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
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.