Stephen D. Hurst

Tectonic rotations of dikes in fast-spread oceanic crust exposed near Hess Deep

Oceanic crust that initially formed at the fast-spreading (130 mm/yr) East Pacific Rise at ∼1 Ma is exposed in the walls of Hess Deep rift at the northern edge of the Galapagos microplate. Steep fault scarps expose a dismembered vertical sequence of pillow basalts, sheeted diabase dikes and gabbroic rocks. Diabase dikes in these units are dominantly vertical; however, extensive outcrops of steeply to moderatley dipping dikes exist in some places. The nonvertical attitude of the dikes may be the result of tectonic rotations attending sea-floor spreading at the East Pacific Rise or the result of complex rotations of fault blocks during the opening of the Hess Deep rift at ∼0.65 Ma.

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.

Structural and geophysical expression of the Solea graben, Troodos Ophiolite, Cyprus

Field studies show that extensional structures in the north-central part of the Troodos Ophiolite, including steep normal faults, grabens, dikes, and low-angle detachment faults are related to E-W (present coordinates) spreading. The Solea graben, directly north of the mafic-ultramafic rock outcrops around Mount Olympus, consists of a 10- to 15-km-wide western portion that is underlain by a low-angle detachment fault between the sheeted dike complex and the plutonic section. Structural and paleomagnetic evidence indicates that the sheeted dikes above this detachment have been rotated 40° or more about subhorizontal axes. The eastern portion of the graben contains sheeted dike blocks with varying axes of rotation and significantly different style of intrusive activity. Most structural features in the graben formed simultaneously with or shortly after cessation of spreading at the graben axis based on cross-cutting ophiolitic dikes and hydrothermal mineralization. Timing of the uplift of the central part of the Troodos Ophiolite is uncertain but is probably related to the tectonic thinning of the upper crust by low-angle normal faulting. The Solea graben spreading center provides a model for the structure of modern mid-ocean ridges that are spreading amagmatically.

Paleomagnetic constraints on the formation of the Solea graben, Troodos ophiolite, Cyprus

Abstract The Solea graben, on the northern flank of the Troodos ophiolite, Cyprus, is a 15–20-km-wide asymmetric structure primarily defined by dike attitudes in the sheeted dike complex. The wider western portion of the graben consists mainly of gently dipping dikes while the eastern portion is narrow and consists of blocks of dikes with widely varying attitudes. Using a method which restores the sampled dikes to vertical and the primary remanent magnetizations to a previously known stable remanent magnetization direction for the Troodos complex, we have calculated rotation axes and amounts of rotation for the dikes. The rotation axes for dikes in the western portion of the Solea graben are subhorizontal and subparallel to the original strikes of the dikes. In the eastern portion the rotation axes have varying attitudes, including some which are very steep. Our results are consistent with field observations that suggest that the western Solea graben formed by listric normal faulting near the axis of a spreading center, with rotation of the dikes above a detachment surface. Across the graben axis on the eastern portion, more intense intrusive activity and minor normal faulting resulted in less rotation about subhorizontal axes and some rotations about vertical axes.