The calculation of bulk strain in oblique and inclined balanced sections

The calculation of bulk strain from balanced sections is a technique commonly used in the study of orogenic belts. The validity of a balanced section is often enhanced by controls such as seismic and/or borehole data. The construction of a conventional vertical section perpendicular to orogenic strike (i.e. in the transport direction) may not allow control data to be directly incorporated into the section. The control data can be projected into the plane of the section or, alternatively, the section may be constructed in an orientation such that it contains the available control data. In the case of a vertical section constructed at an oblique angle to the transport direction the bulk strain value obtained is a minimum and can be corrected to the true value of bulk strain in the perpendicular, vertical section by a simple trigonometric calculation. If the section is perpendicular to strike but inclined at an angle to the vertical, the correction of the bulk strain values is mathematically unique to the section. This problem can be overcome by using a digitizing table in conjunction with a simple computer program. The general case of a section both oblique to the strike direction and inclined to the vertical can be resolved by removing the effects of the inclination to produce a vertical, oblique section from which the bulk strain can be obtained and corrected. In realistic geological problems, the oblique section will be a more common problem than the inclined section. In both cases deviations of 10° and less introduce such small errors in the estimates of bulk strain that their effects can be ignored.