs.]: kilometers deep. From middle Paleozoic paper are the outgrowth of a tectonics EOS (Am. Geophys. Union Trans.), v. 55, time through middle Mesozoic time, the study supported by Dames and Moore, p. 436. peripheral Ouachita orogeny and closing consultants in the applied and environmen- Faure, H., 1971, Relations dynamiques entre la proto-Atlantic subjected the rift area to tal earth sciences, but the authors are solely croûte et le manteau d'après l'étude de only mild tectonism. The region was gener- responsible for its contents. Computing l'évolution paléogéographique des bassins sedimentaires: Acad. Sei. Comptes Rendus, ally shallow or slightly emergent, with the facilities were provided by Northern Illinois ser. D., v. 265, p. 3239-3242. rising Pascola arch and subsiding Illinois University. 1972, Paléodynamique du craton africain: basin representing the primary tectonic ac- Research was sponsored by Northern Il- Internat. Geol. Cong., 24th, Montreal tivity. The arch, decoupled from the basin linois University and the U.S. Geological 1972, sec. 3, Proc., p. 44-50. by faulting, continued to rise and erode Survey, Department of the Interior, under Ferguson, J., and Currie, K. L., 1971, Evidence through middle Mesozoic time. U.S. Geological Survey Grant no. of liquid immiscibility in alkaline ultrabasic By late Mesozoic time, tensional forces 14-08-000 l-G-139. dikes at Callander Bay, Ontario: Jour. Pe- due to subsidence in the Gulf of Mexico trology, v. 12, p. 561-585. were sufficient to breach the Ouachita front Griffiths, D. H., King, R. F., Khan, M. A., and and initiate a renewal of activity along the REFERENCES CITED Blundell, D. 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