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When Plates Collide

Elizabeth J. Catlos1 and Ibrahim Çemen2

1 Jackson School of Geosciences, Department of Geological Sciences, The University of Texas at Austin, Austin, Texas, USA

2 Department of Geological Sciences, The University of Alabama, Tuscaloosa, Alabama, USA

ABSTRACT

Compressional and contractional tectonics are of interest to various researchers, from rock mechanics and engineering to those studying the hazards, dynamics, and evolution of plate boundaries. We summarize here the terminology regarding deformation associated with compressional and contractional tectonics. We describe the now largely discarded geosyncline theory, which has its roots in contraction. Today, plate tectonics is the primary theory for explaining the processes shaping the Earth, including earthquakes, volcanoes, and mountain ranges. We emphasize the importance of subduction zones, the most extensive recycling system on the planet, and suture zones, complex boundaries marking the collision zone between two plates. The effects and hazards associated with convergent and collisional plate boundaries are felt far afield and for long distances.

1.1. INTRODUCTORY NOTES ABOUT TERMINOLOGY

Compressional tectonics is associated with terminology that will be defined here and in other sections. Rock deformation is divided into basic components: translation (change position), rotation (change orientation), dilation (change size passively), dilatation (change size in response to an active force), and distortion (change shape). In basic terms, compressive forces are directed toward each other () and work to squeeze and shorten rock volumes (Fig. 1.1a). A rock responds to stress (s), including compressional stress, by changing volume or form. Stress has units of force per area (N/m2 or lb/in2 or Pa, pascals) and is characterized by both a magnitude and an orientation on the surface in which it acts (Fig. 1.1). Deformed rocks result from total (finite) deformation over time, from which the forces and mechanisms that created rock textures or structures are interpreted.

Stress can be normal (perpendicular to the surface) or shear (parallel). Anderson (1905, 1951) linked the orientation of the causative stress tensor relative to the Earth's surface relation to fault types in the upper, shallower levels of the crust (see reviews in Simpson, 1997; Sorkhabi, 2013). The magnitude of stress may not be the same in all directions and thus is defined as maximum s1> intermediate s2> minimum s3.

A rock experiences uniaxial or unconfined compression when stress is directed toward the center of a rock mass, but more force is applied in one direction, and lateral component forces are zero (s1 > 0, s2 = s3 = 0) (Fig. 1.1a). Shortening strain is the change in rock volume due to compressive stress. Compressional stress results in shortening features in rocks from the microscale to mesoscale, depending on the pressure-temperature (P-T) environment and the nature of the materials composing the rock.

Rock composition and temperature are critical factors in evaluating how rocks respond to compressional stress. The initial deformation rock experiences during gradually increasing stress is elastic. During this time, changes in stress induce an instantaneous change in sample dimensions as measured by strain. With elastic deformation, the strain completely disappears when the stress is removed, and strain is recoverable (Twiss & Moores, 1992). Brittle materials fracture under compressive stress to release stored energy, whereas ductile materials deform and compress without failure. Rock layers may fold, or objects change shape, as evidenced by distributed strain. Plastic materials flow readily without fracture when the applied stress reaches conditions at or above specific yield stress (Twiss & Moores, 1992).

Figure 1.1 Relationship between stress axes and fault types (after Butler, 2021). (a) Rocks are displaced by contraction, (b) extension, and (c) shear. The principal stress axes are identified.

This book focuses on the processes that occur when the maximum compressive stress is in a horizontal orientation (contraction) (Fig. 1.1a). In this case, thrust faulting or folding occurs, shortening and thickening a rock or rock layers. Contraction is also observed as rocks lose volume through crushing, consolidation, or shear. In rock mechanics, contraction is a term that results in a reversible reduction in size, whereas compression results in a density increase. Contraction is exposed in the rock record as the shortening of rock layers, thrust or reverse faults, and folds. Thrust faults occur when rocks break along low angles and result in large earthquakes due to the large surface area affected by the process. In this volume, the dynamics of thrust faulting are described by Pashin (Chapter 10, Stratigraphic and Thermal Maturity Evidence for a Break-Back Thrust Sequence in the Southern Appalachian Thrust Belt, Alabama, USA) and Çemen and Yezerski (Chapter 11, Strain Partitioning in Foreland Basins: An Example from the Ouachita Fold-Thrust Belt Arkoma Basin Transition Zone in Southeastern Oklahoma and Western Arkansas). Reverse faults result from the rock breaking at high angles in response to compression (Fig. 1.1a). Normal faults occur when the maximum compressive stress is vertical, horizontally extending, and vertically thinning rock (Fig. 1.1b). We cover extensional tectonics in the second volume and strike-slip tectonics (Fig. 1.1c) in the third volume of this series.

1.2. SETTING THE STAGE: GEOSYNCLINE THEORY

The origin of mountains on the Earth has always been debated among philosophers, geographers, and Earth scientists. Since the late 1960s, plate tectonics has been a unifying theory of mountain building (see section 1.3). Although many theories before plate tectonics were proposed regarding the formation of mountains, one that received wide recognition is the geosynclinal or geosyncline theory, commonly attributed to James Hall and his coworkers (Hall, 1859; Dana, 1873; see Fisher, 1978; Frankel, 1982; Friedman, 2012; De Graciansky et al., 2011; Kay, 2014). James Hall and coworkers based their theory on field observations in the Appalachian Mountains of New York and Pennsylvania, where they observed features characteristic of shallow-water sedimentation, such as ripple marks, mud cracks, and shallow-water fossils in sedimentary units that were over 10,000 m in thickness. But they knew these sediments were deposited in basins where water was only about 100 m deep. Consequently, Hall proposed that these thick Paleozoic shallow-water sediments must have been deposited in a slowly subsiding basin, receiving a thick succession of shallow-water sediments as it subsided. They coined the term geosyncline for this subsiding basin (Fig. 1.2) (Glaessner & Teichert, 1947; De Graciansky et al., 2011). The formation can be further divided into miogeosynclines, eugeosynclines, and orthogeosynclines, depending on the rock strata, location, and nature of the mountain system.

To explain the deformation that they observed in the Appalachian Mountains, Hall and his coworkers proposed that after thick sediments accumulated, horizontal compressional forces directed from the seaward side of the geosyncline squeezed the sediments, shortened, and thickened the crust, and produced a high-standing mountain chain while pushing much of sediments into the crust. In the 1873, Dana proposed that the deeply buried sediments melted in high temperature and pressure conditions and generated magma that intruded into the sediments. During the 1890s and early 1990s, geosynclinal theory was widely recognized for explaining the formation of mountain chains, like the Appalachians, Ouachitas, Cordillerans, Urals, Alps, and Himalayas (see Mark, 1992; Sengör, 2021). However, Schaer and Sengör (2008) indicate that the geosyncline theory is not a "made in America" concept. For example, geologists in the Alps had noted the behavior of sediments in deep-water basins and ascribed their formation to synclines (e.g., 1828, Elie de Beaumont) (Schaer, 2010).

Figure 1.2 A diagram showing an imagined cross section of the northern Appalachians before the Appalachian Orogeny (after Kay, 1948). A geanticline is a ridge that separates two belts of sedimentary rocks. A eugeosyncline is a deep-water trough with abundant volcanic rocks and deep-water sediments. A miogeosyncline is a basin of mainly shallow-water sediments (De Graciansky et al., 2011).

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