TEMPERATURE AS A FIRST-ORDER CONTROL OF FOLD FORMATION

Fold formation cannot be understood by wavelength-thickness relationships alone, as layer anisotropies and boundary condition variations affect fold development. Fluctuating boundary conditions during folding play a significant role in the resulting geometries and shapes of folds. Temperature exerts a primary control on rock rheology and behavior, and therefore strongly influences folding. Folds are ideal structures for studying rheology as they can record temporal and spatial temperature changes over time. Temperature effects can be divided into three main categories that complexly interact: 1) rheologic domain (elastic, plastic, or viscous) and therefore relative competence of layers; 2) the dominant deformation mechanisms operating at all scales; and 3) metamorphic reactions, which may change bulk composition, grain size, or modify layering.

In naturally deformed folds, temperature-induced competence contrast between folding layers may reverse over time. For example, in the folds surrounding the Tertiary Adamello pluton, the more competent limestone and less competent marl reverse competence within the thermal aureole, inverting the observed sense of boudin and cuspate-lobate structures. This reversal is caused by thermal metamorphism of the limestone and marl to relatively weak marble and much stronger pyroxenes and idocrase, respectively (Ramsay and Huber, 1987, 400-401). This alteration is also significant enough to affect consequent fold geometries.

The Cascades crystalline core, Washington, preserves a unique crustal section that allows examination of rheology, deformation mechanisms, and metamorphic processes related to the development of folds. The presence of folds throughout the section provides a window into how varied boundary conditions affect fold development. Field and microstructural study indicate significant differences between low-temperature and high-temperature folds: fold shapes, strains, competence contrasts, and deformation mechanisms all vary. Temperature and its effects provide a valuable way to approach and understand differing fold characteristics, and therefore rheology, throughout a metamorphic gradient.