THE TECTONIC SIGNIFICANCE OF MESOSCOPIC SUBFABRICS IN THE SOUTHERN ROCKY MOUNTAINS OF ALBERTA AND BRITISH COLUMBIA

Deformation throughout much of the southern Rocky Mountains was characterized by brittle failure in a strongly anisotropic layered sequence of non-metamorphic rocks. On a megascopic scale, the overall structure is dominated by an interlocking system of imbricate thrust plates that have moved relatively eastward or northeastward and upward. On a mesoscopic scale, the principal elements in the fabric of these rocks are fractures that are statistically parallel or perpendicular to the bedding, or else intersect it at preferred angles of approximately 25° or 70°. During deformation many of these fractures obviously were kinematically active, as discrete surfaces of slip that became slickensided, as zones of dilation that became filled with vein minerals, or as surfaces of pressure solution that are now marked by stylolites. Each of these fractures provides a partial record of the kinematics of some stage of the deformation, even when they are considered individually rather than as components in a fracture array whose symmetry is related to that of the movement picture during deformation. Each defines a unique line of slip and axis of rotation for slip, or a unique direction of relative extension or compression. Collectively, they provide a direct and succinct record of the kinematic history of an individual fabric domain, and a sound basis for dynamic analyses of deformation.Some preliminary results of a reconnaissance study of these mesoscopic subfabrics illustrate their tectonic significance.A movement picture can be established for the deformation that occurs within an individual thrust plate during its development and translation. Kinematic relationships between and among the interlocking thrust plates can. be studied.Within a broad area centered along the prominent structural reentrant that crosses the Rocky Mountains near Crowsnest Pass, two different movement pictures occur in superposition. Movement about both northerly and north westerly trending axes can be outlined on the basis of the mesoscopic subfabrics of rocks which, on a megascopic scale, have either a northwesterly or a northerly trending fabric axis. Movement patterns for the deformation associated with each of two regional structural salients converge in the vicinity of the reentrant.The mesoscopic subfabrics associated with transverse faults in parts of the Front Ranges outline a pattern of movement which indicates that they did not originate as tear faults related to the translation of the thrust plates, but instead are probably older gravity faults, whose orientation may be controlled by the fabric of the Hudsonian basement extending beneath the mountains from the Canadian Shield.