Roy B. Van Arsdale

The Mississippi Embayment, North America: a first order continental structure generated by the Cretaceous superplume mantle event

Abstract The Mississippi Embayment of North America, a northward extension of the Gulf of Mexico coastal plain, is a southwestward-plunging trough containing ∼1.5 km of Cretaceous and Cenozoic sediments. The Embayment is underlain by the early Paleozoic Mississippi Valley graben basement fault complex. Previous authors have attributed Embayment subsidence to the opening of the Gulf of Mexico. However, the Embayment subsided 60 million years after cessation of the sea-floor spreading in the Gulf. We have previously argued that the Mississippi Embayment formed as a result of the westward passage of faulted crust (Mississippi Valley graben) over the Bermuda hotspot in mid-Cretaceous. More recently published age data clarify age progressive (northwest-to-southeast) mid-Cretaceous volcanism that crosses the Mississippi Embayment, beginning ∼115 Ma in eastern Kansas and ending ∼65 Ma in central Mississippi. This line of volcanism coincides with the predicted Bermuda hotspot path and has isotopic signatures consistent with a mantle hotspot source. We propose that during mid-Cretaceous, the weak crust of the Mississippi Valley graben complex was uplifted 1–3 km as it passed over the Bermuda plume, and this upland was eroded. As the Mississippi Valley graben complex moved west of the hotspot, it subsided, and the eroded region became a topographic low that filled with fluvio-marine sediments, the Mississippi Embayment. Supporting evidence for mid-Cretaceous uplift and erosion of the Embayment region includes: (1) an angular unconformity on pre-Late Cretaceous rocks with ∼2 km eroded at mid-Cretaceous along the hotspot path; (2) a broad anticline in the Embayment at mid-Cretaceous (revealed by unfolding the down-warped basal Late Cretaceous unconformity); (3) exhumation and weathering of mid-Cretaceous plutons before burial by Late Cretaceous sediments; and (4) a mid-Cretaceous change in the northern part of the Gulf of Mexico sedimentation from a continuous carbonate platform to a large influx of deltaic clastics. We now suggest that magmatic activity and pronounced uplift in the Mississippi Valley graben region may have been a result of increased hotspot flux of the typically weak Bermuda hotspot during the Cretaceous superplume mantle event (∼120–80 Ma).

Late Cretaceous and Cenozoic Geology of the New Madrid Seismic Zone

Structure-contour maps constructed from well, seismic reflection, and outcrop data of the tops of the Paleozoic section, Upper Cretaceous section, Paleocene Midway Group, and Eocene section illustrate the post-Paleozoic structure of the New Madrid seismic zone region. Isopach maps of the Late Cretaceous section, Midway Group, and Eocene section help constrain the timing of the structural events. These maps, which encompass much of the northern Mississippi embayment, reveal reactivation of the underlying late Precambrian/Cambrian Reelfoot rift during Midway Group deposition but no reactivation during Late Cretaceous or Eocene deposition. The structure-contour maps also indicate a subtle, south-plunging depression on the tops of the Paleozoic, Upper Cretaceous, and Midway Group along the axis of the northern Mississippi embayment that we have called a trench. This trench is 50-km wide, has a maximum depth of 100 m, and appears to have formed during the Eocene. The trenchs western boundary coincides with the Blytheville arch/Lake County uplift and its southeastern margin underlies Memphis, Tennessee. The Blytheville arch/Lake County uplift is the structure responsible for the New Madrid seismic zone, and thus it is possible that the southeastern margin of the trench is also a fault zone. Northern Mississippi embayment post-Paleozoic stratigraphy consists of sands, silts, and clays that thicken from 477 m at New Madrid, Missouri, to 987 m near Memphis, Tennessee. The uniformity of these sediments indicates their elastic properties and therefore seismic velocities are very similar; however, variations in cementation and unconformities within the section may influence seismic-wave propagation. Manuscript received 17 June 1999.

Identification of possible Quaternary deformation in the northeastern Mississippi Embayment using quantitative geomorphic analysis of drainage-basin asymmetry

To investigate neotectonism in the Mississippi Embayment east of the New Madrid seismic zone, we identified geomorphic domains that show evidence of ground tilting during Quaternary time. Transverse basin profiles were converted to two-dimensional vectors that denote channel position with respect to basin divides. These basin-asymmetry vectors record the net direction and degree of lateral migration of trunk streams. More than 2500 vectors were measured and spatially averaged within 400 km 2 bins. This field of 300 mean vectors delineates several domains that show preferred directions of stream migrations possibly driven by ground tilting. The timing of stream migration was interpreted using across-valley distributions of Quaternary alluvial terraces. Comparison of our mean vector field with subsurface structures suggests that some domain boundaries may be related to reactivated faults. Late Quaternary activity is suggested for two northeast-striking faults of the southeastern Reelfoot Rift margin. We acquired two seismic profiles showing near-surface faulting beneath scarps that follow the domain boundary associated with one of these northeast-striking faults (Big Creek fault zone). Reelfoot thrust seismicity ends on the south against this fault, suggesting that the rift margin has dextral slip accommodating northeastward movement of the thrust hanging wall. Our vector field also suggests late Quaternary movement on the Reelfoot thrust and on two other northwest-striking faults, here termed the Hatchie River fault and the Wolf River fault. Several other weak domains may imply minor elements of neotectonism. Our results demonstrate that morphometric analysis of drainage-basin asymmetry can be an effective reconnaissance tool within neotectonic settings.

Reelfoot rift and its impact on Quaternary deformation in the central Mississippi River valley

Geophysical and drill-hole data within the Reelfoot rift of Arkansas, Tennessee, Missouri, and Kentucky, USA, were integrated to create a structure contour map and threedimensional computer model of the top of the Precambrian crystalline basement. The basement map and model clearly defi ne the northeast-trending Cambrian Reelfoot rift, which is crosscut by southeast-trending basement faults. The Reelfoot rift consists of two major basins, separated by an intrarift uplift, that are further subdivided into eight subbasins bound by northeast- and southeast-striking rift faults. The rift is bound to the south by the White River fault zone and to the north by the Reelfoot normal fault. The modern Reelfoot thrust fault, responsible for most of the New Madrid seismic zone earthquakes, is interpreted as an inverted basement normal fault. Geologic interpretation of 5077 shallow borings in the central Mississippi River valley enabled the construction of a structure contour map of the Pliocene‐Pleistocene unconformity (top of the Eocene‐base of Mississippi River alluvium) that overlies most of the Reelfoot rift. This map reveals both river erosion and tectonic deformation. Deformation of the Pliocene‐Pleistocene unconformity appears to be controlled by the northeast- and southeast-trending basement faults. The northeast-trending rift faults have undergone and continue to undergo Quaternary dextral transpression. This has resulted in displacement of two major rift blocks and formation of the Lake County uplift, Joiner ridge, and the southern half of Crowley’s Ridge as compressional stepover zones that appear to have originated above basement fault intersections. The Lake County uplift has been tectonically active over the past ~2400 yr and corresponds with a major segment of the New Madrid seismic zone. The aseismic Joiner ridge and the southern portion of Crowley’s Ridge may refl ect earlier uplift, thus indicating Quaternary strain migration within the Reelfoot rift.

Neotectonics of the upper Mississippi embayment

Abstract Although the upper Mississippi embayment is an area of low relief, the region has been subjected to tectonic influence throughout its history and continues to be so today. Tectonic activity can be recognized through seismicity patterns and geological indicators of activity, either those as a direct result of earthquakes, or longer term geomorphic, structural, and sedimentological signatures. The rate of seismic activity in the upper Mississippi embayment is generally lower than at the margins of tectonic plates; the embayment, however, is the most seismically active region east of the Rocky Mountains, with activity concentrated in the New Madrid seismic zone. This zone produced the very large New Madrid earthquakes of 1811 and 1812. Geological and geophysical evidence of neotectonic activity in the upper Mississippi embayment includes faulting in the Benton Hills and Thebes Gap in Missouri, paleoliquefaction in the Western Lowlands of Missouri, subsurface faulting beneath and tilting of Crowleys Ridge in northeastern Arkansas and southeastern Missouri, subsurface faulting along the Crittenden County fault zone near Memphis, Tennessee, faulting along the east flank of the Tiptonville dome, and numerous indicators of historic and prehistoric large earthquakes in the New Madrid seismic zone. Paleoearthquake studies in the New Madrid seismic zone have used trenching, seismic reflection, shallow coring, pedology, geomorphology, archaeology, and dendrochronology to identify and date faulting, deposits of liquefied sand, and areas of uplift and subsidence. The cause of todays relatively high rate of tectonic activity in the Mississippi embayment remains elusive. It is also not clear whether this activity rate is a short term phenomenon or has been constant over millions of years. Ongoing geodetic and geological studies should provide more insight as to the precise manner in which crustal strain is accumulating, and perhaps allow improved regional neotectonic models.

New Madrid seismic zone fault geometry

The New Madrid seismic zone of the central Mississippi River valley has been interpreted to be a right-lateral strike-slip fault zone with a left stepover restraining bend (Reelfoot reverse fault). This model is overly simplistic because New Madrid seismicity continues 30 km southeast of the stepover. In this study we have analyzed 1704 earthquake hypocenters obtained between 1995 and 2006 in three-dimensional (3-D) space to more accurately map fault geometry in the New Madrid seismic zone. Most of the earthquakes appear to align along fault planes. The faults identifi ed include the New Madrid North (29°, 72° SE), Risco (92°, 82° N), Axial (46°, 90°), Reelfoot North (167°, 30° SW), and Reelfoot South (150°, 44° SW) faults. A diffuse zone of earthquakes exists where the Axial fault divides the Reelfoot fault into the Reelfoot North and Reelfoot South faults. Regional mapping of the top of the Precambrian crystalline basement indicates that the Reelfoot North fault has an average of 500 m and the Reelfoot South fault 1200 m of down-to-the-southwest normal displacement. Since previously published seismic refl ection profi les reveal reverse displacement on top of the Paleozoic and younger strata, the Reelfoot North and South faults are herein interpreted to be inverted basement normal faults. The Reelfoot North and Reelfoot South faults differ in strike, dip, depth, and displacement, and only the Reelfoot North fault has a surface scarp (monocline). Thus, the Reelfoot fault is actually composed of two left-stepping restraining bends and two faults that together extend across the entire width of the Reelfoot rift.

Neotectonics of the southeastern Reelfoot rift zone margin, central United States, and implications for regional strain accommodation

The northeast-striking New Madrid fault system of central North America has been described as a right-lateral strike-slip system with a left-stepping restraining-bend thrust. The fault system has one of the highest rates of seismic energy release in an intraplate setting, and it has been regarded as a zone of significant earthquake hazard. The New Madrid fault system is the central part of the wider Reelfoot rift fault system, a northeast- striking basement fault zone. Although component faults of the New Madrid system are well defined by microseismicity, recent geodetic surveys suggest that little if any strain is accumulating on the principal (southern) strike-slip arm of the fault system. Seismicity and geologic data show that the “restraining-bend” thrust continues past the southern strike-slip arm to the southeastern Reelfoot rift margin. Thus, we suggest that earthquakes defining the southern arm of the New Madrid fault system are primarily aftershocks of an earthquake on that arm during the last sequence of great earthquakes (A.D. 1811– 1812), and it is the southeastern rift-margin fault system that is currently accommodating right-lateral strain along the boundary of the thrust block. This interpretation is consistent with recent geodetic results. The southeastern rift margin coincides with a 150-km-long linear topographic scarp from near Memphis to the Tennessee-Kentucky line, and S-wave reflection profiles, auger data, and a trench excavation reveal late Wisconsin–early Holocene surface faulting and late Holocene liquefaction associated with this fault-line scarp. Variation in sense of throw along strike and flower-structure geometry suggest that this is a strike-slip fault. Recognition of this rift margin as an important element of active tectonism in the Mississippi embayment has broad implications for assessment of the seismic hazard of this and similar intraplate settings. Temporal shifts in strain accommodation may give rise to short-term seismicity patterns and/or geodetic velocities that do not reveal long-term tectonic patterns.

Quaternary faulting in the southern Mississippi embayment and implications for tectonics and seismicity in an intraplate setting

The recently recognized Saline River fault zone in the southwestern Mississippi embayment (strike = 135°) is characterized by moderate historic seismicity. We document paleoseismicity (possibly strong) on the Saline River fault zone that is outside the known region of neotectonism in the northern embayment. Six surface faults were excavated within the Saline River fault zone, and all faults displace marine Eocene units and fluvial Pliocene–Pleistocene units; five show post-Wisconsin loess movement (thermoluminescence [TL] age = 23.6 ± 3 ka), and three deform middle to late Holocene eolian silt (TL age = 5.1 ± 0.6 ka and 3.6 ± 0.5 ka) as fault-tip flexures. Along the principal excavated fault, fault-plane braiding and plunging drag folding suggest a strong component of left slip, and subsidiary faults show both normal and reverse displacement (<1m) of Pliocene−Pleistocene, Wisconsin, and Holocene units. We interpret these faults as components of a left-slip flower structure. A shallow seismic reflection profile acquired across the principal fault and a subsidiary fault shows Eocene stratigraphy separated in a reverse sense below 120 m depth and separated in a normal sense above 120 m, supporting a strike-slip interpretation. In addition, the only focal plane mechanism available in the area indicates left slip with a small reverse component. We conclude that the Saline River fault zone is a Paleozoic–Mesozoic basement fault zone reactivated as a left-slip system in a Quaternary east-west compressive stress field. Field evidence suggests that earthquakes with several meters of strike slip may be characteristic, and the width of a liquefaction field on the Saline River fault zone suggests an event of magnitude 5.5–6. Considering a similarity of strike and sense of Holocene movements of the Meers fault of southern Oklahoma and of the Saline River fault zone, there may be additional active fault zones that pose seismic hazards concealed by sediments in southern North America.

Earthquake signals in tree-ring data from the New Madrid seismic zone and implications for paleoseismicity

Severe ground shaking and the formation of Reelfoot Lake during the great New Madrid earthquakes of a.d. 1811–1812 had a profound effect on baldcypress trees that still survive in Reelfoot Lake of northwestern Tennessee. Inundation greatly increased baldcypress radial growth from 1812 to 1819 and permanently decreased wood density after 1811. Ground shaking fractured the baldcypress stems that were present during the 1811–1812 event, but fractures are absent in the post-1811 growth. In contrast, the growth of old baldcypress trees in the St. Francis sunkland of northeastern Arkansas was severely suppressed for almost 50 yr following the 1811–1812 New Madrid earthquakes. Thus, there are two opposite but profound growth responses to the same earthquake events preserved in baldcypress trees of the New Madrid seismic zone. The tree-ring chronology at Reelfoot Lake extends from a.d. 1682 to 1990, but the 1812–1819 growth surge was the only extreme growth anomaly in this 309 yr period. The St. Francis sunkland chronology extends from a.d. 1321 to 1990, and the 1812–1857 growth suppression is the most severe and prolonged growth anomaly of this entire 670 year period. Thus, the tree-ring record indicates that there was not a great earthquake during the 129 yr prior to 1811 in the Reelfoot Lake basin, nor during the 490 yr prior to 1811 in the St. Francis sunkland.

Origin and age of the Manila high and associated Big Lake “sunklands” in the New Madrid seismic zone, northeastern Arkansas

Uplift of the Manila high and subsidence of the south-flowing Little River during the great New Madrid earthquakes of 1811–1812 formed the present lake in Big Lake basin of northern Arkansas. This is the most recent deformation of the 19-km-long Manila high that began between 11 000 and 5400 yr ago and is time transgressive toward the south. At least 4 m of Holocene uplift occurred on the northern portion of the Manila high prior to initiation of the Little River distributary in mid-Holocene time. The distributary is deflected around the high, and overbank sediment derived from it thins and is locally absent across the northern (highest) portion of the high. Deformation along the southern end of the high began between 3500 and 2000 yr ago. Since 3500 yr B.P., 2 m of uplift appears to have caused a reduction in gradient, anastomosing upstream of the uplift, and incision of the Little River across the uplift. North of the Manila high, in the Big Lake basin, the Little River has an increased aggradation rate ∼10 times that of the preuplift rate. Cores collected in and adjacent to Big Lake indicate that the Little River was ponded twice, apparently in response to deformation in 1811–1812 and between 90 B.C. and A.D. 1640.