HR: 08:00h
AN: T11G-01    [Abstracts]
TI: Lena Trough (Arctic Ocean): An oblique 'amagmatic' rift
AU: * Snow, J E
EM: jesnow@mpch-mainz.mpg.de
AF: Max-Planck Institute for Chemistry, Postfach 3060, Mainz, 55020 Germany
AU: Hellebrand, E
EM: ehelle@mpch-mainz.mpg.de
AF: Max-Planck Institute for Chemistry, Postfach 3060, Mainz, 55020 Germany
AU: Handt, A v
EM: avdh@mpch-mainz.mpg.de
AF: Max-Planck Institute for Chemistry, Postfach 3060, Mainz, 55020 Germany
AU: Nauret, F
EM: nauret@mpch-mainz.mpg.de
AF: Max-Planck Institute for Chemistry, Postfach 3060, Mainz, 55020 Germany
AU: Gao, Y
EM: yjgao@mpch-mainz.mpg.de
AF: Max-Planck Institute for Chemistry, Postfach 3060, Mainz, 55020 Germany
AU: Feig, S
EM: s.feig@mineralogie.uni-hannover.de
AF: Universitaet Hannover , Callinstr. 5, Hannover, 30167 Germany
AU: Jovanovic, Z
AF: Max-Planck Institute for Chemistry, Postfach 3060, Mainz, 55020 Germany
AU: Gauger, S
EM: gauger@fielax.de
AF: Alfred Wegener institut, Columbusstr, Bremerhaven, 27568 Germany
AB: Seafloor spreading is strongly controlled in many important ways by the geometry of the plate boundary. Lena Trough is a recent plate boundary in the Arctic Ocean with a highly oblique plate boundary configuration. It is also, significantly, the gateway for deep water circulation between the Arctic and North Atlantic oceans, and is the most recent and final event in the separation of the North American from the Eurasian continent. Models for the tectonic configuration of Lena Trough have until now differed only in the number and length of fracture zones and spreading segments thought to be present. Most authors admit however, that because of inadequate data coverage, the tectonics of Lena Trough have remained undefined. However, persistent ice cover has prevented any systematic mapping and sampling in most of Lena Trough until now. Here we report new mapping and sampling results from Lena Trough from the ARK XX-2 expedition of PFS Polarstern in summer 2004. Lena Trough is a deep fault-bounded basin with depths of 3800-4800m, and irregular, steep valley sides that are oblique to the spreading direction. Basement horst structures outcrop as sigmoidal ridges with steeply dipping sides project out of the valley floor, and are roughly parallel along flow lines to structures 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 $55\deg$. The basement ridges are composed nearly entirely of mantle peridotite, as are the valley walls. Only at the northern and southern extremities of Lena Trough do basalts appear at all. Peridotite compositions show increasing degrees of melt infiltration near the center of Lena Trough, suggesting magma generation and stagnation in the mantle that defines a first order segmentation at the scale of 100's of kilometers. Otherwise Lena Trough in unsegmented in any normal sense. These results show that Lena Trough belongs to the newly defined amagmatic class of mid-ocean ridge spreading centers, which via an oblique spreading mechanism exposes nearly exclusively ultramafic mantle rocks. At the same time, Lena Trough is a young ocean basin, having only about 9 million years of spreading history, the youngest ocean basin on Earth. Thus Lena Trough represents a transition form between a continental rift and an oceanic one. Without significant basaltic infilling of the nascent ocean basin, subsidence to very great depths (ca. 4500m) probably occurred rapidly after trans-tensional motion began on the plate boundary in the Miocene. The Lena Trough is also the only known modern analog of the Iberia Margin, the conjugate Newfoundland Margin, as well as the ophiolite complexes of the Western Alps.
DE: 9315 Arctic region
DE: 3640 Igneous petrology
DE: 3035 Midocean ridge processes
DE: 1025 Composition of the mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting