Regional electrical resistivity structure of the southern Canadian Cordillera and its physical interpretation

Abstract

Abstract. The regional geoelectric crustal structure of the southern and central Canadian Cordillera of western Canada is interpreted from the inversion of magnetotelluric data along five profiles crossing the physiographic morphogeological belts, with emphasis on the Intermontane and Omineca Belts. Decomposition of the tensor impedance response estimates demonstrates that large-scale regional structures can be reasonably approximated along each profile as two-dimensional, with dominant geoelectric strikes of either -30 ø or +15 ø, depending on profile location. These profile-specific strike directions are consistent with a local clockwise rotation of crustal structures in the southern Intermontane and Omineca Belts suggested by others based on paleomagnetic data and palinspastic reconstructions. Comparing the resistivity models derived from two-dimensional inversions of the distortion-corrected data along each profile allows us to construct an orogen-scale three-dimensional resistivity model for southern British Columbia. Generally, the model shows a resistive upper crust overlying a conductive lower crust. The resistivity of the lower crust beneath the Intermontane Belt is independent of latitude and is similar for all profiles. In stark contrast, a 2 orders of magnitude variation in lower crustal resistivity is observed along strike in the Omineca Belt, with higher conductivities to the south in the region of Eocene extension and lower conductivities to the north in the unextended part of the belt. Such spatial association has noteworthy implications for the cause of lower crustal resistivity in active, or recently active, young regions. Our preferred interpretation of the observed lower crustal resistivities with other geophysical data is in terms of fluids, with brines dominating for the most part but partial melt possible at the base of the crust in specific localities. We attribute the along-strike variation in the Omineca Belt mostly to variation in fluid content and interconnectivity, with the lowermost crust of the southern Omineca Belt being partially molten. This physical state difference is a consequence of degree'of extension and implies that mantlederived fluids are important for lower crustal resistivity.

Structure of the crust in the vicinity of the Banggong‐Nujiang suture in central Tibet from INDEPTH magnetotelluric data

“…MT studies have proven particularly informative in tectonically active regions where fluids and/or melt are present within the lithosphere [e.g., Martinez‐Garcia , 1992; Jones and Dumas , 1993; Jiracek et al , 1995; Wannamaker et al , 1997a, 1997b; Jones , 1998; Partzsch et al , 2000; Ledo and Jones , 2001] because the presence of an interconnected melt or brine increases the bulk conductivity (decreases the resistivity) of the host rock by several orders of magnitude. For example, the laboratory measured resistivity value of dry granite at 500°C is ∼100,000 Ω m, but drops by 5 orders of magnitude when a saline fluid is added [ Olhoeft , 1981; Shankland and Ander , 1983].…”

Deep electrical structure of the northern Cascadia (British Columbia, Canada) subduction zone: Implications for the distribution of fluids

“…With higher temperatures in the backarc, this observation has generally been attributed to partial melting (Brasse et al, 2002). Previous studies in the Cascadia subduction zone have also revealed enhanced electrical conductivities in the backarc of this subduction zone (Gough, 1986;Ledo and Jones, 2001). The depth extent of this conductive zone has remained unresolved because previous magnetotelluric (MT) data were restricted to short periods that failed to sample the lower crust or upper mantle.…”

The electrical resistivity structure of Archean to Tertiary lithosphere along 3200 km of SNORCLE profiles, northwestern Canada

“…(2) There is a significant difference in deep crustal and upper mantle resistivity structure between corridors 2 and 3. The MT data along corridor 2 gives evidence for a conducting lower crust, similar to the southern Cordillera (Jones et al 1992;Jones and Gough 1995;Ledo and Jones 2001). However, the lower crust beneath corridor 3 is unusually resistive (Ledo et al 2004).…”