Application of the Paleomagnetic Fold Test to Determine the Relative Timing of Sill Intrusion and Deformation in the Southwest Helena Salient, Montana
Date of Award
Master of Science
Geological and Environmental Sciences
Dr. Christopher J. Schmidt
Dr. Ronald B. Chase
Dr. Estella Atekwana
Masters Thesis-Open Access
The Doherty Mountain fold complex is a highly folded and faulted region along the southern boundary of the Helena Salient in southwestern Montana. This region has a complex deformation history that is a key to deformation within the Helena Salient as a whole. A structural interpretation of the deformed rocks of this area was conducted using field mapping and cross section construction at 1:24,000 scale providing insight to the internal deformation processes involved. Fourteen folds and three major faults were mapped and analyzed showing the influence of the Southwest Montana Transverse Zone and late stage detachment faulting on fold orientation. Restored cross sections of the detailed map show more shortening than previously estimated for this area. Sills intruded into this complexly folded regions were also studied. In an attempt to determine relative timing of sill emplacement and deformation, the paleomagnetic fold test of McElhinny (1963) was applied to sills which pervasively intrude the sedimentary rocks. The results indicate intrusion occured before or early in the deformation. Combined with an 40Ar/39Ar radiometric age date of 77 Ma, this work provides the first estimate of minimum age of deformation in this area.
Whisner, Stephen Christopher, "Application of the Paleomagnetic Fold Test to Determine the Relative Timing of Sill Intrusion and Deformation in the Southwest Helena Salient, Montana" (1998). Masters Theses. 4424.