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.