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Kansas Geological Survey, Open-file Report 88-25


The fracture pattern of north-central Kansas and its relation to hydrogen soil gas anomalies over the Midcontinent Rift System

by
S. K. Johnsgard

KGS Open File Report 8-25
July 1988


Abstract

Lineament analysis of digitally filtered gravity and aeromagnetic data for that portion of north-central Kansas bounded by 38.5° N. and 40.0° N. latitude and 96.0° W. and 98.0° W. longitude suggests the basement is characterized by a bimodal set of linear compositional discontinuities. These discontinuities are interpreted to reflect an intrabasement fracture pattern with modal orientations of about N. 40-50° W. and N. 10-30° E. These preferred orientations are also expressed as linear elements in a revised contour map of the Precambrian surface configuration, indicating Phanerozoic movement has taken place along many of them. The northwest-oriented fracture set parallels regional basement fault zones associated with the Middle Proterozoic(?) Missouri gravity low and may have resulted from the tensional(?) stresses responsible for formation of that feature. The north-northeast-oriented set parallels the central axis of the Midcontinent rift system and probably reflects extensive tensional faulting inferred to have accompanied the Middle Proterozoic (Keweenawan) rifting. Alternatively, both sets could reflect even older upper-crustal anisotropy. Clustering of historic earthquake and recent microquake epicenters around several of these fracture zones demonstrates that some are still active. Such recurrent seismicity, perhaps in combination with differential isostatic rebound since Pleistocene glaciation, could provide a mechanism for upward propagation of these zones.

Although surface faults are virtually unknown from the study area, correlation between the locations and modal orientations of the inferred basement fractures and both Landsat photolineaments, and linear drainage segments, is good to fair. This may imply that many of these fractures are expressed at the present land surface as zones of preferential weathering, perhaps in the form of linear zones of increased joint density.

Comparison between the magnitudes of nearly 600 hydrogen soil gas measurements and their relative proximity to these suspected fractures indicates that at least some of these zones may extend to the present land surface and act as preferential conduits for vertical migration of free hydrogen gas or gas-charged fluids. However, it is concluded that the present soil-gas sample spacing (0.5 to 1.0 mi.) is too coarse to accurately define the exact surface traces of these fractures. The viability of future exploitation of the hydrogen is still unknown, but will probably depend on more precise definition of these zones, the presence of a suitable trapping mechanism, and drilling and completion practices appropriate for the occurrence.

The complete report is available as an Adobe Acrobat PDF file.

ofr88-25.pdf (1.2 MB)

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