Earth Structure An Introduction to Structural Geology and Tectonics

In the first chapter, we introduced a very general definition for a geologic structure: any definable shape or fabric in a rock body. Most of the scope of structural geology is focused on tectonic structures, meaning those structures that form in response to stresses generated by plate interactions (e.g., convergence, collision, rifting, strike-slip movement, plate drag, crustal-scale buoyancy; see Part D). But before we get to tectonic structures, we want to examine primary structures, i.e., those structures that form during or shortly after the deposition of rocks, and nontectonic structures, i.e., those structures that are not an immediate consequence of plate interactions. Primary sedimentary and igneous structures are also sometimes called nontectonic structures, because it is inferred that they form before the onset of tectonic stresses. Strictly speaking, however, this usage is misleading, because most structures, if not directly, are indirectly a consequence of tectonic activity. For example, the creation of slopes down which sediments slide, the occurrence of volcanic activity leading to the flow of basalt. All of these phenomena are ultimately a manifestation of movements in the Earth. Under the broad masthead of "primary and nontectonic structures" we discuss depositional, penecontemporaneous, intrusive, and gravity-slide structures for both sedimentary and igneous rocks. This heading also includes impact structures, which are discussed at the end of the chapter.