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Processes and Perspectives in Extensional Tectonics

Elizabeth J. Catlos1 and I?brahim Çemen2

1Jackson School of Geosciences, Department of Earth and Planetary Sciences, University of Texas at Austin, Austin, Texas, USA

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

ABSTRACT

The study of extensional tectonics harkens to the early days of plate tectonics, as the search for mechanisms driving large continental blocks to drift apart led to advances in paleomagnetism and geochronology. Divergent plate boundaries form extensive and continuous volcanic systems covering large portions of the Earth and are fundamental to understanding how plates form. The study of extensional dynamics transects traditional geoscience disciplinary boundaries and is critical in the search for the origin and evolution of life. Extensional and normal faults are not restricted to divergent plate boundaries but are also located in regions of plate convergence and within plates. Failed rifts pose significant seismic hazards. This chapter aims to identify the importance of the study of extension and dispel common misconceptions regarding the process. Stress is a critical factor in explaining why extension occurs in the lithosphere and how magma focuses in regions of extensional tectonics.

1.1. INTRODUCTION

Divergent plate boundaries are places where pieces of Earth's lithosphere (crust?+?upper mantle) drift away from each other (Figures 1.1 and 1.2). They are spreading boundaries, where, in some cases, new igneous crust fills space between them or sediments deposit within new accommodation space. Divergent plate boundaries can separate ocean, continental, or transitional lithosphere and have been reviewed elsewhere (Wyllie, 1988; Sibuet & Tucholke, 2013; Acocella, 2014; Philippon & Corti, 2016; Peron-Pinvidic et al., 2019; Acocella, 2021; Zwaan & Schreurs, 2022; Olive, 2023). Divergent plate boundaries separate plates that move in opposite directions due to an extensional stress regime. Plates can extend, stretch out, enlarge in breadth, continue in length, or expand.

Despite an apparent simple observation (two plates move apart), developments in our understanding of plate tectonics have revealed that some common generalizations about these regions are incorrect. At divergent plate boundaries, lithospheric extension is often perceived to be narrow and localized between specific plates. However, the process can also occur within plates due to diverse activity at plate boundaries and mantle dynamics. In addition to its well-known roles at divergent boundaries, extension occurs in both transform and convergent plate boundaries and plate interiors. Misconceptions arise regarding extension because it is usually introduced during introductory geoscience courses without a complete understanding of the fundamentals of stress, strain, and rock physiochemical properties. These misconceptions include discriminating between tension versus extension and the role of magma as a passive outcome versus an active driver. Because introductory geoscience students are exposed to hazards early on at convergent and strike-slip plate boundaries (e.g., the Himalaya, Andes, and San Andreas Fault dominate the coverage in introductory geoscience textbooks), they may not recognize the inherent nature of seismic risk associated with divergent plate boundaries. For example, failed rift systems pose some of the highest risks in intraplate tectonic settings.

Figure 1.1 Global multiresolution topography map.

Source: Adapted from Ryan et al. (2009).

Divergent plate boundaries and extensional back-arc regions are numbered. Numbers in italics are related to subduction systems.

Source: Adapted from Coffin et al. (1998).

See Table 1.1 and Table 1.2 for information.

Figure 1.2 The cartoon cross-section illustrates the main plate boundary types.

Source: Adapted from Kious and Tilling (2016) / U.S. Department of the Interior (USGS) / Public Domain.

Table 1.1 Extensional plate boundaries.

Adapted from Coffin et al. (1998).

No. Plate boundary name Plate separation ?1 South Atlantic Ridge (part of Mid Atlantic Ridge) S. America-Africa ?2 North and Central Atlantic Ridge Axis (also Aegir Ridge) (part of the Mid Atlantic Ridge) N. America-Eurasian ?3 Gulf of California N. America-Pacific ?4 Eurasian Basin Spreading Ridge N. America-Eurasia ?6 Falcon Basin Fault Maricaibo-Perija ?7 Cayman Trough (Trench, Spreading Center, Rise) N. America-Caribbean Plate ?9 Reykjanes-Nansen Ridge N. America-Eurasia 10 Azores Spreading Center and Triple Junction N. America-Eurasian-Africa 11 Ridge north of Taimyr, Arctic Ocean N. America-Eurasia 12 Ridge in northeast Asia N. America-Eurasia 13 Ridge near Okhotsk Sea Okhotsk-N. America 14 Baikal Rift Zone Amur-Eurasia 15 Iberia-Africa Plate Boundary Iberia-Africa 16 Carlsberg Ridge Africa-Indo-Australia 17 Central Indian Ridge Africa-Indo-Australia 18 Gulf of Aden Spreading Ridge Africa (Somali)-Arabian 19 Red Sea Ridge Axis Africa (Somali)-Arabian 22 Ayu Trough Philippine Sea-Caroline 24 Sorol Trough NE of Eauripik Rise Pacific-Caroline 25 Caroline Ridge East-West Caroline 27 Southeast Indian Ridge Australia-Antarctic 28 Australia-Antarctic Spreading Ridge Australia-Antarctic 29 East American-Antarctic Ridge S. America-Antarctic 30 Pacific Antarctic Spreading Ridge Pacific-Antarctic 32 Juan Fernandez-Antarctic Spreading Center (part of Chile Rise) Juan Fernandez-Antarctic 36 Pacific-Galapagos Spreading Center Pacific-Galapagos 37 Pacific-Cocos Spreading Center Pacific-Cocos 38 Pacific-Nazca Spreading Center Pacific-Nazca 39 Juan Fernandez-Pacific S Spreading Center Juan Fernandez-Pacific 42 Pacific-Rivera Ridge Pacific-Rivera 43 Explorer Ridge Juan de Fuca-Gorda 44 Galapagos-Nazca Spreading Center Galapagos-Nazca 45 Easter-Nazca Spreading Center Easter-Nazca 46 Juan Fernandez-Nazca Spreading Center (part of Chile Rise) Juan Fernandez-Nazca 47 Cocos-Nazca Spreading Center (Cocos Ridge) Cocos-Nazca 48 Nazca-Antarctic Spreading Center (part of Chile Rise) Nazca-Antarctic

Advances in our understanding of lithospheric extension have also led to significant debate about the processes involved in developing regions of extended lithosphere and new and exciting ideas about how the Earth works. This chapter aims to clarify some of the basics of extensional tectonics, emphasize the importance of the study regions of crustal extension, dispel some common misconceptions, and set the stage for the chapters that focus on extensional tectonics in this volume.

1.2. THE IMPORTANCE OF EXTENSIONAL TECTONICS

Extensional tectonics plays a vital role in understanding an array of Earth processes that range from fundamental tectonics to the evolution of life. Continental drift requires extension and the rifting apart of large land masses (Wegener, 1912; Davies, 2022). These were the first plate boundaries used as evidence for plate tectonic theory, which also led to the application and further development of the fields...