Jim Craven

Deep electrical conductivity structures of the Appalachian Orogen in the southeastern U.S.

Long period magnetotelluric data across the southeastern Appalachians image deep crustal and upper mantle relics of ancient orogenic events. Inversions of the responses show : (1) Beneath the Appalachian mountains there is a sub-horizontal conductor at 15-20 km depth which dips to the southeast at the surface trace of the Brevard fault. (2) At the location of the Central Piedmont suture, there is a crustal conductor which dips towards the southeast, interpreted as a structure related to the Acadian suture. (3) Upper mantle conductors were found at 80 km depth northwest of the Blue Ridge and at 140 km depth southeast of the Eastern Piedmont. Between these, there is a northwest-dipping resistive gap, possibly representing the remnant structure of the Alleghanian collision.

Regional geoelectrical complexity of the Western Canada Basin from magnetotelluric tensor invariants

Magnetotelluric data from 323 sites in the Western Canada Basin have provided impedance tensors from which invariants based on Mohr circle analysis are calculated. From thresholds determined from an initial examination of the data, the invariant responses are used to explore the dimensionalities of subsurface structures. This leads to a classification of the electrical structure of the region with respect to dimensionality wherein zones with sites of similar nature are delineated, and demonstrates that the invariants can be used as a dimensionality indicator to infer regional geoelectrical complexity. It is observed that the dimensionality appears to increase with period and this result is consistent with the response from sounding a zone of increasing areal extent and may indicate more laterally complex structure at depth, but may also be due to fundamental limitations of galvanic distortion analysis.

Interpretation of resistivity and magnetic anomalies from the Fox River Sill, Trans Hudson Orogen, Canada

The Fox River Belt is a sequence of rocks at the margin of the Proterozoic Trans Hudson Orogen in Canada that have been intruded by the Fox River Sill, a stratiform ultramafic-mafic sill. An earlier 2-D magnetotelluric (MT) study of the sill revealed a conductor that is spatially correlated with a sheared serpentinite unit in the Lower Central Layered Zone of the sill. Re-analysis of the data from 10 MT sites lying on a 1.4 km north-south profile, approximately perpendicular to geological strike, across a 1 km wide portion of the sill produced a resistivity model containing a conductor with an average resistivity of <1 ohm.m. Using aeromagnetic data from a profile subparallel to the MT profile, a geologically constrained magnetic model of the sill was constructed. Empirical susceptibility-magnetic mineral content relationships were used to estimate the magnetite content of the different geological units from the magnetic model. The results indicated a susceptibility of 0.2 SI for the sheared serpentinite unit, suggesting a magnetite content of ~5% which compares with petrological estimates of up to 10%. The bulk resistivity of geological units in the resistivity model was interpreted in terms of metallic mineral content using published resistivity relationships and a range of connectivity models. Integration of these results with magnetic and geological analyses suggests the enhanced conductivity in the sheared serpentinite is a result of a higher degree of magnetite interconnectivity due to the shear fabric. The analysis also reveals that although portions of the adjacent Marginal Zone in the sill contain concentrations of magnetite similar to those in the sheared serpentinite, the significantly higher resistivity of the Marginal Zone can be explained by a lower degree of magnetite interconnectivity.

Electromagnetic imaging of the Alberta Paleo‐Proterozoic basement

Lithoprobe electromagnetic experiments have imaged Paleo-Proterozoic basement structures buried beneath the Western Canada Sedimentary Basin (WCSB). Several highly conductive structures were identified which are predominantly located above the seismically reflective lower crust. The EM data show clear associations between tectonic domains and conductive anomalies. Localized, highly conductive anomalies are interpreted as the euxinic facies deposited in a synorogenic (Paleo-Proterozoic) foredeep. The basis for this assertion is found in the available geophysical, geochronologic and geological information and from contemporaneous analogues on the exposed shield. The model is consistent with the timing, the deformation history and the known geology of the WCSB basement. Interpreting a foredeep places strong constraints on the geodynamic reconstruction of the Early Proterozoic history of the domains beneath the basin. Moreover, the long period EM data are sensitive to the conductivity structure of the upper mantle and suggest a strong correlation between plate tectonic processes and upper mantle structure. When combined, these data may be useful for understanding the basement controls on Phanerozoic sedimentation.