Geophysical Interpretations of the Libby Thrust Belt, NW Montana U.S.G.S. Professional Paper 1546


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Regional geophysical studies conducted by the U.S. Geological Survey in the northern Rocky Mountains during the past 25 years provide new insights about the geologic framework and mineral resources of the region. In this report, the emphasis is on interpretation of geophysical data compiled for the Libby thrust belt in northwestern Montana (fig. 1). Interpretations complement the results of geologic mapping described by Harrison and Cressman (1993), who also discussed the geology and structural framework of the Libby thrust belt. During the course of their studies, Harrison and Cressman constructed geologic cross sections across the Libby thrust belt on the basis of measured sections and projections of surface geology. Depth cutoff of these sections was 4.3 km (14,000 ft) below sea level.

In the study described herein modeling of gravity anomaly data was done along two of the geologic cross sections to provide information about structure and lithology of the Purcell anticlinorium and Sylvanite anticline from depths greater than 4.3 km to at least as deep as Precambrian crystalline basement in the middle part of the crust. No magnetic modeling was done because of the absence of magnetic anomalies having sources likely in deeply buried crystalline basement.

Included in this report are sections on interpretation of seismic and magnetotelluric data. J.E. Harrison describes interpretations of data from about 70 km of COCORP (The Consortium for Continental Reflection Profiling) seismic reflection surveying along two profiles that cross the Sylvanite anticline and Purcell anticlinorium. W.D. Stanley describes analysis of five magnetotelluric soundings along a profile that extends across the Libby thrust belt and the Purcell anticlinorium. The lines of section of the gravity models and the COCORP profiles and the locations of the magnetotelluric soundings are shown in figure 2. Data from 13 audiomagnetotelluric soundings in two profiles on the Sylvanite anticline provide information about resistivities of rocks in the upper few kilometers of the crust (Long, 1988).

The geology shown on plate 1 is a simplified version of the 1:125,000-scale geologic map compiled by Harrison and Cressman (1993). The western parts of geologic maps for the Wallace and Kalispell 1°X2°quadrangles (Harrison and others, 1986, 1992) provide the regional geologic context of the Libby thrust belt.

Magnetic and gravity anomaly data are also shown on plate 1. To provide a broader perspective of the geophysical setting, magnetic and gravity anomaly maps (pl. 2) were compiled at a scale of 1:500,000 for the Libby thrust belt and adjacent areas, extending from near lat 47°20 ' N. to lat 49°00 '; N. and from near long 114°45 '; W. to long 116°15 ';W.

Previous studies of gravity and magnetic anomaly data applied to geologic framework and mineral resource investigations of the region are described in King and others (1970), Harrison and others (1972, 1980, 1985), Kleinkopf and others (1972, 1982, 1988), Wynn and others (1977), Kleinkopf (1977, 1981, 1983, 1984), Kleinkopf and Long (1979), Kleinkopf and Wilson (1981), Kleinkopf and Bankey (1982), Kleinkopf and Harrison (1982), Fountain and McDonough (1984), and Harris (1985).

The author thanks the many U.S. Geological Survey colleagues who contributed to this project. Jack Harrison offered many constructive suggestions and provided many stimulating discussions during various phases of the project and report preparation. The sections on seismic and magnetotelluric investigations by Jack Harrison and Dal Stanley provide substantive contributions to the conclusions of this paper. The geology was digitized and compiled under supervision of Stanton H. Moll; the geologic map of plate 1 was completed in final digital form by Nancy Shock. Viki Bankey and Mike Brickey collected gravity data in the field. Viki Bankey compiled and edited the gravity and magnetic data and prepared early versions of the gravity and magnetic anomaly maps, and Gerda Abrams prepared later versions of the gravity and magnetic anomaly maps used in this report. Several colleagues offered valuable constructive comments during the technical review process.

USGS Professional Papers

For more information about this report, contact M. Dean Kleinkopf logo