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TitleMagnetic modelling in the St. Mary Block, Purcell anticlinorium, Canadian Cordillera, British Columbia
AuthorThomas, M D
SourceGeological Survey of Canada, Current Research (Online) 2018-2, 2018, 22 pages,
PublisherNatural Resources Canada
Mediaon-line; digital
File formatpdf
ProvinceBritish Columbia
NTS82F/01; 82F/08; 82F/09; 82G/04; 82G/05; 82G/12
Lat/Long WENS-116.5000 -115.5000 49.6500 49.1333
Subjectsregional geology; structural geology; geophysics; crustal studies; crustal structure; modelling; geophysical surveys; magnetic surveys; aeromagnetic surveys; magnetic anomalies; total field magnetics; magnetic susceptibility; magnetic interpretations; seismic surveys; seismic reflection surveys; seismic interpretations; bedrock geology; basement geology; structural features; faults; folds; anticlinoria; anticlines; synclines; structural trends; lithology; igneous rocks; intrusive rocks; mafic intrusive rocks; gabbros; trend surface analyses; tectonic history; intrusions; sills; unconformities; Canadian Cordillera; Purcell Anticlinorium; St. Mary Block; Hall Lake Block; Moyie Block; Mesoproterozoic; Aldridge Formation; Creston Formation; Perry Creek Fault; Kitchener Formation; Rocky Mountain Trench Fault; Old Baldy Fault; McNeil Fault Zone; Moyie Sills; Precambrian
Illustrationsgeoscientific sketch maps; tables; profiles; cross-sections; models; seismic profiles
ProgramDirector office, GSC Central Canada Division, GSC Central Canada Division
Released2018 05 01
AbstractThe St. Mary Block is a broad half anticline within the Purcell anticlinorium, southeast Cordillera, formed largely of the sedimentary Mesoproterozoic Aldridge and Creston formations, the former intruded by mafic Mesoproterozoic Moyie sills. The Creston Formation is generally associated with a relatively strong magnetic field that includes prominent linear highs. Elsewhere the field is relatively weak and little perturbed. A northeastward-increasing regional magnetic gradient spans much of the northeastern half of the block.
Modelling of magnetic profiles provides insight into upper crustal structure. Modelled sections, some 5-6 km deep, crossing the northwestern half of the area consistently display northwest-dipping (45° to 65°) magnetic units. Magnetic unit boundaries coinciding with faults, e.g. Perry Creek Fault, support a view that faults are northwest-dipping, contrary to an opinion advocating vertical faults. Modelling in localities coinciding with the Kitchener Formation suggests the presence of underlying older Creston Formation at shallow depth, and questions a conformable relationship between the two formations.
Some prominent linear magnetic highs within the Aldridge Formation are modelled as vertical sheet-like bodies, having relatively high magnetic susceptibilities uncharacteristic of the formation. These are interpreted to be gabbroic intrusions distinct from Moyie sills intrusions that generally do not produce a noticeable magnetic high. The northeastern half of the St. Mary Block displays a northeastward-increasing magnetic field. A magnetic model partially constrained by a seismic-reflection model indicates the increase relates to decreasing depth of the basement from roughly 20 km at the outset of the gradient to about 11 km near the Rocky Mountain Trench fault.
Summary(Plain Language Summary, not published)
The St. Mary Block is an half-anticline within the dominantly metasedimentary Mesoproterozoic Purcell anticlinorium, southeast Canadian Cordillera. It is comprehensively mapped geologically, but knowledge of its deeper structure is limited essentially to segments of two seismic lines. Widespread magnetic coverage affords a means to investigate upper crustal structure. Modelling of prominent magnetic anomalies along the northwest flank of the block indicates magnetic units dip between about 45O and 65O northwest consistent with observed surface dips. Units are modelled to depths of roughly 5-6 km. Near and within the centre of the block vertical dyke-like bodies are modelled within weakly magnetic rocks. These are probably mafic intrusions that differ from a group of widespread and relatively weakly magnetic mafic intrusions within the central area of the block. A regional northeastward increase in the magnetic field over the northeast half of the block is explained by a concomitant decrease in depth to the basement.