Area Investigation Report on Benedict Arnold Scenic Road, Maine

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Magnetization of 0–29 Ma ocean crust on the Mid-Atlantic Ridge, 25°30′ to 27°10′N

Author Posting. © American Geophysical Union, 1998. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 103, No. B8 (1998): 17807–17826, doi:10.1029/98JB01394.A sea-surface magnetic survey over the west flank of the Mid-Atlantic Ridge from 0 to 29 Ma crust encompasses several spreading segments and documents the evolution of crustal magnetization in slowly accreted crust. We find that magnetization decays rapidly within the first few million years, although the filtering effect of water depth on the sea-surface data and the slow spreading rate (<13 km/m.y.) preclude us from resolving this decay rate. A distinctly asymmetric, along-axis pattern of crustal magnetization is rapidly attenuated off-axis, suggesting that magnetization dominated by extrusive lavas on-axis is reduced off-axis to a background value. Off-axis, we find a statistically significant correlation between crustal magnetization and apparent crustal thickness with thin crust tending to be more positively magnetized than thicker crust, indicative of induced magnetization in thin inside corner (IC) crust. In general, we find that off-axis segment ends show an induced magnetization component regardless of polarity and that IC segment ends tend to have slightly more induced component compared with outside corner (OC) segment ends, possibly due to serpentinized uppermost mantle at IC ends. We find that remanent magnetization is also reduced at segment ends, but there is no correlation with inside or outside corner crust, even though they have very different crustal thicknesses. This indicates that remanent magnetization off-axis is independent of crustal thickness, bulk composition, and the presence or absence of extrusives. Remanence reduction at segment ends is thought to be primarily due to alteration of lower crust in OC crust and a combination of crustal thinning and alteration in IC crust. From all these observations, we infer that the remanent magnetization of extrusive crust is strongly attenuated off-axis, and that magnetization of the lower crust may be the dominant source for off-axis magnetic anomalies.M. Tivey was supported by ONR grant N00014-94-1-0467 and NSF grant OCE-9200905 and B. Tucholke was supported by ONR grant N00014-94-1-0466 and NSF grant OCE-9503561. Data were collected under ONR grant N00014-90-JI612

Source of oceanic magnetic anomalies and the geomagnetic polarity time scale

Marine magnetic anomalies provide the framework for the geomagnetic polarity timescale for the Late Jurassic to Recent (since 160 Ma). Magnetostratigraphic records confirm that the polarity reversal sequence interpreted from magnetic anomalies is complete to a resolution of better than 30 ky. In addition to this record of polarity reversals, magnetic anomalies also appear to preserve information on geomagnetic intensity fluctuations. The correspondence of coherent near-bottom anomaly variations with independent estimates of field intensity provides strong evidence that geomagnetic intensity modulates the magnetization of the ocean crust. Indeed, many short wavelength anomaly variations in sea-surface magnetic profiles over fast-spreading ridges are likely attributable to geomagnetic intensity variations. Although longer-term geomagnetic field behavior may also be reflected in anomaly amplitudes, documenting such a signal requires a better understanding of time-dependent changes in the magnetic source (e.g., from low-temperature alteration) that may also affect magnetic anomalies. The extrusive layer, with an average remanence of ∼ 5 A m−1, is the largest contributor to magnetic anomalies. However, enhanced sampling of oceanic gabbros (average remanence ∼ 1 A m−1) and, to a lesser extent, dikes (average remanence ∼ 2 A m−1) reveals that these deeper (and thicker) layers likely generate anomalies comparable to those from the lavas. Lava accumulation at intermediate- and fast-spreading ridges typically occurs over a narrow (1–3 km) region and dike emplacement is even more narrowly confined, resulting in a relatively high fidelity record of geomagnetic field behavior. The slow cooling of the gabbroic layer, however, results in gently dipping polarity boundaries that significantly affect the skewness of the resulting anomalies, which is also a sensitive measure of net rotations of the source layer(s). The magnetizations of the dikes and gabbros are characterized by high stability and are not expected to significantly change with time, although there are insufficient data to confirm this. The lavas, however, typically show evidence of low-temperature alteration, which has been long regarded as a process that progressively reduces the magnetization (and degrades the geomagnetic signal) in the extrusive layer and reduces the amplitude of magnetic anomalies. Sufficient data have become available to examine this conventional wisdom. There is a substantial (∼ 4x) reduction in magnetization from on-axis samples to immediately off-axis drillsites (∼ 0.5 My), but little further change in half-dozen or so deep crustal sites to ∼ 160 Ma. High paleointensity that characterizes the last few thousand years may contribute significantly to the high on-axis magnetization. The task of evaluating changes in remanence of the extrusive layer is made more difficult by substantial cooling-rate-dependent changes in magnetic properties and the systematic variation in remanence with iron content (magnetic telechemistry). The commonly cited magnetic anomaly amplitude envelope is in fact not systematically observed – the Central Anomaly is elevated at slow-spreading ridges but is not as prominent at faster spreading rates. Nonetheless, magnetic anomaly amplitudes are consistent with magnetization change is poorly constrained. Direct determinations of the degree of low-temperature oxidation reveal the presence of highly oxidized titanomagnetite in samples less than 1 My old, suggesting a short (∼ 105 years) time constant though the effects of low-temperature oxidation are quite heterogeneous. While low-temperature oxidation does have some affect on lava magnetization and anomaly amplitudes, there is increasing evidence that marine magnetic anomalies are capable of recording a broad spectrum of geomagnetic field behavior, ranging from millennial-scale paleointensity variations to polarity reversals to apparent polar wander to, more speculatively, long-term changes in average field strength. Several emerging tools and approaches – autonomous vehicles, oriented samples, absolute paleointensity of near-ridge lavas, and measurements of the vector anomalous field – are therefore likely to significantly advance our understanding of the geomagnetic signal recorded in the oceanic crust, as well as our ability to utilize this information in addressing outstanding problems in crustal accretion processes

Inversions of magnetic anomalies in the South Atlantic and quantification of observed variations in magnetization solutions

A method for inverting magnetic anomalies in the presence of topography (Parker and Huestis, 1974) has been used to analyze and describe the earth's magnetic field along 12 profiles in the South Atlantic. The profiles, on the average, measure 2000 km and are perpendicular to the ridge axis. A bandpass filter has been used to filter out most of the longer wavelengths which are not known to be associated with seafloor spreading anomalies. A signal composed of wavelengths between 200 and 3 km (after inversion) produces magnetization solutions which are successful at reproducing the positive and negative aspects of the reversal sequence. Our model utilizes a 6 km crust in order to produce magnetizations which are consistent with those measured from dredged samples of the oceanic crust. A reoccurring pattern of amplitude variation has been observed in the magnetization solutions, A quantification of this variation reveals that the highest magnetizations in the solutions for both sections of the study area are associated with the ridge crest (1.3S-1.74 -1 A·m ) • Within the western section of the study area, a sharp decrease in magnetization occurs within the first 100 km or by 4.5 mybp. A minimum in the magnetization is reached at 400 km from the axis or by 18 mybp (.S1-.57 -1 A•m ) . The magnetization then gradually increases to 1100 km or by SO mybp (.91-.93 A· -1 m ) . This high is once again followed by an overall pronounced decrease to 1900 km or by 86 mybp ( .3 4- .43 -1 A•m ) • Within the eastern section of the study area, a sharp decrease in magnetization occurs within the first 100 km or by 5 mybp. A minimum in the magnetization occurs at approximately 400 km -1 from the ridge axis or by 20 mybp (.54-.56 A·m ) . A gradual increase in the magnetization then occurs at 1200 km from the ridge axis or by 60 -1 mybp (1.1S-1.24 A•m ) . An overall pronounced decrease then occurs to -1 1900 km from the ridge axis or by 9S mybp (.36-.40 A•m ) . Speculation as to a source for the observed amplitude variations in the magnetization solutions includes long term thickening of the oceanic crust, local thickening of the oceanic crust and/or variations in the intensity of the geomagnetic field through time

Area Investigation Report on Benedict Arnold Scenic Road, Maine

https://digitalmaine.com/doi_feddocs/1005/thumbnail.jp