Eugene S. Schweig

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.

Bootheel lineament: A possible coseismic fault of the great New Madrid earthquakes

A remote sensing examination of the New Madrid seismic zone has revealed a feature, the Bootheel lineament, that may be the surface expression of one of the coseismic faults of the great New Madrid earthquakes of 1811 and 1812. The lineament extends about 135 km in a north-northeast direction through northeastern Arkansas and southeastern Missouri. The morphology and pattern of the lineament suggest that it reflects a fault with strike-slip displacement. Field data indicate that liquefied sand was injected along the lineament, probably in 1811 and 1812. The Bootheel lineament does not coincide with any of the major arms of New Madrid seismicity, possibly indicating that the current seismicity does not precisely reflect the faults that ruptured in 1811 and 1812.

Subsurface structure in the vicinity of an intraplate earthquake swarm, central Arkansas

Abstract Over 40,000 events have been recorded in the Arkansas earthquake swarm since its inception in 1982. The earthquakes occur at depths between 3 and 6 km and cluster in a volume of about 25 km 3 beneath the easternmost Arkoma basin, near the town of Enola, Arkansas. A study of proprietary reflection seismic lines reveals that the earthquakes cluster within a graben formed in Mississippian time. This graben is part of a system of steeply dipping normal faults that trends ENE across the region. The regional strikes of the basement faults are not favorably oriented for activation under the regional stress regime. In the swarm area, however, these faults bend to form a 2.5-km long segment trending WNW. The small WNW striking segments are well oriented for left-lateral strike slip and focal mechanisms are consistent with this sense of slip. Additionally, a subset of the most accurately located earthquakes do appear to lie along a WNW trend. The length of the WNW trending fault segments is sufficient to have generated the largest of the swarm events. The reflection data reveal a loss of coherent reflectors within the swarm hypocentral volume. Reflectors above the graben have been uplifted about 30 m in post-Atokan (Pennsylvanian) time. Third order leveling surveys show 14.3 cm of uplift between 1961 and 1986 at a benchmark over the graben relative to a benchmark outside of the graben.

Subsurface structure of the eastern Arkoma Basin

Analysis of 425 km of seismic reflection data in the eastern Arkoma basin reveals a structure and history quite different from those previously reported for the Arkoma basin. The eastern Arkoma basin has three structural styles that formed in the following chronological order: deep, steep normal faults; shallow listric normal faults; and thrust faults. The origin of the structural styles and the Paleozoic history of the eastern Arkoma basin may be summarized as follows. Late Proterozoic or Early Cambrian rifting was followed by deposition of Cambrian through Upper Mississippian strata on a passive plate margin. The Mississippian-Pennsylvanian boundary marks a time of major down-to-the-south normal faulting with coincident folding of the footwall blocks and truncation of t e faulted and folded terrain by a pre-Morrowan (Mississippian-Pennsylvanian) unconformity. The unconformity slopes southward and steepens locally across the erosionally truncated footwall blocks. During the Pennsylvanian, down-to-the-south listric normal faults cut the Atokan and Morrowan sections and merged with, but did not displace, the steeper segments of the sub-Morrowan unconformity. Surface folds north of the Ross Creek thrust are rollover anticlines overlying these listric normal faults. Major deformation in the eastern Arkoma basin terminated with emplacement of the Ross Creek thrust in the Late Pennsylvanian.

Basin-range tectonics in the Darwin Plateau, southwestern Great Basin, California

The Darwin Plateau, in the southwestern Great Basin of California, is underlain by upper Cenozoic basalt flows, pyroclastic rocks, and alluvial deposits. Many of the volcanic units are laterally extensive and can be correlated across the study area, using hand-sample characteristics and magneto-stratigraphy. The age of the section, from potassium-argon dating and magnetic polarities, is largely 5.3 to 5.7 m.y., although the oldest basalt flow is 7-8 m.y. old. Structural, radiometric, and sedimentologic data all suggest that basin-range crustal extension was underway locally by 7-8 Ma, and the earliest high-angle normal faulting predates 5.8 Ma. This chronology is consistent with westward migration of tectonism in the southwestern Great Basin. Additionally, structural data suggest two extension directions, the current west-northwest-east-southeast direction and an earlier west-southwest-east-northeast one. The change in extension direction occurred after 5.7 Ma. Local and regional data also indicate that the maximum compressive and intermediate stresses in the latest stress regime have been approximately equal in magnitude. Paleomagnetic data do not indicate significant rotation of the Darwin Plateau between the many large strike-slip faults in the area.

Liquefaction induced by historic and prehistoric earthquakes in western Puerto Rico

Dozens of liquefaction features in western Puerto Rico probably formed during at least three large earthquakes since A.D. 1300. Many of the features formed during the 1918 moment magnitude (M) 7.3 event and the 1670 event, which may have been as large as M 7 and centered in the Anasco River Valley. Liquefaction features along Rio Culebrinas, and possibly a few along Rio Grande de Anasco, appear to have formed ca. A.D. 1300–1508 as the result of a M ≥ 6.5 earthquake. We conducted reconnaissance along Rio Culebrinas, Rio Grande de Anasco, and Rio Guanajibo, where we found and studied numerous liquefaction features, dated organic samples occurring in association with liquefaction features, and performed liquefaction potential analysis with geotechnical data previously collected along the three rivers. Our ongoing study will provide additional information regarding the age and size distribution of liquefaction features along the western, northern, and eastern coasts and will help to improve estimates of the timing, source areas, and magnitudes of earthquakes that struck Puerto Rico during the late Holocene.

Extent and Character of Soil Liquefaction during the 1811‐12 New Madrid Earthquakes

The great 181 1-12 New Madrid earthquakes produced extensive liquefaction which is still very much in evidence today. Visible as a myriad of light-colored and often irregular shapes against the dark brown soils of the Mississippi embayment, sands liquefied and extruded during the 181 1-12 earthquakes are still readily recognized both in the field and on aerial photographs. No systematic studies of the geological effects of the earthquake were undertaken immediately after the earthquakes. Consequently, the extent, magnitude, and style of liquefaction produced by the earthquakes can be assessed from compilations of graphic accounts of contemporaries who witnessed the events and from later studies of evidence registered in the geologic record. Our intent here is not to duplicate those efforts. Rather, we will limit ourselves to a brief synopsis of work bearing on liquefaction which took place during 181 1-12 as a basis for discussing the potential that still exists to advance our understanding of liquefaction processes and seismic hazard in the Mississippi embayment through further study of the geologic record.