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

A geomagnetic record over the last 3.5 million years from deep-tow magnetic anomaly profiles across the Central Indian Ridge

International audienceHigh-resolution records of the geomagnetic field intensity over the last 4 Myr provided by paleomagnetic analyses of marine sediments have shown the occurrence of short-lived low field intensity features associated with excursions or short polarity intervals. In order to evaluate the ability of marine magnetic anomalies to record the same geomagnetic events, we have collected six deep-tow (-500 m above the seafloor) and several sea surface magnetic anomaly profiles from the Central Indian Ridge across the Brunhes, Matuyama, and Gauss chrons (i.e., from the ridge axis to anomaly 2A). After removal of topography, latitude, and azimuth effects, we converted distances into time sequences using well-dated polarity reversal anomalies as tie points. We calculated the average signal to test the robustness of the short-wavelength anomalies. The resulting stacked profile is very similar to stacked sea surface and downward continued profiles from the Central Indian Ridge, the East Pacific Rise, and the Pacific-Antarctic Ridge. Our results suggest that in addition to polarity reversals, to previously suggested geomagnetic events (subchrons or excursions) within the Brunhes and Matuyama chrons. A new small-scale magnetic anomaly, likely generated by several closely spaced geomagnetic field intensity variations represent the major contributor to the detailed shape of recent marine magnetic anomalies in investigated areas. We observe a dense succession of microanomalies that are correlated excursions (Ontong Java 1 and 2, and Gilsa), is found after the Olduvai chron. The near-bottom results support the existence of three geo-magnetic features between the Gauss-Matuyama boundary and Olduvai. They also suggest three geomagnetic events during the C2A. I n subchron within the Gauss chron. This study emphasizes the potential of deep-tow magnetic surveys in detecting fluctuations in geo-magnetic field intensity and, in particular, short-lived excursions, a poorly constrained part of the geomagnetic field temporal variation spectrum

Fault rotation and core complex formation : significant processes in seafloor formation at slow-spreading mid-ocean ridges (Mid-Atlantic Ridge, 13°–15°N)

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 9 (2008): Q03003, doi:10.1029/2007GC001699.The region of the Mid-Atlantic Ridge (MAR) between the Fifteen-Twenty and Marathon fracture zones displays the topographic characteristics of prevalent and vigorous tectonic extension. Normal faults show large amounts of rotation, dome-shaped corrugated detachment surfaces (core complexes) intersect the seafloor at the edge of the inner valley floor, and extinct core complexes cover the seafloor off-axis. We have identified 45 potential core complexes in this region whose locations are scattered everywhere along two segments (13° and 15°N segments). Steep outward-facing slopes suggest that the footwalls of many of the normal faults in these two segments have rotated by more than 30°. The rotation occurs very close to the ridge axis (as much as 20° within 5 km of the volcanic axis) and is complete by ∼1 My, producing distinctive linear ridges with roughly symmetrical slopes. This morphology is very different from linear abyssal hill faults formed at the 14°N magmatic segment, which display a smaller amount of rotation (typically <15°). We suggest that the severe rotation of faults is diagnostic of a region undergoing large amounts of tectonic extension on single faults. If faults are long-lived, a dome-shaped corrugated surface develops in front of the ridges and lower crustal and upper mantle rocks are exposed to form a core complex. A single ridge segment can have several active core complexes, some less than 25 km apart that are separated by swales. We present two models for multiple core complex formation: a continuous model in which a single detachment surface extends along axis to include all of the core complexes and swales, and a discontinuous model in which local detachment faults form the core complexes and magmatic spreading forms the intervening swales. Either model can explain the observed morphology.D. Smith and H. Schouten were supported in this work by NSF grant OCE-0649566. J. Escartın was supported by CNRS

Crustal Evolution of the Mid-Atlantic Ridge near the Fifteen-Twenty Fracture Zone in the last 5 Ma

Author Posting. © American Geophysical Union, 2003. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 4 (2003): 1024, doi:10.1029/2002GC000364.The Mid-Atlantic Ridge around the Fifteen-Twenty Fracture Zone is unique in that outcrops of lower crust and mantle rocks are extensive on both flanks of the axial valley walls over an unusually long distance along-axis, indicating a high ratio of tectonic to magmatic extension. On the basis of newly collected multibeam bathymetry, magnetic, and gravity data, we investigate crustal evolution of this unique section of the Mid-Atlantic Ridge over the last 5 Ma. The northern and southern edges of the study area, away from the fracture zone, contain long abyssal hills with small spacing and fault throw, well lineated and high-amplitude magnetic signals, and residual mantle Bouguer anomaly (RMBA) lows, all of which suggest relatively robust magmatic extension. In contrast, crust in two ridge segments immediately north of the fracture zone and two immediately to the south is characterized by rugged and blocky topography, by low-amplitude and discontinuous magnetization stripes, and by RMBA highs that imply thin crust throughout the last 5 Ma. Over these segments, morphology is typically asymmetric across the spreading axis, indicating significant tectonic thinning of crust caused by faults that have persistently dipped in only one direction. North of the fracture zone, however, megamullions are that thought to have formed by slip on long-lived normal faults are found on both ridge flanks at different ages and within the same spreading segment. This unusual partitioning of megamullions can be explained either by a ridge jump or by polarity reversal of the detachment fault following formation of the first megamullion.This work was completed while T. Fujiwara was a Guest Investigator at Woods Hole Oceanographic Institution with funding from Japan Marine Science and Technology Center (JAMSTEC), National Science Foundation, and the JAMSTEC Research Overseas Program. J. Lin’s contributions to this research were supported by NSF Grant OCE-9811924. B. E. Tucholke’s contributions were supported by NSF Grant OCE-9503561 and by the Andrew W. Mellon Endowment Fund for Innovative Research and the Henry Bryant Bigelow Chair at Woods Hole Oceanographic Institution

Central Anomaly Magnetization High documentation of crustal accretion along the East Pacific Rise (9°55′–9°25′N)

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 9 (2008): Q04015, doi:10.1029/2007GC001611.Near-bottom magnetic data collected along the crest of the East Pacific Rise between 9°55′ and 9°25′N identify the Central Anomaly Magnetization High (CAMH), a geomagnetic anomaly modulated by crustal accretionary processes over timescales of ∼104 years. A significant decrease in CAMH amplitude is observed along-axis from north to south, with the steepest gradient between 9°42′ and 9°36′N. The source of this variation is neither a systematic change in geochemistry nor varying paleointensity at the time of lava eruption. Instead, magnetic moment models show that it can be accounted for by an observed ∼50% decrease in seismic Layer 2A thickness along-axis. Layer 2A is assumed to be the extrusive volcanic layer, and we propose that this composes most of the magnetic source layer along the ridge axis. The 9°37′N overlapping spreading center (OSC) is located at the southern end of the steep CAMH gradient, and the 9°42′–9°36′N ridge segment is interpreted to be a transition zone in crustal accretion processes, with robust magmatism north of 9°42′N and relatively low magmatism at present south of 9°36′N. The 9°37′N OSC is also the only bathymetric discontinuity associated with a shift in the CAMH peak, which deviates ∼0.7 km to the west of the axial summit trough, indicating southward migration of the OSC. CAMH boundaries (defined from the maximum gradients) lie within or overlie the neovolcanic zone (NVZ) boundaries throughout our survey area, implying a systematic relationship between recent volcanic activity and CAMH source. Maximum flow distances and minimum lava dip angles are inferred on the basis of the lateral distance between the NVZ and CAMH boundaries. Lava dip angles average ∼14° toward the ridge axis, which agrees well with previous observations and offers a new method for estimating lava dip angles along fast spreading ridges where volcanic sequences are not exposed.The research project was funded by National Science Foundation under grants OCE-9819261 and OCE- 0096468

Revised tectonic boundaries in the Cocos Plate off Costa Rica: Implications for the segmentation of the convergent margin and for plate tectonic models

The oceanic Cocos Plate subducting beneath Costa Rica has a complex plate tectonic history resulting in segmentation. New lines of magnetic data clearly define tectonic boundaries which separate lithosphere formed at the East Pacific Rise from lithosphere formed at the Cocos-Nazca spreading center. They also define two early phase Cocos-Nazca spreading regimes and a major propagator. In addition to these sharply defined tectonic boundaries are overprinted boundaries from volcanism during passage of Cocos Plate over the Galapagos hot spot. The subducted segment boundaries correspond with distinct changes in upper plate tectonic structure and features of the subducted slab. Newly identified seafloor-spreading anomalies show oceanic lithosphere formed during initial breakup of the Farallon Plate at 22.7 Ma and opening of the Cocos-Nazca spreading center. A revised regional compilation of magnetic anomalies allows refinement of plate tectonic models for the early history of the Cocos-Nazca spreading center. At 19.5 Ma a major ridge jump reshaped its geometry, and after ∼14.5 Ma multiple southward ridge jumps led to a highly asymmetric accretion of lithosphere. A suspected cause of ridge jumps is an interaction of the Cocos-Nazca spreading center with the Galapagos hot spot