NORMAL FAULTS AND ROTATING BLOCKS IN TRANSTENSIONAL BASINS

Many analyses of the geometrical features of basins influenced by strike slip have been based on, and limited by, hypothetical orientations of stress axes, and infinitesimal strains. Such models neglect the effects of incremental rotations in progressive strain. In oblique rifts and pull-apart basins, intra-basinal normal faults are common. As deformation proceeds, the heaves of extensional faults provide a means of quantifying this rotation.

For ideal strike-slip (simple shear), the apparent stretch due to fault heaves can be related simply to shear strain. At angles of shear above 45°, previously formed extensional faults begin to show inversion as reverse faults, becoming fully inverted when the angle of shear reaches 90°.

In transtensional basins the angle of divergence (alpha) represents the relative importance of shear and extension. Two methods are suggested for determining alpha: shortening can be estimated from bed-lengths across folds and/or by using strain markers, and compared with the extension represented by normal faults. Alternatively, it is possible to use the initial orientations of normal faults as an indication of the incremental extension direction, and thus estimate alpha. Once a value of alpha is obtained, a relationship between fault heaves and angle of shear can be derived.

In the Stellarton basin of Nova Scotia, Canada, fault heaves and orientations can be measured from subsurface mine plans, which provide unparalleled 3-dimensional control on basin geometry. Bed-lengths can be used to quantify shortening due to folding. Measurements of these quantities indicate that strain was only mildly transtensional, with a small (<10°) angle of divergence, and an angle of shear of about 13°.

The relationship between the degree of shearing and fault heave can assist the development of basin models wherever strike-slip or transtensional basins have been explored in detail by seismic or other subsurface methods.