No abstract

Gravity fields in the continental interior reveal little relation to late Paleozoic structure and often show an inverse relation to early Paleozoic structure. Because of the inversion, displacements have been interpreted by several investigators as being caused by an isostatic mechanism; however, structural development recorded by geologic evidence indicates a more complex history than simple isostasy would allow. Detailed gravity surveys covering large areas in Illinois are used to determine mechanisms of crustal movements more definitively. From these detailed studies interpretations are extended to other areas having similar geologic‐gravity relationships within the midcontinent region. In the interior, a mosaic pattern of gravity highs and lows outlines mass concentrations that formed during the evolution of the craton. The midcontinent gravity high, the central anomaly lying within a linear, symmetrical belt of gravity anomalies, is superimposed over the mosaic pattern and trends through the Lake Superior‐Keweenawan rift system. Similar anomalies, although of smaller intensity, trend through centers of intracratonic basins. From the relation between linear, symmetrical anomalies and structure displayed by the Lake Superior rift system, intracratonic basins are interpreted as representing the terminal stage of more subdued rifting. Initial depression of basins is believed to be caused by the collapse of incipient rift systems. The presence of numerous ancient rift systems in the craton suggests that mantle convection played an important part in the deformation of the youthful continental crust. Erosion from uplifts and sedimentation in depressions resulted in further deflections, but of an isostatic nature, with subsequent faulting and plutonism. Subcrustal convection continued throughout Paleozoic time, but its effects on crustal bending became less pronounced. Examples of these structures and related gravity fields are presented.

Electrical resistivity field measurements and subsequent laboratory analyses of the electrical properties of soils and rocks, both frozen and unfrozen, are used to describe the type and configuration of permafrost in Wright and Taylor Valleys, Antarctica. Rock and soil samples saturated with potable water have resistivities that increase four orders of magnitude when cooled from +21°C to −25°C. Resistivities greater than 10,000 ohm-m are associated with an impermeable variety of permafrost referred to in this paper as confining permafrost. Confining permafrost is continuous throughout the ice-free valleys except near McMurdo Sound and under saline lakes. A hydrologic connection between lakes and the groundwater reservoir beneath the lakes is inferred; thus, part of the brines contained in the lakes may be derived from groundwater.

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