GP33B-01
Paleomagnetic Poles From the Skewness of Marine Magnetic Anomalies
The skewness of marine magnetic anomalies due to seafloor spreading depends strongly on the direction of the paleomagnetic field as oceanic lithosphere is created and cooled. Thus, anomaly skewness can be used to estimate paleoomagnetic poles and is attractive, in part, because the ages of the poles can easily be related to the geomagnetic polarity time scale. Here we review methods for determining paleomagnetic poles from anomaly skewness and examine the accuracy of the poles. A concern has been the influence of ''anomalous" skewness, recognized in pioneering studies of anomaly skewness (Cande 1976). Anomalous skewness can be thought of as a systematic difference between observed skewness and that expected from simple models of the marine magnetic source that assume vertical boundaries between reversals. Anomalous skewness is now understood to be the consequence of non-vertical curving reversal boundaries expected from the thermal and magmatic evolution of oceanic lithosphere (Dyment & Arkani-Hamed 1995). Anomalous skewness depends on spreading rate if spreading is slow, but is independent of spreading rate if spreading is fast (Dyment & Arkani-Hamed 1995). The profiles we examine for paleomagnetic analysis dominantly record fast spreading and thus we are able to approximate anomalous skewness as being independent of spreading rate. We can therefore treat anomalous skewness as a third adjustable parameter, along with pole latitude and pole longitude, when determining paleomagnetic poles from skewness estimates. At any one crossing of a magnetic anomaly, local effects could cause the estimate of the paleomagnetic direction to be in error. Aside from anomalous skewness, however, errors in skewness estimates are expected to be uncorrelated from site to site except at exceptionally closely spaced sites. Thus we can treat misfits as independent random errors and reduce the statistical error in the pole position by obtaining many estimates of skewness from widely separated sites. The plate geometry of the Pacific plate throughout much of Late Cretaceous and Tertiary time, in particular the long north-south-striking paleo-East Pacific Rise, provides a nearly ideal geometry for recording critical information on the location of Pacific plate paleomagnetic poles. Moreover, a large skewness gradient with paleolatitude is expected, which helps to limit the location and, to a lesser degree, the strike of the paleoequator and thus strongly constrains the pole. If the accuracy of all skewness estimates is uniform, estimates of skewness near the paleo-equator can be shown to contain much more information than those from higher paleo-latitudes. On total-intensity magnetic profiles the amplitudes of anomalies near the paleo-equator are very low and likely have higher than average uncertainties. On vector aeromagnetic profiles, however, clear anomalies are recorded near the paleo-equator and provide strong constraints on the pole position. The assumptions made in determining paleomagnetic poles from skewness are subject to several critical tests including comparisons of anomalous skewness estimated from the single-plate method with those determined from cross-ridge analysis, comparisons with other skewness poles of similar age, and comparisons with poles determined from other types of paleomagnetic data. We present several such tests and show that the skewness results pass all these tests of consistency. Thus, there are many reasons to believe that poles determined from skewness give unbiased and accurate paleomagnetic poles.
GP33B-02 INVITED
Marine Magnetic Anomalies, Oceanic Crust Magnetization, and Geomagnetic Time Variations
Since the classic paper of Vine and Matthews (Nature, 1963), marine magnetic anomalies are commonly used to date the ocean floor through comparison with the geomagnetic polarity time scale and proper identification of reversal sequences. As a consequence, the classical model of rectangular prisms bearing a normal / reversed magnetization has been dominant in the literature for more than 40 years. Although the model explains major characteristics of the sea-surface magnetic anomalies, it is contradicted by (1) recent advances on the geophysical and petrologic structure of the slow-spreading oceanic crust, and (2) the observation of short-term geomagnetic time variations, both of which are more complex than assumed in the classical model. Marine magnetic anomalies may also provide information on the magnetization of the oceanic crust as well as short-term temporal fluctuations of the geomagnetic field. The "anomalous skewness", a residual phase once the anomalies have been reduced to the pole, has been interpreted either in terms of geomagnetic field variations or crustal structure. The spreading-rate dependence of anomalous skewness rules out the geomagnetic hypothesis and supports a spreading-rate dependent magnetic structure of the oceanic crust, with a basaltic layer accounting for most of the anomalies at fast spreading rates and an increasing contribution of the deeper layers with decreasing spreading rate. The slow cooling of the lower crust and uppermost mantle and serpentinization, a low temperature alteration process which produces magnetite, are the likely cause of this contribution, also required to account for satellite magnetic anomalies over oceanic areas. Moreover, the "hook shape" of some sea-surface anomalies favors a time lag in the magnetization acquisition processes between upper and lower magnetic layers: extrusive basalt acquires a thermoremanent magnetization as soon as emplaced, whereas the underlying peridotite and olivine gabbro cool slowly and pass through serpentinization to bear a significant magnetization. Our analysis of the amplitude of Anomaly 25 shows a sharp threshold at the spreading rate of 30 km/Ma, which corresponds to the transition between oceanic lithosphere built at axial domes and axial valleys. The twice lower amplitudes are in agreement with a much disrupted and altered basaltic layer at slow rates and a significant contribution from the deeper layers. Oceanic lithosphere created at fast and slow spreading rates therefore exhibits contrasted magnetic structures. High resolution magnetic anomaly measurements carried out with deep tows and submersibles show that the magmatic (fast spreading and parts of the slow spreading) crust is a good recorder of short-term geomagnetic time variations, such as short polarity intervals, excursions, or paleointensity variations. Surface and deep-sea magnetic anomalies therefore help to confirm or infirm geomagnetic findings obtained by other means. Many excursions and paleointensity variations within Brunhes and Matuyama periods are confirmed, but the "saw tooth pattern" inferred from sediment cores - a possible candidate to explain the anomalous skewness - is not, which suggests a bias in the sedimentary approach.
GP33B-03 INVITED
Toward a minimum change model for recent plate motions: Calibrating seafloor spreading rates for outward displacement
We use seafloor spreading distances derived from dense magnetic surveys of young magnetic anomalies flanking seven seafloor spreading centers and the velocities of 398 continuous GPS sites on the plates bordering these spreading centers to study outward displacement, a phenomenon in which seafloor spreading magnetic lineations are displaced outward from their idealized locations as a consequence of extrusive and intrusive emplacement of new magma across a several-km-wide zone centered on the spreading axis and outward sloping reversal boundaries. Linear regressions of age-opening distance series derived from crossings of magnetic reversals 1n-3An.2 (0.78 Ma-6.72 Ma) yield positive Y-intercepts for 42 out of 53 seafloor spreading segments, corresponding to displacement of reversals outward from the seafloor spreading axis. The improvement in the least-squares fit of a model that allows for outward displacement relative to a model in which outward displacement is assumed to be zero is significant at a very high confidence level. Separate inversions of 13 age-distance series derived from magnetic anomaly crossings grouped by plate boundary yields 12 estimates of outward displacement that range from 0.5-3 km and unusually wide outward displacement of 6.1+-0.4 km along the Reykjanes Ridge. Detailed analysis of numerous crossings of Anomaly 1n from the Southeast Indian ridge suggests there is a correlation between axial morphology and the magnitude of outward displacement; however, too few data are available from axial rise segments along other seafloor spreading centers to confirm whether this correlation is characteristic of other seafloor spreading centers. Our results corroborate previous estimates of magnetic polarity transition zone widths derived from near-bottom magnetic measurements, which range from 1-8 km and average 2 km. Statistical tests of our age-distance series indicate that all but two are consistent within errors with a globally averaged value for outward displacement of 1.9+-0.2 km. Seafloor spreading rates corrected for outward displacement agree better with instantaneous rates estimated from GPS-derived plate angular velocity vectors than do uncorrected long-term rates, underscoring the need to correct seafloor spreading rates for outward displacement before attempting to interpret differences between geodetic and geologic estimates of plate motions.
GP33B-04 INVITED
Geomagnetic paleointensity results from 9° - 10°N on the East Pacific Rise and models of lava accretion at fast-spreading ridges
Our understanding of the neovolcanic zone at fast spreading ridges has become more sophisticated in recent years, but placing accurate age constraints on many near-axis flows remains difficult. Geomagnetic paleointensity recorded in submarine basaltic glass (SBG) holds promise for placing quantitative age constraints on near-axis flows. Based on the results of over 500 SBG paleointensity estimates from 189 near-axis (< 4 km) sites at the East Pacific Rise, 9° - 10°N, we evaluate the temporal and spatial variability in lava accretion. Paleointensities range from 6 - 53 μT and are distributed spatially as would be expected from known temporal variations in the geomagnetic field: samples within and adjacent to the axial summit trough (AST) have values approximately equal to or slightly higher than the present day; off-axis samples out to 1-3 km from the AST have values higher than the present day; and samples farther off-axis have values lower than the present day. Paleointensity values are also generally consistent with rough age interpretations based on sidescan data. While along-axis data (< 500 m from the AST) provide constraints on eruptive recurrence intervals, the cross-axis data contain valuable information regarding variability in spatial scales of lava accretion. Spatial patterns in paleointensity data suggest that the width of the lava emplacement zone must vary significantly in time and in space, locally remaining quite narrow (< 1 km half-width) for extended periods of time. This implies that extensive off-axis flow emplacement may occur infrequently, with recurrence intervals of 10-20 kyr. Results of a stochastic model of lava emplacement show that this can be achieved by modeling the eruptive recurrence interval as a Poisson process, which results in a standard gamma distribution of wait times. Flow size is directly or indirectly linked to wait time through an available magma budget. A median wait time of 70 years most closely matches the patterns seen in the paleointensity data, and such a flow distribution results in a flow > 3.5 km total width once ever ~20 kyr on average.
GP33B-05
Why is the Remanent Magnetic Intensity of Cretaceous MORB so Much Higher Than That of Mid- to Late Cenozoic MORB?
The fact that the natural remanent magnetization (NRM) intensity of MORB samples shows systematic variations as a function of age has long been recognized: maximum as well as average intensities are generally high for very young samples, falling off rather rapidly to less than half the recent values in samples between 10 and 30 Ma, whereupon they slowly rise in the early Tertiary and Cretaceous to values that approach those for the very young samples. NRM intensities measured in this study follow the same trends as those observed in previous publications. In this study we take a statistical approach and examine whether this pattern can be explained by variations in one or more of all previously proposed mechanisms: chemical composition of the magnetic minerals, abundance of these magnetization carriers, vectorial superposition of parallel or antiparallel components of magnetization, magnetic grain or domain size patterns, low-temperature oxidation to titanomaghemite, or geomagnetic field behavior. We find that the samples do not show any compositional, petrological, rock-magnetic or paleomagnetic patterns that can explain the trends. Geomagnetic field intensity is the only effect which can not be directly tested on the same samples, but shows a similar pattern as our measured NRM intensities. We therefore conclude that the geomagnetic field strength was, on-average, significantly greater during the Cretaceous than during the Oligocene and Miocene, although it was perhaps rather variable on a short time scale.
GP33B-06 INVITED
Origin of deep-sea magnetic lineations: magnetized oceanic crust or serpentinized exhumed mantle
Since a long time, it has been established that sea-floor spreading at mid-oceanic ridges gives rise to oceanic crust at medium or high spreading rates (magmatic segments) and mostly exhumed mantle at slow and ultraslow spreading rates (amagmatic segments). Whatever the extension rates, the nature of the crust and the process of crustal magnetization are, magnetic lineations appear to be symmetrical or roughly symmetrical allowing to determine spreading rates and to convincely reconstruct the positions of plates in the past. Thus, the origin of magnetic anomalies was never a real concern for the Ridge community. A few observations can be made concerning the emplacement of crustal material at mid- oceanic ridges by using constraints established in the Iberia/Newfoundland transitional crusts and at or close to mid-oceanic ridges: 1) The contribution of abyssal peridotites to marine magnetic anomalies becomes significant when they are affected by a larger than 75 per cent degree of serpentinization (NRM values larger than 5 A/m). Such highly serpentinized peridotites are characterized by Vp seimic velocities of 5-5.5 km/s and down to 1.5-2 km in the Mid-Atlantic ridge axial discontinuity at 35 °N where serpentinized peridotites have been dredged. 2) Magnetic anomalies, resembling those generated by seafloor spreading processes, have been observed at the base of several passive continental margins where mantle is exhumed as serpentinized peridotites. In the Iberia Abyssal Plain, ages of gabbroic intrusions within the exhumed mantle are identical to ages of basement obtained from modeling of magnetic anomalies as seafloor spreading anomalies. We suggest that these anomalies are caused by the magnetization of titano-magnetite grains formed during the serpentinization of peridotites, which starts at lower crustal level during mantle exhumation and continues until most of the exhumed mantle reaches the sea bottom. Thus, serpentinization and magnetization of exhumed mantle take place in time and space resembling a process of formation of seafloor spreading crust at mid-oceanic ridges formed at medium to high spreading rates. This also explains why seafloor spreading type anomalies are usually observed across slow and ultraslow spreading ridges, which mostly contain exhumed mantle material. Thus, sequences of magnetic anomalies can provide informations concerning the timing of the emplacement of crust, but generally not on its nature (oceanic crust versus exhumed mantle).
GP33B-07
New Insights Into the Farallon Plate Break-Up
The break-Up of the Farallon plate was the most important event during the Early Miocene plate tectonic reorganization of the East Pacific. The opening of a new oceanic spreading center perpendicular to the existing Pacific spreading is unique and probably had far-reaching consequences for the active continental margins of Central- and South America. Most of the original fissure where the Farallon plate split into the Cocos plate and the Nazca plate has already been subducted beneath Central- and South America together with much of the oceanic crust that was formed during the early phase of Cocos-Nazca spreading. This made a full reconstruction of plate kinematics in the area back to the time of the opening poorly constrained and left many questions open about the mechanisms involved and the subduction zones affected by this event. During a R/V Sonne cruise in late 2004 the area conjugate to the Farallon remains offshore Costa Rica and Ecuador was investigated in the Central Pacific around 120° W, just south of the Equator. With the new magnetic data it was possible to identify seafloor spreading anomalies between chrons 7 and 5A for a large area in the Central Pacific. In combination with picks of the same anomalies from the Cocos- and Nazca plates it was possible to trace back the history of the Farallon break-Up in a three-plate reconstruction. The plate motion of the Pacific plate in the hotspot reference frame provides the absolute position of the initial triple junction and the strike direction of the newly formed Cocos-Nazca spreading center, revealing its close relation to the Galapagos hotspot and the Central American subduction zone. The multibeam bathymetry data from the research area highlight the details of an increased magmatic activity near the newly formed triple junction and the reorganization of the seafloor spreading that followed the break-Up and which finally resulted in a major ridge jump at the East Pacific Rise during chron 6.