Earth Structure  An Introduction to Structural Geology and Tectonics

15. Extensional Tectonics

Two-hundred and fifty million years ago, a dinosaur could have walked from North America to Africa. At that time, most of the continental crust on the planet was sutured together to form one large supercontinent called Pangea . Beginning in Late Triassic, however, Pangea began to stretch and thin along discrete belts. One of these belts roughly followed the trace of the Appalachian orogen, a collisional mountain range in eastern North America, which formed during the Paleozoic assembly of Pangea. By Cretaceous time, North America had separated from Africa and Europe along this belt, and sea-floor spreading at the new Mid-Atlantic Ridge began to form the Atlantic Ocean. African dinosaurs bade farewell to their American cousins. The drama that we just described is an example of rifting. Simply put, rifting is the process by which continental lithosphere stretches. A rift or rift system is the belt of continental lithosphere that is stretching. In active rifts, which are places where extensional is currently taking place, the ground surface is cut by faults, earthquakes rumble with unnerving frequency, and volcanic eruptions occasionally bury the countryside. Typically, active rifts display a topography that is characterized by the occurrence of linear ridges or mountain ranges that separate non-marine or shallow-marine sedimentary basins. In inactive rifts, meaning rifts at which extensional deformation has ceased, the record of rifting is manifested by the occurrence of normal faults in association with thick sequences of redbeds, conglomerates, evaporites and volcanics. When extension continues until a continent separates into two pieces and a new oceanic spreading ridge forms, we say that the rifting was successful. But not all rifts are successful. If extension ceases 5efore the continent separates into two pieces, we say that the rifting was unsuccessful. Unsuccessful rifts that cut into cratonic areas of continents at a high angle to the continental margin are also known as aulacogens (from the Greek for "furrow").

Typically, 20 to 60 million years pass between inception of a rift and the time at which active rift faulting ceases and sea-floor spreading begins (called the rift-drift transition). The amount of lithospheric stretching that takes place prior to the rift-drift transition is variable. Typically, continental lithosphere stretches by a factor of 2 to 4 times before separation, meaning that the lithosphere of the rift region eventually becomes 2 to 4 times its original width and therefore about one half to one quarter of its original thickness. The amount of stretching prior to rifting at a given locality, as well as the overall width of the rift, depends on such variables as pre-rift strength of the lithosphere and syn-rift heat flow at the base of the lithosphere. During the rift-drift transition, movement on rift-related faults ceases, and the thinned continental lithosphere underlying the newly formed continental margins begins to subside (= sink to greater depths). As subsidence occurs, relicts of the now inactive rift are progressively buried beneath a thick accumulation of marine sedimentary rock. Continental margins at which this process is occurring are known as passive margins, to distinguish them from active continental margins at which you find strike-slip motion and/or convergence. The trough containing the generally thick sedimentary accumulation is a passive-margin basin. Passive-margin basins, which underlie the continental shelves of many continents, are not really static, but rather are the locus of gravity-driven deformation causing the sedimentary wedge to gradually slide seaward. Rifts and passive margins are fascinating regions, not only because of the complex structural and stratigraphic assemblages that occur within them, but also because of the petroleum resources that they contain. In this chapter, we survey the principal structural, petrologic, and topographic features of rifts and passive margins, and will introduce some of the current speculations about how and why rifting occurs.