Paleomagnetism of tilted dikes in fast spread oceanic crust exposed in the Hess Deep Rift: Implications for spreading and rift propagation

Tilted dikes occur within the pillow lava, sheeted dike, and gabbroic sections of 1 million‐year‐old, fast spread oceanic crust exposed on the walls of the Hess Deep Rift. During recent Alvin dives, oriented samples from both tilted and vertical dikes were collected from several of the outcrops. These are the first oriented paleomagnetic samples ever collected directly from outcrops on the ocean floor and were obtained with the help of a specially constructed geological compass. Remanent magnetic inclinations measured on the dike samples are significantly different from the near‐horizontal inclinations expected. The present attitudes of the dikes and their remanent magnetizations indicate that the dikes have been technically rotated since their intrusion. Submersible observations from other dive programs that examined parts of nearby escarpments suggest that the structural complexity observed in the Alvin study areas may be only a local phenomenon, or at least, not a pervasive characteristic of oceanic crust. Although tectonic rotations such as those observed may have taken place as a result of the opening of the Hess Deep Rift, crosscutting vertical dikes and other geologic considerations suggest that substantial tectonic rotations occurred at or very near the East Pacific Rise axis, probably as a result of intermittent axial collapse. The observed structures suggest that tectonic processes beneath fast spreading ridges and the internal structure of crust produced at them may be significantly more complex than previously assumed.