Huismans and Beaumont present some vividly illustrated examples of the variety of both symmetrical and asymmetrical responses to variable lithosphere rheology and rifting velocity, and transformation from one form to the other during deformation.
A third paper (Cochran and Karner) discusses the transition from rifting to drifting in the Red Sea, concluding that rift systems that are characterized by large rotated fault blocks rarely lead to seafloor spreading, but may be left stranded alongside adjacent margins or survive as failed rifts.
After millions of years as a divergent margin, around 200 million years ago the West Coast became a convergent margin and the site of active volcanic arcs (this was about the same time that the American-African supercontinent was rifting to form the Atlantic Ocean).
This ocean formed during a rifting event more than 600 million years ago, and the Iapetus passive margin lasted about 150 million years before becoming involved in oceanic subduction.
Commencing about 750 Ma and lasting for at least 150 million years, global rifting
was the overriding control on sedimentary basin formation, basin-fill stratigraphies and glaciation.
The nature and style of the earliest rifting is preserved in thick strata of the Wernecke Supergroup (ca.
Evidence of a major rifting and opening of an ocean is well preserved in the Humber Zone.
Yet the newly collected sediments indicate Broken Ridge was sinking immediately prior to breaking from the rest of the plateau, which means active rifting
did not cause the split, says Leg 121 co-chief investigator Jeffrey Weissel of Columbia University's Lamont-Doherty Geological Observatory in Palisades, N.
For this reason, these reflectors are thought to be key to understanding the rifting
and seafloor spreading processes.
Leg 103: By drilling in the Galicia margin off Spain, scientists on Leg 103 took an important stab trying to document the rifting
process that tore apart Africa and Europe from North America 150 Myr ago as the Atlantic Ocean opened.
According to Henry Mullins, a marine geologist at Syracuse University in New York, who first proposed this theory and who also participated in Leg 101, the rifting
left both topographical highs and deep gulleys.