Bootstrapped Total Least Squares orocline test: A robust method to quantify vertical axis rotation patterns in orogens, with examples from the Cantabrian and Aegean oroclines.
Most mountain belts on Earth show some degree of curvature in plan-view: from a slight bend to horse-shoe shapes. Such curvatures may occur on different scales, from individual thrust sheets to whole plate boundaries. Curvature may be acquired by vertical axis rotation during or after orogenesis, or reflect primary lateral variations in shortening directions or physiographical features. Quantifying the amount of vertical axis rotations of plan-view curvature is therefore helpful to our understanding of orogenesis, geodynamics, and paleogeography. The orocline test assesses to what extent vertical axis rotations have played a role in the acquisition of an orogen’s curvature. The test quantifies through linear regression the relationships between changes in structural trends and the orientations of a geologic fabric. However, the current mathematical approaches to the orocline test show potential biases.
In this paper, we aim to overcome such biases by developing a novel orocline test that applies Total Least Squares (TLS) regression combined with a novel approach to bootstrapping. This bootstrap TLS orocline test can be used with all types of directional data, acquired from structural geology, paleomagnetism or sedimentology. It quantifies, for the first time, secondary curvature with confidence bands. We also provide several graphical and analytical tests to evaluate the statistical significance of the result. An open source online application implementing this method is available for use on www.paleomagnetism.org. We illustrate the use of the methodology by re-analyzing published data sets from two well-known oroclines in the Cantrabrian (NW Iberia) and Aegean (Greece) regions.