While data is available on the lithospheric and crustal structure of the Andes region of South America, there is limited knowledge of these aspects of the eastern portion of the continent. For this reason, a surface wave dispersion study of the area was initiated. Long period seismograms were obtained for a tripartite analysis of dispersion. A flow chart of the analysis to be conducted is presented along with a preliminary geologic/tectonic map that was prepared. Efforts to characterize the provinces identified in terms of their geological and geophysical parameters continue.
A preliminary crustal structure map and a bibliography of crustal and upper mantle studies for South America are given. Work on the surface wave analysis and tectonic characterizations continues.
An integrated geophysical/geologic program is being conducted to evaluate the rift complex hypothesis as an explanation for the earthquake activity in the New Madrid Seismic Zone and its extensions, to refine our knowledge of the rift complex, and to investigate the possible northern extensions of the New Madrid Fault Zone, especially its possible connection to the Anna, Ohio seismogenic region. Drillhole basement lithologies are being investigated to aid in tectonic analysis and geophysical interpretation, particularly in the Anna, Ohio area. Gravity and magnetic modeling combined with limited seismic reflection studies in southwest Indiana are interpreted as confirming speculation that an arm of the New Madrid Rift Complex extends northeasterly into Indiana. The geologic and geophysical evidence confirm that the basement lithology in the Anna, Ohio area is highly variable reflecting a complex geologic history. The data indicate that as many as three major Late Precambrian tectonic features intersect within the basement of the Anna area suggesting that the seismicity may be related to basement zones of weakness.
The Midcontinent of North America is commonly characterized as a stable cratonic area which has undergone only slow, broad vertical movements over the past several hundreds of millions of years. This tectonically stable crust is an unfertile area for hot dry rock (HDR) exploration. However, recent geophysical and geological studies provide evidence for modest contemporary tectonic activity in limited areas within the continent and, therefore, the possibility of localized thermal anomalies which may serve as sites for HDR exploration. HDR, as an energy resource in the Midcontinent, is particularly appealing because of the high population density and the demand upon conventional energy sources. Five generalized models of exploration targets for possible Midcontinent HDR sites are identified: (1) radiogenic heat sources, (2) conductivity-enhanced normal geothermal gradients, (3) residual magnetic heat, (4) sub-upper crustal sources, and (5) hydrothermal generated thermal gradients. Three potential sources of HDR, each covering approximately a 2/sup 0/ x 2/sup 0/ area, were identified and subjected to preliminary evaluation. In the Mississippi Embayment test site, lateral thermal conductivity variations and subcrustal heat sources may be involved in producing abnormally high subsurface temperatures. Studies indicate that enhanced temperatures are associated primarily with basement rift features where vertical displacement of aquifers and faults cause the upward migration of hot waters leading to anomalously high local upper crustal temperatures. The Western Nebraska test site is a potential low temperature HDR source also related, at least in part, to groundwater movement. The Southeast Michigan test site was selected for study because of the possible presence of radiogenic plutons overlain by a thickened sedimentary blanket.
