Magnetotelluric soundings over a Precambrian contact in Australia

Abstract

The Willyama Complex near Broken Hill represents a major exposure of the Precambrian terrain in Australia. However, the eastern limits are obscured by recent cover in the Murray/Darling basin. The nature and extent of the contact with the younger basement to the east is unknown.Magnetotelluric data have now been obtained as an aid to regional mapping. Profiles are based on observations at 14 sites in a traverse east of Broken Hill. Apparent resistivities presented as pseudosections reflect the major structural units identified from surface geology. Continuity between sites is indicated from similarity in apparent resistivities and lateral changes east of the Block are generally gradational.Characteristic divergences in orthogonal resistivity components are consistent with 2-D structure. The foliated Precambrian complex appears to form a semi-vertical contact at the margin of the Murray/Darling basin, thus generating the required anisotropy at depths from 15-35 km. Seismic evidence indicates an equivalent structural contact possibly extending into the mantle.

Origin of High Electrical Conductivity in the Lower Continental Crust: A Review

“…Probably, this is one of the main reasons that relatively fewer field surveys for electrical anisotropy than for electrical depth structure have been carried out. It is not known whether electrical anisotropy in the lower continental crust appears on a global scale; however, the few reports available do show horizontal electrical anisotropy beneath several places in conjunction with high electrical conductivity up to 0.01-0.1 S/m along certain directions, e.g., at Australia shield, Fennoscandian (Baltic) shield, Saxothuringia in central Germany, and Rhenish shield in western Germany (Cull 1985;Rasmussen 1988;Kellett et al 1992;Bahr et al 2000;Leibecker et al 2002).…”

“…This is basically consistent with the reported values of *3 to [20 in the lower continental crust, especially in recent years (Rasmussen 1988;Kellett et al 1992;Bahr et al 2000;Leibecker et al 2002), considering the uncertainty in MT and GDS field surveys and the technical problems in resolving electrical conductivity and layer thickness of conductive zones as pointed out above. In one of the earliest studies by Cull (1985), an anisotropy factor of 400 was reported for the lower crust (at depths of 15-35 km) beneath the Australia shield, which was probably caused by serious technical problems in both the measurement and inversion modeling involved in this very early work. So far, no systematic studies have been conducted on the preferred orientations of plagioclase in the lower continental crust, e.g., on representative mafic xenolith granulites, and thus the possible contribution of hydrous plagioclase to geophysically resolved deep crustal electrical anisotropy requires further investigations.…”

“…1; Hyndman and Shearer 1989); (5) although the spatial resolution of electrical conductivity, especially the vertical depth mapping, needs improving, some scholars have suggested an apparent correlation between high electrical conductivity and seismic reflectivity in some regions (Jones 1987;Hyndman and Shearer 1989;Marquis and Hyndman 1992;Hyndman et al 1993;Warner 2004); and (6) electrical anisotropy along horizontal directions has been reported in many regions, with anisotropy factors ranging from *3 to more than 20 in some areas (Cull 1985;Rasmussen 1988;Kellett et al 1992;Bahr et al 2000;Leibecker et al 2002).…”

“…However, there are still some questions that remain poorly constrained, and one of the most significant concerns electrical properties. It has been recognized for more than *30 years that the lower continental crust is globally characterized by a remarkably high electrical conductivity, e.g., up to 0.01-0.1 S/m, with a magnitude that can vary dramatically from one region to another (Shankland and Ander 1983;Haak and Hutton 1986;Jones 1992;Korja and Hjelt 1993) and whose distribution is probably anisotropic (Cull 1985;Rasmussen 1988;Kellett et al 1992;Bahr et al 2000;Leibecker et al 2002). Nevertheless, the possible origin of these electrical anomalies is a long-standing and controversial issue (e.g., Gough 1986;Jones 1992;Hyndman et al 1993;Glover 1996;Yardley and Valley 1997).…”

Towards an electrical conductivity model for Australia

“…The source used for information on ocean depth is the GEBCO (1982) 1:10000000 chart of the oceans in the Australian region. (Cull & Spence 1983) and, on other con- Figure 1 which shows a simple configuration of 60 m of weathered zone (o= 10"' S.m' 1 ) overlying a crystalline craton (a = 2X 10~4 S.m" 1 ) adjacent to a sediment-filled graben 5000 m thick (a = 10"' S.m" 1 ); the graben is underlain by a resistive basement (a = 10~3 S.m" 1 )-The base of the sheet is at depth 10 km.…”