Kansas Geological Survey, Open-file Report 1987-35
by Don W. Steeples
Greg M. Hildebrand
Brett C. Bennett
Richard D. Miller
Young-Jun Chung
Ralph W. Knapp
Kansas Geological Survey
KGS Open-file Report 1987-35
October 1987
At least 30 felt earthquakes with epicenters in Kansas have been documented since 1867 (DuBois and Wilson, 1978). For comparison, historical earthquakes are shown in Figure 1 and microearthquakes from 10 years' recording are shown in Figure 2. The most serious of the historical earthquakes were Modified Mercalli Intensity VII events that occurred in 1867 and 1906 in the vicinity of Manhattan. Although the geologic structure or structures responsible for these two earthquakes have not been positively identified, the following paragraphs outline the tectonic setting and provide some insight to the probable geologic structure or structures involved.
The proximity of several MM Intensity VII/VIII earthquake epicenters in Kansas, Nebraska, and Oklahoma to the Nemaha Ridge (a buried Precambrian granitic uplift) or the Humboldt Fault (the eastern boundary of the Nemaha Ridge, Figure 3) led early investigators (Lugn, 1935; Lee, 1954) to ascribe the source of seismic activity to movement on these structures. This view is also reflected in the Seismic Risk Map of the United States (Algermissen, 1969) which shows a zone two (moderate damage expected) designation in the vicinity of the Nemaha Ridge.
Docekal (1970) analyzed the isoseismal patterns of Intensity VII/VIII historical earthquakes in the midcontinent and related them to basement configuration, structure, and lithology. From this, he delineated the Midcontinent Seismic Trend, with earthquake occurrence in central Texas; central Oklahoma; northeast Kansas; southeast Nebraska; southeast Minnesota; and the Keweenaw Peninsula, Michigan. He concluded that the stronger earthquakes of the region were genetically related to the Arbuckle, Nemahal/Humboldt, and Keweenawan Mafic Belt (MGA) structures or combinations of them.
More recently, the Midcontinent Geophysical Anomaly (MGA) has been recognized as representing an important structural feature in the Central Stable Region (Synder, 1973). The MGA (Figure 4) extends from the LakeSuperior region southwestward at least to central Kansas and into Oklahoma (King and Zietz, 1971; Chase and Gilmer, 1973).
Yarger (1981) has shown without doubt that the MGA extends southward beyond the Kansas-Oklahoma border. Data presented on page 8532 of Guinness et al. (1982) suggest that the MGA extends as far south as near 32.5N, 99.6W, near Abilene, Texas. The MGA is the largest positive gravity anomaly in North America with a length of more than 1,000 km, a width of 50 to 100 km, and a maximum peak-to-peak amplitude of160 mgal. It marks a thick sequence of mafic igneous rocks emplaced along a zone of major late Precambrian rifting (Ocola and Meyer, 1973). The structure is bounded by faults at the surface in the Lake-Superior region, and similar bounding faults are inferred from geophysical data in the area to the south where the structure is deeply buried below younger sediments (King and Zietz, 1971).
An offset of 50 to 60 km to the northwest in the Nebraska section of the MGA (near the Kansas-Nebraska border) is interpreted by Chase and Gilmer (1973) as a transform fault of the Precambrian rifting. F.W. Wilson (oral communication) has noted that epicenters of all Intensity-VII earthquakes in Kansas, Nebraska, Iowa, and Minnesota lie very close to the transform faults hypothesized by Chase and Gilmer (1973).
There are surface structures associated with the MGA in the Manhattan, Kansas, area. The Abilene Anticline (Jewett, 1941) parallels its southeast flank and the Riley County kimberlite intrusives (Brookings, 1970) lie along the same structural trend. Emplacement of the kimberlites has been associated with right lateral strike-slip movement on a buried fault at the east flank of the Abilene Anticline (Chelikowsky, 1972). The direction of movement is inferred from rotation of rock joints in the area of the possible fault. The relation of the kimberlites to this strike-slip hypothesis is in doubt because the long axis of the intrusions is oriented northwest-southeast, perpendicular to the Abilene Anticline (Cook, 1955). This indicates that the direction of least horizontal compressive stress was perpendicular to the Abilene Anticline at the time the kimberlites were emplaced, a condition incompatible with strike slip parallel to the Abilene Anticline. The Elk Creek, Nebraska, carbonatite (Brookins et al., 1975) also lies along the southeast flank of the MGA not far from the Kansas-Nebraska border. It appears from aeromagnetic evidence (Yarger, 1981) all of these ultramafic intrusions are controlled or at least influenced by the faults bounding the MGA.
Recently, deep seismic reflection data have been gathered by the Consortium for Continental Reflection Profiling (COCORP) across the MGA in Kansas. These data ultimately will go far in assisting in regional interpretation of the relationship of the MGA to the Nemaha Ridge (Serpa et al., 1984) and in constructing a geologic cross-section of the MGA itself. Data from the eastern section of the COCORP line show the relatively flat-lying sediments in the Forest City basin and complex structures within the Precambrian basement at depths of 10 to 20 km (Brown et al., 1983). The deep Precambrian structures become more shallow in the vicinity of the MGA and the Nemaha Ridge, suggesting 2 to 3 km of uplift since the mid-Precambrian. The age of the uplift may well be Keweenawan, associated with the rifting.
The COCORP data from the MGA vicinity (Serpa et al., 1984) indicate that the Rice Formation reaches a thickness of as much as 3 to 4 km along the flanks of the MGA. The basalt flows within the MGA itself reach a thickness of about 8 km, also. Preliminary interpretation of these latest COCORP data suggest that he Humboldt Fault zone dips about 20-30 to the east and that the Humboldt may be a reactivation of late Precambrian aged faults associated with the formation of the MGA. Faults with similar dips are present on both sides of the central portion of the MGA.
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