Continental Rifts
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Continental Rifts A. M. Cel^al Şengör Faculty of Mines, Department of Geology and Eurasia Institute of Earth Sciences, Istanbul Technical University, Istanbul, Turkey
Synonyms Graben; Taphrogen
Definition A continental rift (Gregory 1894; Quennell 1982, 1985) is a fault-bounded elongate trough under or near which the entire thickness of the lithosphere (▶ “Lithosphere, Continental” and ▶ “Lithospere, Mechanical Properties”) has been reduced in extension during its formation. Just as old mountain ranges may no longer have any topographic expression because of tectonic and/or erosional events, some, especially old, rifts may no longer appear as troughs for the same reasons, but their original trough shape is recognized by their stratigraphically younger fills, or metamorphically lower grade of their down-dropped central blocks (▶ “Sedimentary Basins”).
Introduction Rifts form one of the three main categories of lithosphericscale structures resulting from differential motion of parts of the lithosphere. Lithospheric shortening creates orogens, simple shear creates keirogens, and stretching creates taphrogens that are collections of rifts (Fig. 1). However, at present only 20.5% of the active plate boundaries show normal convergence and 21% normal divergence. Some 8% show pure
dextral strike-slip and some 6% pure sinistral strike-slip. The remaining 58.5% display some deviation from the three end-members, with relative motion vector angles to boundary strikes varying between 0 and 67 (Woodcock 1986). Plate boundaries must have shown a similar behavior in the past, so only about half of all the rifts a geologist may encounter is likely to show normal extension (▶ “Plate Motions in Time: Inferences on Driving and Resisting Forces” and ▶ “Plates and Paleoreconstructions”). Rifts are important structures from the viewpoints of our understanding of the behavior of our planet and our exploitation of its resources. They record evidence of continental fragmentation in diverse tectonic settings including all three types of plate boundaries and in plate interiors (▶ “Plate Motions in Time: Inferences on Driving and Resisting Forces” and ▶ “Plates and Paleoreconstructions”). Also, at different stages of their evolution they present opportunities of studying the continental crust (and in extreme cases even the upper mantle) from its surficial sedimentary rocks down to the crust-mantle interface (▶ “Lithosphere, Continental”). Especially the lacustrine sedimentary sections of rifts are useful for studies on the past climates (e.g., Olsen and Kent 1999; Kravchinsky et al. 2003; Felton et al. 2007), and they have enabled geologists to refine stratigraphic correlations down to thousands of years as far back as in the early Mesozoic (e.g., Olsen and Kent 1999). Rifts house important economic reserves such as hydrocarbons (the Sirt rift in Libya being the most productive with about 45 billion barrels of oil and 33 trillion cubic feet of gas; for the hydrocarbon potential of rifts, see: Harding 1983; MacKenzie and McKenzie 1983: a cla
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