HR: 16:45h
AN: T34A-04 INVITED    [Abstracts]
TI: Lena Trough (Arctic Ocean): Active mantle exhumation on a continental rifted margin
AU: * Snow, J E
EM: jesnow@uh.edu
AF: University of Houston, S&R 1, Houston, TX 77204, United States
AU: * Snow, J E
EM: jesnow@uh.edu
AF: Max-Planck Institut fuer Chemie, Postfach 3060, Mainz, 55020, Germany
AU: Hellebrand, E
EM: ericwgh@hawaii.edu
AF: SOEST - University of Hawaii, Dept. of Geology and Geophysics 1680 East-West Road, POST612B, Honolulu, HI 96822, United States
AU: Hellebrand, E
EM: ericwgh@hawaii.edu
AF: Max-Planck Institut fuer Chemie, Postfach 3060, Mainz, 55020, Germany
AU: von der Handt, A
EM: avdhandt@mpch-mainz.mpg.de
AF: SOEST - University of Hawaii, Dept. of Geology and Geophysics 1680 East-West Road, POST612B, Honolulu, HI 96822, United States
AU: von der Handt, A
EM: avdhandt@mpch-mainz.mpg.de
AF: Max-Planck Institut fuer Chemie, Postfach 3060, Mainz, 55020, Germany
AU: Nauret, F
EM: nauret@ipgp.jussieu.fr
AF: Institut de Physique du Globe-Paris, Laboratoire de Geochimie-Cosmochimie Boite 89 - 4 place Jussieu, Paris, 75252, France
AU: Nauret, F
EM: nauret@ipgp.jussieu.fr
AF: Max-Planck Institut fuer Chemie, Postfach 3060, Mainz, 55020, Germany
AB: Lena Trough is the northern continuation of the Mid-Atlantic Ridge through Fram Strait and into the Arctic Ocean. The rifting of Lena Trough began in the Miocene, and significantly, is the final and the most recent event in the separation of the North American from the Eurasian continent. Lena Trough was mapped in 1999, 2001 and 2004 by PFS Polarstern (Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany), revealing sea floor structures that are inconsistent with any normally conceived mid-ocean ridge spreading, and instead indicative of late continental rifting. Lena Trough is shown to be a deep, fault-bounded basin with depths of 3800-4200m, and irregular, steep valley sides that are oblique to the spreading direction. Basement horst structures that outcrop as sigmoidal ridges with steeply dipping sides project out of the valley floor. These basement ridges are roughly parallel along flow lines to the valley walls on either side. Ridge-orthogonal topography is simply absent (ie no segments trending parallel nor fracture zones perpendicular to Gakkel Ridge). Most faults trend approximately SSE-NNW, an obliquity with respect to Gakkel Ridge (SW-NE) of about 55°. The basement ridges are composed nearly entirely of fertile mantle peridotite, as are the valley walls. Only at the northern and southern extremities of Lena Trough do basalts appear at all. The peridotites compositions are consistent with either continental or oceanic (asthenospheric) mantle. They show evidence of low-degree mantle melting, followed by high-level stagnation in a thick lithosphere. This evidence (veining, impregnation) is more evident where little or no basaltic cover is present, while peridotites dredged in the vicinity of basalts tend to be more residual. This may indicate some degree of magmatic focusing in the absence of a basaltic crust per se. Lena Trough contains rare, highly alkaline basalts that are unlike any compositions dredged from mid-ocean ridges. While nearly all alkaline E-MORB have less then 49 wt. % SiO2 and less than 15% Al2O3, the Lena Basalts have nearly 52% SiO2, and 18% Al2O3. This suggests that the melts formed at the quartz eclogite peritectic with residual garnet rather than at the peridotitic peritectic usual for MORB. Their trace element and isotopic characteristics moreover reflect a ubiquitous enriched component found in the Western Gakkel Ridge and the Mohn’s Ridge. We suggest that the Lena Trough has undergone almost no partial melting, and that the rare basalts found there are nearly uniquely the result of melting of early-melting heterogeneities (veins) in the upwelling asthenosphere.
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 8105 Continental margins: divergent (1212, 8124)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting