Special Report 056

Author(s) Date 2003-07-31

Steep crystalline-basement faults in the Alberta and Athabasca basins are commonly expressed as potential-field lineaments. A practical tool for identifying brittle basement faults in northern Alberta was created by detailed processing of public-domain gravity and magnetic data to highlight subtle linear features. Lineaments can be gradient zones, alignments of separate local anomalies of various types and shapes, straight breaks or discontinuities in the anomaly pattern, etc.

Two fundamentally different types of crystalline-basement structure, formed in different tectonic conditions, are recognized in the cratonic, platformal Alberta Basin:

Archean and Early Proterozoic (Hudsonian and older) ductile orogenic structures

Middle Proterozoic to Recent cratonic ones

The latter are usually brittle, high-angle, block-bounding faults. Brittle cratonic structures sometimes follow the older orogenic ones, but commonly cut across them. Brittle block-bounding faults, far more than ancient ductile basement structures, had a controlling influence on basin evolution. In the search for steep brittle faults in Alberta, many of the largest gravity and, especially, magnetic anomalies are undesirable because they represent the ductile, healed, ancient orogenic basement structures. These undesirable, strong anomalies commonly obscure the desirable subtle features. Steep brittle faults, formed and reactivated at different times after cratonization, are marked by potential-field anomalies that tend to be straight and very subtle. Detection of subtle gravity and magnetic lineaments was the primary purpose of making this atlas.

The common subtlety of the desirable, fault-related anomalies necessitates detailed and careful processing of potential-field data, using a wide range of anomaly-enhancement techniques and display parameters. Different processing and display methods reveal different aspects of the anomaly field. Which methods of anomaly enhancement will yield the most geologically meaningful results is often hard to predict in advance. A good practice is to process the data with a multitude of procedures and parameters. Such extensive experimentation, coupled with prior experience, helps to reveal many anomalies of practical interest.

The most geologically meaningful maps, which best highlight subtle potential-field lineaments and anomaly fabrics that could be related to faults, were selected for inclusion in this compilation. This atlas contains 17 gravity and 17 magnetic maps. Each map is accompanied by a text annotation, explaining the processing methods employed and suggesting practical uses of these maps for geological interpetation.

Lyatsky, H.V. and Pana, D.I. (2003): Catalogue of selected regional gravity and magnetic maps of northern Alberta; Alberta Energy and Utilities Board, EUB/AGS Special Report 56, 43 p.