Publication 91

Lithospheric structure of the Yukon, Northern Canadian Cordillera, obtained from magnetotelluric data

Juanjo Ledo, Alan G. Jones, Ian J. Ferguson and Lisa Wolynec

Abstract

Two goals of Lithoprobe∆s geoscientific studies in the Phanerozoic accretionary cordillera of western North America were to define the subsurface geometries of the terranes and to infer the physical conditions of the crust. These questions were addressed in Canada∆s southern cordillera a decade ago, and have more recently been addressed in the northern cordillera, of which one component of the new studies is magnetotelluric (MT) profiling from ancestral North American rocks to the coast. We present a resistivity cross-section, and its interpretation, of the northern cordillera derived from modeling data from forty-two MT sites along a 470-km-long NE-SW profile. Beneath the Coast Belt (southwestern end of the profile) a deep crustal low resistivity layer dips inland; we interpret the crustal part of this conductor as being due to metasedimentary rocks emplaced and metamorphosed during Paleocene Kula plate subduction. A strong lateral transition in lithospheric mantle resistivity exists below the Intermontane Belt that is spatially coincident with changes in chemical and isotopic characteristics of Tertiary to recent alkaline lavas, suggesting that isotopically-enriched lithosphere related to the Coast Belt basalts extends partly beneath the Intermontane Belt. The unusually high lower crustal resistivity in the Intermontane and Omineca belts, similar in value to the resistivity found in the unextended part of central British Columbia, excludes the presence of fluids or conducting metasediments. Finally, our resistivity model displays strong lateral variation of the middle and lower crust between different terranes within the same belt which contrasts with a Westward Tapering Wedge (WTW) containing a uniform pattern of seismic reflectivity. We take this resistivity variation as indicative of the complex structural evolution of the lithosphere due to superimposed episodes of accretion, subduction, sea-floor spreading and transform faulting.

Source

Journal of Geophysical Research, 109, B04410-1 - B04410-15, doi: 10.1029/2003JB002516, 2004.

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Alan G Jones / 10 June 2004 / alan-at-cp.dias.ie