Chen Yongshun J.

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Chen
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Yongshun J.
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  • Article
    A reduced crustal magnetization zone near the first observed active hydrothermal vent field on the Southwest Indian Ridge
    (American Geophysical Union, 2010-09-21) Zhu, Jian ; Lin, Jian ; Chen, Yongshun J. ; Tao, Chunhui ; German, Christopher R. ; Yoerger, Dana R. ; Tivey, Maurice A.
    Inversion of near-bottom magnetic data reveals a well-defined low crustal magnetization zone (LMZ) near a local topographic high (37°47′S, 49°39′E) on the ultraslow-spreading Southwest Indian Ridge (SWIR). The magnetic data were collected by the autonomous underwater vehicle ABE on board R/V DaYangYiHao in February-March 2007. The first active hydrothermal vent field observed on the SWIR is located in Area A within and adjacent to the LMZ at the local topographic high, implying that this LMZ may be the result of hydrothermal alteration of magnetic minerals. The maximum reduction in crustal magnetization is 3 A/M. The spatial extent of the LMZ is estimated to be at least 6.7 × 104 m2, which is larger than that of the LMZs at the TAG vent field on the Mid-Atlantic Ridge (MAR), as well as the Relict Field, Bastille, Dante-Grotto, and New Field vent-sites on the Juan de Fuca Ridge (JdF). The calculated magnetic moment, i.e., the product of the spatial extent and amplitude of crustal magnetization reduction is at least −3 × 107 Am2 for the LMZ on the SWIR, while that for the TAG field on the MAR is −8 × 107 Am2 and that for the four individual vent fields on the JdF range from −5 × 107 to −3 × 107 Am2. Together these results indicate that crustal demagnetization is a common feature of basalt-hosted hydrothermal vent fields at mid-ocean ridges of all spreading rates. Furthermore, the crustal demagnetization of the Area A on the ultraslow-spreading SWIR is comparable in strength to that of the TAG area on the slow-spreading MAR.
  • Article
    Crustal thickness anomalies in the North Atlantic Ocean basin from gravity analysis
    (American Geophysical Union and the Geochemical Society, 2011-03-31) Wang, Tingting ; Lin, Jian ; Tucholke, Brian E. ; Chen, Yongshun J.
    Gravity-derived crustal thickness models were calculated for the North Atlantic Ocean between 76°N and the Chain Fracture Zone and calibrated using seismically determined crustal thickness. About 7% of the ocean crust is <4 km thick (designated as thin crust), and 58% is 4–7 km thick (normal crust); the remaining 35% is >7 km thick and is interpreted to have been affected by excess magmatism. Thin crust probably reflects reduced melt production from relatively cold or refractory mantle at scales of up to hundreds of kilometers along the spreading axis. By far the most prominent thick crust anomaly is associated with Iceland and adjacent areas, which accounts for 57% of total crustal volume in excess of 7 km. Much smaller anomalies include the Azores (8%), Cape Verde Islands (6%), Canary Islands (5%), Madeira (<4%), and New England–Great Meteor Seamount chain (2%), all of which appear to be associated with hot spots. Hot spot–related crustal thickening is largely intermittent, suggesting that melt production is episodic on time scales of tens of millions of years. Thickened crust shows both symmetrical and asymmetrical patterns about the Mid-Atlantic Ridge (MAR) axis, reflecting whether melt anomalies were or were not centered on the MAR axis, respectively. Thickened crust at the Bermuda and Cape Verde rises appears to have been formed by isolated melt anomalies over periods of only ∼20–25 Myr. Crustal thickness anomalies on the African plate generally are larger than those on the North American plate; this most likely results from slower absolute plate speed of the African plate over relatively fixed hot spots.