M. Dean Kleinkopf

MAGNETIC ANOMALY MAP OF NORTH AMERICA

Data presented on the magnetic anomaly map in the continental areas have largely been derived from published map sources. However, these maps for the most part have been digitized and subjected to various processing steps in preparation for compositing. The marine areas of the map consist mostly of digital data track, except in the Gulf of Mexico, Caribbean Sea and the continental shelf areas of Alaska where the data were obtained from digitized maps.

GRAVITY ANOMALY MAP OF NORTH AMERICA

The Gravity Anomaly Map of North America is the product of a 12-year international effort to compile, critically edit and merge gravity anomaly data on a continental and global scale. This color‐pixel map, printed on four quadrant sheets at a scale of 1:5 000 000 and including a fifth sheet showing a color index map with data references, is the first such map at this large scale to include several hundreds of thousands of precise surface data of the United States, Canada, Mexico and Central America as well as other high‐quality surface data from neighboring continental and oceanic areas. The map, which shows Bouguer gravity anomalies on land and free‐air gravity anomalies over oceans, is remarkable for its detail. Sixty‐six colors or shadings have been used in a carefully conceived nonlinear scheme to show anomalies at a 5 or 10 mGal interval over a dynamic range from about −300 mGal to +130 mGal.

Phanerozoic thrusting in Proterozoic belt rocks, northwestern United States

Newly identified listric thrust faults show eastward translation north of the Lewis and Clark line across all 800 km of Belt terrane from Spokane, Washington, on the west to Glacier National Park on the east. Right-lateral slippage on the line was accompanied by clockwise rotation on the thrusts. These movements were probably in response to the complex plate interactions that began about 200 m.y. ago along the western continental margin. Gravity and magnetic data suggest that basement rock is involved in the thrusting. Reconcentration of strata-bound copper sulfides along bedding-plane shears may help form ore near one thrust. West of the Rocky Mountain Trench, Phanerozoic strata that elsewhere have oil and gas potential may have been stepped up by the thrusting, and eroded, rather than extending beneath most of the thrust belt.

Audio-frequency magnetotelluric and gravity traverse across the crest of the Purcell anticlinorium, northwestern Montana

A series of audio-frequency magnetotelluric (AMT) and gravity measurements were made across the Purcell anticlinoriurn of the Belt basin in Montana. The purpose of the study was to gain information about the electrical properties of the Belt Supergroup strata before using techniques that probe deeper but are more expensive. The Bouguer gravity data indicate an anomaly of about 20 mgal that can be modelled as a block uplifted 11 km above a flat basement surface that lies at a depth of 17 km. The AMT measurements did not penetrate to the crystalline basement, but they did delineate several intermediate structural features, including a deep layer of unusually high conductivity. Both the uplifted block and the conductive unit are positioned beneath the crest of the anticlinoriurn, approximately beneath the outcrop of the Prichard Formation.

GEOPHYSICAL STUDIES OF THE CRIPPLE CREEK MINING DISTRICT, COLORADO

Integrated geophysical, geochemical, and geological interpretations expand the knowledge about the localization of the ore deposits in the Cripple Creek district, Colorado. The principal gold deposits occur in a Tertiary volcanic subsidence basin within Precambrian granite, gneiss, and schist. The basin is filled with volcanic breccia and is intruded by dikes and irregular masses of phonolite, latite‐phonolite, syenite, trachydolerite, and basalt. The volcanic complex gives rise to a broad 10 mgal gravity minimum anomaly upon which are superimposed local minima believed to be related to deep mineralized fissure zones. A negative magnetic anomaly over the volcanic subsidence basin probably reflects the degree of alteration of rocks in the subsurface. Two local closed magnetic lows may represent highly altered volcanic centers in the bottom of the basin. The gravity and magnetic anomalies of the basin correlate geographically with positive geochemical anomalies for gold, silver, and tellurium. Just east of ...

Geophysical evidence for a volcanic subsidence feature near Silver Cliff, Colorado

A prominent negative gravity and magnetic anomaly across outcrops of volcanic tuff coupled with geologic evidence for collapse and hydrothermal alteration suggest a cauldron subsidence feature on the edge of the Silver Cliff, Colorado, volcanic area.

Principal facts for gravity stations of the Broadwater geothermal area, Montana

Two complete Bouguer anomaly values were calculated for each of the 67 stations assuming average rock densities of 2.67 g/cm/sup 3/ and 2.45 g/cm/sup 3/. The corrections and anomaly values are listed. A hand contoured Bouguer gravity map is included.

EXPLORATION GEOPHYSICS IN THE U.S.S.R.

Five American exploration geophysicists visited the USSR for a month during the late summer of 1971 as members of an official U.S.-USSR exchange program. The group was hosted by the USSR Ministry of Geology and saw 15 technical activities in six cities: Moscow, Leningrad, Tyumen, Kiev, Krasnodar, and Baku. During their stay, the Ministry of Geology assisted the American delegation in participating in the Proceedings of the 15th General Assembly of the International Union of Geodesy and Geophysics held at Moscow State University.