Revised Pacific-Antarctic plate motions and geophysics of the Menard Fracture Zone

A reconnaissance survey of multibeam bathymetry and magnetic anomaly data of the Menard Fracture Zone allows for significant refinement of plate motion history of the South Pacific over the last 44 million years. The right-stepping Menard Fracture Zone developed at the northern end of the Pacific-Antarctic Ridge within a propagating rift system that generated the Hudson microplate and formed the conjugate Henry and Hudson Troughs as a response to a major plate reorganization ∼45 million years ago. Two splays, originally about 30 to 35 km apart, narrowed gradually to a corridor of 5 to 10 km width, while lineation azimuths experienced an 8° counterclockwise reorientation owing to changes in spreading direction between chrons C13o and C6C (33 to 24 million years ago). We use the improved Pacific-Antarctic plate motions to analyze the development of the southwest end of the Pacific-Antarctic Ridge. Owing to a 45° counterclockwise reorientation between chrons C27 and C20 (61 to 44 million years ago) this section of the ridge became a long transform fault connected to the Macquarie Triple Junction. Following a clockwise change starting around chron C13o (33 million years ago), the transform fault opened. A counterclockwise change starting around chron C10y (28 millions years ago) again led to a long transform fault between chrons C6C and C5y (24 to 10 million years ago). A second period of clockwise reorientation starting around chron C5y (10 million years ago) put the transform fault into extension, forming an array of 15 en echelon transform faults and short linking spreading centers

Cenozoic Reconstructions of the Australia-New Zealand-South Pacific Sector of Antarctica

Reconstructions are presented documenting the relative motion of the Australia. Antarctic and Pacific plates since Chron 27 (61.1 Ma). In addition to the motion of the major plates, the reconstructions show the relative motion between East and West Antarctica and the continental fragments that make up the South Tasman Rise. Recent observations that are used in making these reconstructions include the mapping of seafloor spreading magnetic anomalies in the Adare basin, northeast of Cape Adare, which recorded roughly 150 km of opening between East and West Antarctica between Chrons 20 (43.8 Ma) and 8 (26.6 Ma). In addition, magnetic and bathymetric observations from the lselin Rift, northeast of the Iselin Bank, and from the Emerald Fracture Zone, along the western boundary of Pacific-Antarctic spreading, document the rotation of the Iselin Bank between Chrons 27 and 24 (53.3 Ma). Our reconstructions indicate that there was a total of about 200 km of separation between East and West Antarctica in the northern Ross Sea region in the Cenozoic. These reconstructions document the development of a deep-water passageway between Australia and Antarctica as the South Tasman Rise clears the final piece of the Antarctic continental margin around Chron 13 (33.5 Ma)

Constraints Imposed by the Shape of Marine Magnetic Anomalies on the Magnetic Source

A two-layer source model for marine magnetic anomalies can accommodate several observations made on the shapes of anomalies in the Pacific and southeast Indian oceans. The layers are defined on the basis of cooling history and magnetic properties. The upper layer consists of rapidly cooled basalts, which acquire a strong magnetization near the ridge axis. This layer, with narrow transition zones, can account for the observed short polarity events. The lower layer consists of moderately magnetized, slowly cooled intrusive rocks in the lower oceanic crust. The transition zones in this layer are broad, sloping boundaries reflecting the delayed acquisition of magnetization with depth as, for example, along a sloping Curie point isotherm. The lower layer can account for a skewness discrepancy of 10°-15° in the observed skewness of some anomalies. It is shown that the upper layer has to contribute about three quarters of the total amplitude of magnetic anomalies in order for this model to simulate the observed shape of the anomalies. The model predicts that a deep drill hole located just to the older side of a reversal boundary in the upper part of the oceanic crust should encounter a magnetization polarity reversal within the lower oceanic crust

Ultrahigh Resolution Marine Magnetic Anomaly Profiles: A Record of Continuous Paleointensity Variations?

A distinctive pattern of small-scale marine magnetic anomalies (25-100 nT amplitude, 8-25 km wavelength: tiny wiggles) is superimposed on the more generally recognized seafloor spreading pattern between anomalies 24 and 27 in the Indian Ocean. By normalizing and stacking multiple profiles, it is demonstrated that this pattern of tiny wiggles is a high-resolution recording of paleodipole field behavior between chrons C24 and C27. The pattern of tiny wiggles between anomalies 26 and 27 is compared to an ultrafast spreading (82 mm/yr half rate) profile from the southeast Pacific where a similar signal is observed, confirming the paleodipole field origin of the anomalies. Two basic models are considered in which the tiny wiggles are attributed either to short polarity intervals or to paleointensity fluctuations. We conclude that tiny wiggles are most likely caused by paleointensity fluctuations of the dipole field and are a ubiquitous background signal to most fast spreading magnetic profiles. The implications of this study are that (1) tiny wiggles may provide information on the temporal evolution of the geomagnetic dynamo; (2) the small-scale anomalies observed in the Jurassic quiet zones may be due to paleointensity fluctuations; (3) tiny wiggles are potential time markers in large regions of uniform crustal polarity such as the Cretaceous quiet zones; and (4) much of the variance in anomaly profiles normally attributed to crustal emplacement processes, particularly at fast and ultrafast spreading rates, is actually due to intensity variations in the paleomagnetic field

Crustal structure and rift flank uplift of the Adare Trough, Antarctica

The Adare Trough, located 100 km northeast of Cape Adare, Antarctica, represents the extinct third arm of a Tertiary spreading ridge between East and West Antarctica. It is characterized by pronounced asymmetric rift flanks elevated up to over 2 km above the trough's basement, accompanied by a large positive mantle Bouguer anomaly. On the basis of recently acquired seismic reflection and ship gravity data, we invert mantle Bouguer anomalies from the Adare Trough and obtain an unexpectedly large oceanic crustal thickness maximum of 9–10.5 km underneath the extinct ridge. A regional positive residual basement depth anomaly between 1 and 2.5 km in amplitude characterizes ocean crust from offshore Victoria Land to the Balleny Islands and north of Iselin Bank. The observations and models indicate that the mid/late Tertiary episode of slow spreading between East and West Antarctica was associated with a mantle thermal anomaly. The increasing crustal thickness toward the extinct ridge indicates that this thermal mantle anomaly may have increased in amplitude through time during the Adare spreading episode. This scenario is supported by a mantle convection model, which indicates the formation and strengthening of a major regional negative upper mantle density anomaly in the southwest Pacific in the last 50 million years. The total amount of post-26 Ma extension associated with Adare Trough normal faulting was about 7.5 km, in anomalously thick oceanic crust with a lithospheric effective elastic thickness (EET) between 3.5 and 5 km. This corresponds to an age between 3 and 5 million years based on a thermal boundary layer model and supports a scenario in which the Adare Trough formed soon after spreading between East and West Antarctica ceased, confined to relatively weak lithosphere with anomalously thick oceanic crust. There is little evidence for major subsequent structural activity in the Adare trough area from the available seismic data, indicating that this part of the West Antarctic Rift system became largely inactive in the early Miocene, with the exception of minor structural reactivation which is visible in the seismic data as offsets up to end of the early Pliocene

A New Geomagnetic Polarity Time Scale for the Late Cretaceous and Cenozoic

We have constructed a magnetic polarity time scale for the Late Cretaceous and Cenozoic based on an analysis of marine magnetic profiles from the world's ocean basins. This is the first time, since Heirtzler et al. (1968) published their time scale, that the relative widths of the magnetic polarity intervals for the entire Late Cretaceous and Cenozoic have been systematically determined from magnetic profiles. A composite geomagnetic polarity sequence was derived based primarily on data from the South Atlantic. Anomaly spacings in the South Atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles. Fine-scale information was derived from magnetic profiles on faster spreading ridges in the Pacific and Indian Oceans and inserted into the South Atlantic sequence. Based on the assumption that spreading rates in the South Atlantic were smoothly varying but not necessarily constant, a time scale was generated by using a spline function to fit a set of nine age calibration points plus the zero-age ridge axis to the composite polarity sequence. The derived spreading history of the South Atlantic shows a regular variation in spreading rate, decreasing in the Late Cretaceous from a high of almost 70 mm/yr (full rate) at around anomaly 33-34 time to a low of about 30 mm/yr by anomaly 27 time in the early Paleocene, increasing to about 55 mm/yr by about anomaly 15 time in the late Eocene, and then gradually decreasing over the Oligocene and the Neogene to the recent rate of about 32 mm/yr. The new time scale has several significant differences from previous time scales. For example, chron C5n is ~0.5 m.y. older and chrons C9 through C24 are 2-3 m.y. younger than in the chronologies of Berggren et al. (1985b) and Harland et al. (1990). Additional small-scale anomalies (tiny wiggles) that represent either very short polarity intervals or intensity fluctuations of the dipole field have been identified from several intervals in the Cenozoic including a large number of tiny wiggles between anomalies 24 and 27. Spreading rates on several other ridges, including the Southeast Indian Ridge, the East Pacific Rise, the Pacific-Antarctic Ridge, the Chile Ridge, the North Pacific, and the Central Atlantic, were analyzed in order to evaluate the accuracy of the new time scale. Globally synchronous variations in spreading rate that were previously observed around anomalies 20, 6C, and in the late Neogene have been eliminated. The new time scale helps to resolve events at the times of major plate reorganizations. For example, anomaly 3A (5.6 Ma) is now seen to be a time of sudden spreading rate changes in the Southeast Indian, Pacific-Antarctic, and Chile ridges and may correspond to the time of the change in Pacific absolute plate motion proposed by others. Spreading rates in the North Pacific became increasingly irregular in the Oligocene, culminating in a precipitous drop at anomaly 6C time

Pacific-Antarctic-Australia motion and the formation of the Macquarie Plate

Magnetic anomaly and fracture zone data on the Southeast Indian Ridge (SEIR) are analysed in order to constrain the kinematic history of the Macquarie Plate, the region of the Australian Plate roughly east of 145°E and south of 52°S. Finite rotations for Australia–Antarctic motion are determined for nine chrons (2Ay, 3Ay, 5o, 6o, 8o, 10o, 12o, 13o and 17o) using data limited to the region between 88°E and 139°E. These rotations are used to generate synthetic flowlines which are compared with the observed trends of the easternmost fracture zones on the SEIR. An analysis of the synthetic flowlines shows that the Macquarie Plate region has behaved as an independent rigid plate for roughly the last 6 Myr. Finite rotations for Macquarie–Antarctic motion are determined for chrons 2Ay and 3Ay. These rotations are summed with Australia–Antarctic rotations to determine Macquarie–Australia rotations. We find that the best-fit Macquarie–Australia rotation poles lie within the zone of diffuse intraplate seismicity in the South Tasman Sea separating the Macquarie Plate from the main part of the Australian Plate. Motion of the Macquarie Plate relative to the Pacific Plate for chrons 2Ay and 3Ay is determined by summing Macquarie–Antarctic and Antarctic–Pacific rotations. The Pacific–Macquarie rotations predict a smaller rate of convergence perpendicular to the Hjort Trench than the Pacific–Australia rotations. The onset of the deformation of the South Tasman Sea and the development of the Macquarie Plate appears to have been triggered by the subduction of young, buoyant oceanic crust near the Hjort Trench and coincided with a clockwise change in Pacific–Australia motion around 6 Ma. The revised Pacific–Australia rotations also have implications for the tectonics of the Alpine Fault Zone of New Zealand. We find that changes in relative displacement along the Alpine Fault have been small over the last 20 Myr. The average rate of convergence over the last 6 Myr is about 40 per cent smaller than in previous models

Fast Paleogene Motion of the Pacific Hotspots From Revised Global Plate Circuit Constraints

Major improvements in Late Cretaceous-early Tertiary Pacific-Antarctica plate reconstructions, and new East-West Antarctica rotations, allow a more definitive test of the relative motion between hotspots using global plate circuit reconstructions with quantitative uncertainties. The hotspot reconstructions, using an updated Pacific-hotspot kinematic model, display significant misfits of observed and reconstructed hotspot tracks in the Pacific and Indian Oceans. The misfits imply motions of 5-80 mm/yr throughout the Cenozoic between the African-Indian hotspot group and the Hawaiian hotspot. Previously recognized misfits between reconstructed Pacific plate paleomagnetic poles and those of other plates might be accounted for within the age uncertainty of the paleomagnetic poles, and non-dipole field contributions. We conclude that the derived motion of the Hawaiian hotspot relative to the Indo-Atlantic hotspots between 61 Ma and present is a robust result. Thus, the Pacific hotspot reference frame cannot be considered as fixed relative to the deep mantle. The bend in the Hawaiian-Emperor Seamount chain at 43 Ma resulted from a speedup in the absolute motion of the Pacific plate in a westward direction during a period of southward migration of the hotspot. The relationship between the hotspot motion and plate motion at Hawaii suggests two possible scenarios: an entrainment of the volcanic sources in the asthenosphere beneath the rapidly moving plate while the hotspot source drifted in a plate-driven counterflow deeper within the mantle, or drift of the hotspot source which was independent of the plate motion, but responded to common forces, producing synchronous changes in hotspot and plate motion during the early Tertiary