Abstract

A comparison is made between the use of fracture stations and the subjective selection of prominent fracture orientations in the course of regional mapping as a means of determining the regional fracture pattern in the Precambrian rocks of the Spanish Peaks area of the Madison Range, Montana. Maxima at fracture stations are summarized and compared with summary plots of all fractures measured at the stations and with samples of subjectively selected fracture orientations. Most of the maxima on the various types of plots correspond closely with one of four orientations. All four are nearly vertical (dips of 75-90°) and have trends of N50-65°W, N15-25°W, N45-55°E, and N65-75°E. A fifth direction of N30-35°E appears primarily in the sample collected in the southern part of the area. More maxima appear on the synoptic diagrams of concentrations identified at fracture stations than in the summary plots of the subjective sample. Concentrations representing low-dipping orientations appear on those diagrams of objectively determined orientations, but do not appear in summaries of subjectively selected samples.