Proterozoic Structures in North-Central North Dakota: a Gravity Study

Kathryn C. Luther

The purpose of this study was to determine the structure of the contact between the Superior craton and the Trans-Hudson orogenic belt in north-central North Dakota, between lats. 48° and 49° N and longs. 100° and 102° W, using gravity modeling. Major structural and geochronologic boundaries beneath the eastern flank of the Williston basin in North Dakota and southern Canada appear as coherent patterns on a Bouguer gravity map.

The contribution of the Phanerozoic overburden (density assumed to be 2.48 g cm^-3) to the gravity signal was removed, leaving residual gravity anomalies due to density variations in the Precambrian basement. Four east-west gravity profiles were constructed and modeled using a two-dimensional polygonal method. A fifth profile was constructed and modeled, after gravity modeling of the first four indicated that the Thompson fault strikes approximately N13°E.

Forward gravity modeling of the four east-west profiles suggests that the north-south-striking contact between the Superior province and the Trans-Hudson orogenic belt dips to the west, is slightly concave eastward, and steepens to the south within the study area. Modeling also suggests that the Thompson fault dips west, lies about 45-55 km west of the Superior-Trans-Hudson contact at or near the basement surface, strikes about N13°E, and extends deep into the crust. The fifth profile, constructed and modeled perpendicular to this structure, confirms this orientation. A large circular positive gravity and magnetic anomaly, crossed by profile D, may represent a mafic pluton or a body of some other origin.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988