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

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). © 2002 Elsevier Science Ltd. All rights reserved

Quaternary uplift in the lower mississippi river valley

Discontinuous high-level terrace remnants of the ∼3.1 Ma ancestral Mississippi River floodplain, locally called the Upland Complex (UC), are mapped from Louisiana into Illinois. We interpret the UC to be the basal sand and gravel erosional remnant of a much thicker Pliocene floodplain. The Pliocene Mississippi River, at the latitude of Tennessee, has a base-of-terrace elevation 70 m higher than the base of the Holocene Mississippi River floodplain. This difference cannot be attributed solely to changes in sea level and suggests Quaternary uplift that may be ongoing. Borehole data reveal that the base of the UC and that of the Mississippi River alluvium have been tilted southeast. The geomorphology of the central Mississippi River Valley also supports regional Quaternary uplift. Mississippi and Ohio/Mississippi river terrace distribution reveals that these rivers shifted away from the down-valley axis during the Wisconsinan, and basin asymmetry analysis also indicates Quaternary tributary valley migrations away from this axis. Pliocene sea level is estimated to have been at +25 m, requiring that the UC has risen 45 (70-25) m within the past ∼2.4 My. We believe this 45-m uplift may be isostatic. However, 15 m of overlying Pleistocene loess deposition would result in 8 m of isostatic subsidence, suggesting an original isostatic uplift of 53 (45 + 8) m. A 53-m isostatic uplift response indicates that 141 m of the UC was eroded. Since the current average thickness of the UC is 10 m, its original thickness was 151 m. This proposed isostatic response to Pleistocene erosion of the Mississippi River Valley supports the denudation model for Quaternary reactivation of the underlying Reelfoot rift faults and its New Madrid seismic zone and suggests that uplift mechanisms should be considered in the denudation model

Possible relict meanders of the Pliocene Mississippi River and their implications

Although the late Quaternary history of the Mississippi River, the largest North American river, has been intensely studied, its Pliocene history is largely unknown. We assert that large relict meander bends (oxbows) of the Pliocene Mississippi River are preserved as arcuate valleys of local streams in the northwest area of the state of Mississippi. These arcuate valleys are within the Pliocene Upland Complex stratigraphic unit that occurs as a high-level alluvial terrace along the lower Mississippi River Valley. Outside these arcuate valleys, the Upland Complex is a braidedriver gravel deposit that shows southward (downvalley) paleoflow directions. Interiors of the arcuate valleys are occupied by Upland Complex point bar deposits with large-scale cross-bedding showing a range of paleoflow directions, commonly upvalley flow indicative of a meandering regime. We used measurements of the radii of the putative relict meander bends and of their paleochannel widths to estimate paleodischarge of the Pliocene Mississippi River based on equations empirically derived from modern meandering rivers. These discharge estimates, the first for the Pliocene Mississippi River, show it may have had as much as six to eight times the discharge of the modern river. This result suggests that (1) the North American Pliocene climate was much wetter, and/or (2) the Pliocene Mississippi River’s drainage basin was much larger, extending into southern Canada. Better age constraints and a detrital zircon provenance study of the Upland Complex are needed to test these hypotheses

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 hight-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

Petrology of pliocene Mississippi river alluvium: Provenance implications

Pliocene Mississippi River terrace gravels, the Upland Complex, crop out east and west of the present Mississippi River in the northern Mississippi Embayment. As the only sedimentary unit in the northern Mississippi Embayment deposited between the end of the Eocene and the onset of glaciation, its origin provides ground truth about conditions that existed in the heartland of North America within this 30-m.yr. interval. Recent studies concluded that the Pliocene Mississippi River originated in what is now southern Canada and that the Upland Complex is the remnant of a much larger deposit that once extended from Canada to the Gulf of Mexico. Is there some evidence in the petrology of the Upland Complex that can confirm this conclusion? To determine the provenance of the Upland Complex, we sampled 18 exposures, analyzed ∼50 thin sections, and obtained source terrane age data from its zircons. Upland Complex gravel came from proximal early Paleozoic carbonate rocks containing bedded and nodular chert north and northeast of the Mississippi Embayment with a possible distant contribution from as far away as southcentral Canada. Upland Complex sand came from multiple sources, including the Saint Francois Mountains of Missouri, the Grenville Terrane, and, possibly, southcentral Canada. We conclude that the Pliocene Mississippi River probably drained a much larger area than present Mississippi, an area that extended well north of the US-Canada border into Manitoba and Ontario

A matrix-variate dirichlet process to model earthquake hypocentre temporal patterns

Earthquakes are one of the deadliest natural disasters. Our study focuses on detecting temporal patterns of earthquakes occurring along intraplate faults in the New Madrid seismic zone (NMSZ) within the middle of the United States from 1996–2016. Based on the magnitude and location of each earthquake, we developed a Bayesian clustering method to group hypocentres such that each group shared the same temporal pattern of occurrence. We constructed a matrix-variate Dirichlet process prior to describe temporal trends in the space and to detect regions showing similar temporal patterns. Simulations were conducted to assess accuracy and performance of the proposed method and to compare to other commonly used clustering methods such as Kmean, Kmedian and partition-around-medoids. We applied the method to NMSZ data to identify clusters of temporal patterns, which represent areas of stress that are potentially migrating over time. This information can then be used to assist in the prediction of future earthquakes

Paleoseismology of the southeastern Reelfoot Rift in western Tennessee and implications for intraplate fault zone evolution

The southeastern Reelfoot Rift margin is expressed as a series of collinear scarps across western Tennessee, and shallow S wave reflection profiles, coring, and trenches show late Quaternary faulting along this lineament. At the north end, our seismic profile and coring suggest ≥1.5 m of Wisconsin down-to-the-NW faulting. Coring and trenches across the central lineament show ≥3 m of up-to-the-NW and 8-15 m of right-lateral Late Wisconsin/Holocene faulting. Averaged late Quaternary right-lateral slip has been 0.85-0.37 mm/yr on one fault plane at this locality, suggesting that total slip across all southeast rift margin fault planes may accommodate much of the regional strain. Our seismic profile at this site shows up-to-the-west faulting of Eocene strata folded down-to-the-west, indicating structural inversion. Seismic profiles, electrical surveys, coring, and a trench across the southern lineament segment show Holocene faulting. Trenching exposed a vertical fault with ∼0.5 m of late Holocene down-to-the-NW displacement. Nearby, coring and electrical surveys show ∼0.5 m of late Holocene up-to-the-NW faulting. We conclude the southeastern Reelfoot Rift margin is a right-lateral system with high-angle faults showing both up-to-the-NW and down-to-the-NW separations. Age constraints permit an earthquake circa 2500 to 2000 years B.P. on the southwestern/central segment of this fault system that ruptured ≥80 km. We suggest the northeastern segment of the southeastern rift margin turned off in Holocene when the Reelfoot step over thrust turned on. Copyright 2006 by the American Geophysical Union

Quaternary displacement on the joiner ridge fault, eastern Arkansas

The New Madrid seismic zone of the central United States is an intraplate seismic zone with blind structures that are not seismically active but may pose seismic hazards. The Joiner ridge fault (JRF) is the 35-kilometer-long east-bounding fault of the Joiner ridge blind horst located in eastern Arkansas ∼50 km northwest of Memphis, Tennessee. Shallow S-wave (SH-mode) seismic reflection profiles, continuous cores, and radiometric dating of Quaternary alluvium across the JRF reveal down-to-the-east reverse faulting and folding of Eocene strata and overlying Quaternary Mississippi River alluvium. The base of the Quaternary alluvium has an age of 20.3 ka and is vertically displaced 12 m, resulting in an average slip rate of 0:6 ± 0:1 mm=yr over the past 20.3 ka. The overlying upper Wisconsinan and Holocene alluvial facies are also displaced by the JRF. These facies increase in thickness across the JRF and were used to calculate late Wisconsinan and Holocene slip histories. The JRF slipped 7 m between 20.3 and 17.5 ka, 3 m between 12.3 and 11.5 ka, and 2 m between 11.5 and 8.9 ka. No apparent slip occurred on the JRF within the last 8.9 ka. This research illustrates that slip has been intermittent and that slip magnitudes on the JRF diminished through the late Wisconsinan and early Holocene