Ellen M. Herron

Sea-Floor Spreading and the Cenozoic History of the East-Central Pacific

The east-central Pacific between 20° N. and 45° S. records several patterns of sea-floor spreading. Three distinct spreading centers are now active in the area, and total spreading rates across one of them, the East Pacific Ridge, are the fastest observed anywhere. The present East Pacific Ridge between 20° N. and 45° S. has maintained its present configuration only during the last 9 m.y. Prior to 10 m.y. ago, a north-northwest-trending ridge was active throughout the east-central Pacific, and the fossil crest of this ridge can be identified in the basin east of the present ridge axis between 10° S. and 30° S. and west of the ridge axis between 20° N. and the equator. Between the equator and 20° N., the north-northwest-trending ridge system has been continuously active since at least the Late Cretaceous, but 5 to 10 m.y. ago the position of the axis jumped to the east. South of the Chile Fracture Zone at 36° S., the north-northwest-trending Chile Ridge appears to be a still active segment of this old ridge system. Spreading on the north-northeast-trending East Pacific Ridge began more than 50 m.y. ago at 55° S. and grew northward so that opening at 35° S. began about 20 m.y. b.p. Between 10° N. and 10° S., spreading about the present axis was initiated only during the last 10 m.y. Analysis of magnetic anomalies associated with north-trending ridges near the equator is of limited value because of the extremely low amplitudes of the anomalies generated by the ridge. However, although identification of individual anomalies is tenuous at best, comparison of relative amplitudes and shapes with computed models offers limited support of the hypothesis of Francheteau and others (1970) that the Pacific Plate has migrated northward since the Late Cretaceous.

Plate tectonics synthesis: The displacements between Australia, New Zealand, and Antarctica since the Late Cretaceous☆

Abstract A comprehensive study of the last 75 m.y. of plate tectonics history has been undertaken for the region south of 30°S in the South Pacific, Southeast Indian Ocean and the Tasman Sea. Some aspects of plate boundary evolution have been clarified by our compilation and examination of available marine geophysical data. Reidentification of magnetic lineations in the southern Tasman basin shows that the controversial interval of subduction of Tasman basin crust along the east Australian margin that was previously proposed is no longer necessary. A comparison of Cenozoic magnetic lineations from both sides of the easternmost spreading segment of southeast Indian ridge indicates that a portion of the Indian plate younger than anomaly 10 (32 m.y.B.P.) is missing. We suggest that the missing crust was either subducted beneath or captured by the Pacific plate. Older lineations on the Indian plate out to about anomaly 21 have greater along-strike lengths than their counterparts on the Antarctic plate. The difference is due to an interval of crustal accretion at the Indian—Pacific plate boundary in the Early to Middle Tertiary. In the South Pacific, the Antarctic plate may not have extended northeast of the Eltanin fracture zone system prior to anomaly 29 (69 m.y.B.P.). Subduction of oceanic lithosphere was probably occurring beneath the Antarctic peninsula and eastern Ellsworth Land parts of the Antarctic plate at that time. Between anomaly-29 time and a major reorganization of plate boundaries in the Late Oligocene, plate interactions occurred in the central South Pacific between the Pacific, Farallon, Antarctic, Aluk and possibly a fifth plate. Spreading rate calculations for the Early Oligocene indicate that a simple three-plate system involving the Pacific, Farallon and Antarctic plates is difficult to maintain unless highly asymmetric spreading occurred at the Farallon—Antarctic boundary in the Early Oligocene. Further to the southeast in the Bellingshausen basin, spreading occurred along segments of the Antarctic—Aluk plate boundary beginning at about anomaly-29 time. Collisions of segments of this boundary with the trench along the Antarctic peninsula occurred in the Early and Middle Tertiary and resulted in the total disappearance of portions of Aluk plate. The collisions brought Antarctic—Aluk ridge segments into contact with Antarctic—Aluk trench segments with resulting stabilization of the Antarctic continental margin. From the magnetic lineations mapped in the southern oceans, we calculated new finite rotations which determine the relative positions of the Australian, New Zealand, and Antarctic continental fragments at 5–10 m.y. intervals during the last 75 m.y. For the New Zealand region, finite rotations calculated from magnetic lineations in the Tasman Sea and southwestern Pacific indicate that three plates were active during the Late Cretaceous to Paleocene. Previous workers have proposed that one of the three plate boundaries occurred southward between East and West Antarctica approximately along the trend of the Transantarctic mountains. As no direct geologic or geophysical evidence in Antarctica supports this proposal and since sea-floor spreading magnetic evidence shows that an active plate boundary passed through the New Zealand region after the Paleocene, we prefer to carry the required plate boundary northward. The Indian—Pacific poles of relative motion were always close to this boundary during the Cenozoic and thus corresponding evidence for plate interactions in New Zealand continental geology is generally variable and sometimes subtle.

Plate Tectonics Model for the Evolution of the Arctic

Following a search of critical literature on the geology and geophysics of the Arctic, we have constructed a model for the post-Permian evolution of the Arctic Ocean that follows the tenets of plate tectonics. We consider the history of the Arctic as the study of two separate basins, the Cenozoic Eurasian Basin and the Mesozoic-Cenozoic Amerasian Basin, and we have utilized the detailed pattern of opening of the North Atlantic worked out by Pitman and Talwani (1972) to determine the relative positions of Eurasia and North America during the past 81 m.y. We propose that the Amerasian Basin as we now know it opened by sea-floor spreading during the Jurassic magnetic quiet period, 180 to 150 m.y. ago. We reject the interpretation of the Alpha-Mendeleyev Ridge complex as an early Cenozoic spreading center and show that this feature is better interpreted as a fossil subduction zone-incipient island arc. The Eurasian Basin is an extension of the North Atlantic, which has opened by sea-floor spreading during the past 63 m.y. Prior to 63 m.y., the Lomonosov Ridge formed the seaward edge of the Eurasian continental margin.

Sea-Floor Spreading near the Galapagos

Seismicity, volcanism, and a linear pattern of very large magnetic anomalies that show symmetry about a broad negative anomaly suggest that a type of sea-floor spreading occurs near the Galapagos Islands in the east-equatorial Pacific. This spreading results from the tensile stresses generated by different spreading directions of two adjacent segments of the East Pacific Rise, and it is suggested that the area be called the Galapagos Rift Zone.

Evidence for two fossil spreading ridges in the southeast Pacific

Two different sets of extinct spreading ridge segments (Pacific-Farallon and Pacific-Nazca) are identified from bathymetric and magnetic data in the southeast Pacific. The older set is composed of several topographically subdued segments of a fossil ridge (Pacific-Farallon) that trends northwest, parallel to and about 175 to 550 km distant from the younger side of anomaly 7 (26 m.y. B.P.) The younger set trends north-northeast subparallel to the present East Pacific Rise (EPR), at an angle of about 45° to the older fossil ridge. The younger ridge forms the Galapagos Rise and includes a set of west-northwest–trending fracture zones. This was the original site of the EPR before it jumped westward to its present position. Oligocene magnetic anomalies (7–12) were generated at the older northwest-trending ridge rather than from the younger northeast-trending Galapagos Rise. The change in ridge orientation from the older Pacific-Farallon to the younger Pacific-Nazca direction is likely to be associated with the birth of the Cocos-Nacza spreading ridge system, about 20 to 25 m.y. ago.