Structural or climatic control in granite landforms? The development of sheet structure, foliation, boudinage, and related features

Abstract

Granite landforms have been interpreted in terms of climatic geomorphology, or morphogenetic regions, but the field evidence overwhelmingly points to structural control. Some features are developed after the exposure of the granitic bodies, for joggling of the brittle crust continues and external agencies also achieve change but the origin of some forms can be traced to the emplacement of the granite bodies, and to strains and stresses developed in magmatic bodies during their intrusion. Various mineral, magmatic and magnetic fabrics are produced. The consolidation of the magma begins at the contact between the emplaced body and the host rock. At this stage, the marginal zone is already crystalline and brittle. Arguably, shearing consequent on continued emplacement causes deformation and the development of planar fractures, some aligned roughly parallel with the cooling and crystallisation surface developed in the uppermost zone of the intrusive body, others imposed by lateral stresses. Differential movements between the sheets produced by shear causes the development of stretching and/or shortening movements. Extension produces a structural fabric that is later exploited by weathering and thus may contribute to the generation of such forms as pseudobedding, foliation, polygonal cracking, boudinage, and where deformation is pronounced, the formation of spheroidal cores within cubic or quadrangular blocks. The second type of fabric, due to shortening, generates folding or buckling of the previously defined planar structures and the formation of sheet structures. Both deformational signals, though of opposite sign, are a continuum in a close spatial relationship. This indicates a simultaneous or at least sequential development of the two types of planar fabrics at the end of the emplacement stage. Once the rock is at the land surface, it is affected by external processes, and the structural fabric determines the planes of ready water access in the rocky massif thus determining the progress of weathering and significantly influencing the evolution of granitic landscapes.