Introduction

Continental rifts are regions of extensional deformation where the entire thickness of the lithosphere has deformed under the influence of deviatoric tension. The term "rift" thus applies only to major lithospheric features and does not encompass the smaller-scale extensional structures that can form in association with virtually any type of deformation.

Rifts represent the initial stage of continental break-up where extension may lead to lithospheric rupture and the formation of a new ocean basin. If it succeeds to the point of rupture the continental rift eventually becomes inactive and a passive or rifted continental margin forms. These margins subside below sea level as a result of isostatic compensation of thinned continental crust and as the heat that was transferred to the plate from the asthe-nosphere during rifting dissipates. However, not all rifts succeed to the point where new ocean crust is generated. Failed rifts, or aulacogens, become inactive during some stage of their evolution. Examples of failed rifts include the Mesozoic Connecticut Valley in the northeastern United States and the North Sea Basin.

Studies of active rifts show that their internal structure, history, and dimensions are highly variable (Ruppel, 1995). Much of this variability can be explained by differences in the strength and rheology of the lithosphere (Section 2.10) at the time rifting initiates and by processes that influence these properties as rifting progresses (Section 7.6.1). Where the lithosphere is thick, cool, and strong, rifts tend to form narrow zones of localized strain less than 100 km wide (Section 7.2). The Baikal Rift, the East African Rift system, and the Rhine Graben are examples of this type of rift (Fig. 7.1). Where the lithosphere is thin, hot, and weak, rifts tend to form wide zones where strain is delocalized and distributed across zones several hundreds of kilometers wide (Section 7.3). Examples of this type of rift include the Basin and Range Province and the Aegean Sea. Both varieties of rift may be associated with volcanic activity (Section 7.4). Some rift segments, such as those in Kenya, Ethiopia, and Afar, are characterized by voluminous magmatism and the eruption of continental flood

140oW 100oW 60oW 20oW 20oE 60oE 100oE 140oE

140oW 100oW 60oW 20oW 20oE 60oE 100oE 140oE

Figure 7.1 Shaded relief map showing selected tectonically active rifts. Map constructed using digital seafloor topography of Smith & Sandwell, 1997, USGS Global 30 arc second elevation data (GTOPO30) for land areas (data available from USGS/EROS, Sioux Falls, SD, http://eros.usgs.gov/), and software provided by the Marine Geoscience Data System (http://www.marine-geo.org), Lamont-Doherty Earth Observatory, Columbia University. BR, Basin and Range; RG, Río Grande Rift; R, Rhine Graben; AG, Aegean Sea; B, Baikal Rift; E, Main Ethiopian Rift; A, Afar depression; K, Kenya Rift. Box shows location of Fig. 7.2.

140oW 100oW 60oW 20oW 20oE 60oE 100oE 140oE

Figure 7.1 Shaded relief map showing selected tectonically active rifts. Map constructed using digital seafloor topography of Smith & Sandwell, 1997, USGS Global 30 arc second elevation data (GTOPO30) for land areas (data available from USGS/EROS, Sioux Falls, SD, http://eros.usgs.gov/), and software provided by the Marine Geoscience Data System (http://www.marine-geo.org), Lamont-Doherty Earth Observatory, Columbia University. BR, Basin and Range; RG, Río Grande Rift; R, Rhine Graben; AG, Aegean Sea; B, Baikal Rift; E, Main Ethiopian Rift; A, Afar depression; K, Kenya Rift. Box shows location of Fig. 7.2.

Karonga Basin L = 120 km

Livingstone fault

Livingstone fault

Albert Basin

Bunia fault

Bunia fault

Toro-Bunyoro fault

Manyara Basin L = 87 km

Omo Chew Bahir Basin L = 65 km Basin

I Cenozoic volcanic rock

Late Cenozoic sedimentary rock

Figure 7.2 (a) Shaded relief map and geodynamic setting of the East African Rift system constructed using digital topography data and software cited in Fig. 7.1. White arrows indicate relative plate velocities. Black arrows indicate absolute plate motion in a geodetic, no-net-rotation (NNR) framework (Section 5.4). (b-e) Cross-sections showing fault and half-graben morphology (after compilation of Ebinger et al., 1999, with permission from the Royal Society of London). M, Manyara basin (from Foster et al., 1997); K, Karonga basin (from van der Beek et al., 1998); A, Albert basin (from Upcott et al., 1996); CB, Chew Bahir basin (from Ebinger & Ibrahim, 1994); EAP, East African Plateau; EP, Ethiopian Plateau; MER, Main Ethiopian rift; L, the length of the border fault.

basalts. Others, such as the Western branch of the East African Rift system (Fig. 7.2) and the Baikal Rift, are magma starved and characterized by very small volumes of volcanic rock.

In this chapter, several well-studied examples of rifts and rifted margins are used to illustrate how strain and magmatism are distributed as rifting proceeds to sea floor spreading. The examples also show how geoscientists combine different data types and use spatial and temporal variations in the patterns of rifting to piece together the tectonic evolution of these features.

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