Geological Controls in the Evolution of Granite Areas

The preceding chapters have already indicated that granite properties and structures play a key role in the progress of rock weathering, the development of many medium and small-scale landforms, and in the patterns of mass movement phenomena on slopes. But the influence of geological factors sensu lato is by no means limited to these, rather restricted spatial scales. Geotectonic settings, modes of emplacement, and long-term geological histories are all relevant to the understanding of the diversity of granite landforms and landscapes. The aim of this section is show the variety of geological controls and how they are reflected in granite landscapes, moving progressively from large to small spatial scales. If plate tectonics is used as a framework, then granite intrusions form in two major settings: orogenic, including transitional, and anorogenic (see Chapter 2). Geographically, the former take place at convergent plate margins, whereas the latter take place at divergent plate margins (rift zones) and within continental interiors, at hot spots. However, for the purpose of a geomorphological approach to granite landscapes of the world, the time-independent plate tectonics framework is less useful. This is because many granite intrusions occur in settings different than those in which they formed millions of years ago, and it is their post-emplacement long-term geological history and current location that are crucial to understanding the landscapes that have developed upon them. For example, late Paleozoic granite intrusions in central and western Europe took place within the Hercynian orogenic belt, hence in a convergent plate margin setting, but their present-day morphology is mainly the legacy of long-term evolution in an anorogenic regime and late Cainozoic rejuvenation, including plateau uplift and faulting. In a somewhat similar manner, ancient orogenic granite intrusions have been incorporated into shield interiors and passive margins. Figure 8.1 is an attempt to relate the tectonic settings of granite intrusions to the distribution of granite areas, as we see them today, against the background of global tectonics. From this point of view, granite landscapes occur within the following five main geodynamic settings: (1) orogenic zones along convergent plate margins, (2) eroded and rejuvenated ancient orogenic belts, subject to geologically recent plateau uplift, (3) passive margins at divergent plate boundaries, (4) stable shield interiors, and (5) oceanic islands.