Global Tectonic Models and Case Histories

These models are large scale and are primarily intended to help identify site scale initial models and to put these models into the broader setting. No anticipation list has been given with the global Tectonic models for this reason; anticipation annotation is only supplied with the site scale models. Therefore, the case histories are not specifically based on the global models although the relevant global model is indicated in each case history; where more than one model pertains, this is also indicated.

Identification of the appropriate Tectonic model for any particular site would be made by the geologist, either from his local knowledge or reading of the literature. Since the theory of plate tectonics was developed in the 1960s, many of the subsequent geological papers published of a particular area identify the relevant tectonic setting. Such papers can be used to identify one or more of the models. Appropriate modern text books carry maps of the world divided into various plate-related subdivisions, which can be used to help identify the relevant model or models. Our Figures 2.1, 2.2 and 2.3 and Sections 2.0 and 3.0 are to help broadly in this.

Each Tectonic model contains a list of the principal lithologies associated with the model, the gross structure, examples and, in particular, related site scale Geological models. However, the site scale list must not be considered fully comprehensive as, within reason, each Tectonic model has the potential to contain parts of the great majority of the site scale Geological models and, therefore, only the most likely models have been listed. The engineering geologist using this system should be aware of this.

Each figure portraying the model is meant to be essentially self-contained and requires little further comment. The way the figures relate to each other is broadly indicated in Section 2.2, in Figure 2.2 , and in Tables 2.2 and 3.1. Discussion of the fundamental geology of the global structure and processes is given in Section 2.0. The following are short notes on each global Tectonic model and indicate their relevant case histories.

Cratons - Refer especially to our section showing major tectonic elements to see how these ancient land masses merge into adjacent systems: see Case Histories 16 and 17 below.

Mobile belts - These rocks are the remains of ancient collision zones: see Case Histories 14, 26 and 27.

Platform sediments and basins - This model covers large parts of the world and produced many case histories. For example, see Case Histories 4, 12, 18, 19, 20, 21, 22, 24, 26 and 31.

Continental rifts - We thought that this would produce several case histories: in fact, we could not find a good one - no doubt they exist.

Oceanic rifts - Much of this model exists on sea floors and where these emerge, for example in Iceland, conventional case histories become possible. See Case History 6.

Accretionary prisms - In the formative stage of this model the system is under deep water and therefore did not produce conventional case histories, and we did not find any good examples from the older parts of the stratigraphic column.

Fold and Thrust belt - This model is generally found as a distinct zone in orogenic belts, after the deep water sequences of forearc or backarc basins have been subjected to orogenesis (see Figs. 2.2 and 2.3). See Case History 27 which is one of a few that we have tested ourselves by work in the field, using key words and check lists that we developed and which were very successful in identifying problems, some of which were hitherto unforeseen (although not unforeseeable) in the original site investigation. Though the model and key words were only used after the original site investigation had been completed, problems were quickly identified. Note that this same case history is used to illustrate the magmatic arc model as the site setting contained elements of both models.

Magmatic arc - Modern examples of this are found in the world's active island arc zones and are illustrated by our Case History 27.

Collision complexes - This proved one of the easier models for which to find case histories as much engineering and building construction seems to have been carried out in such zones. See Case Histories 1, 11, 13, 14, 23 and 28.

Foreland basins - Again, another area in which construction is active. See, for example, Case Histories 3, 4, 5, 10 and 15.