ABSTRACT

A new plate tectonics structural model is proposed for the development of the Western Canada Sedimentary Basin (WCSB). This model is based upon the recognition of complex regional rhomboidal fault patterns using Thematic Mapper imagery. Identification of detailed megafracture-fault (zone) patterns by applying selective DEMOFRAC methodology is the essential element in this modelling.

Recognizable patterns represent the regional system of fault zones which delimit the tectonic megablocks, thus forming a distinctive regional rhomboidal to off-orthogonal pattern which characterizes each particular fault or a zone. The complex regional system of faults and fault zones as revealed on the surface reflects the long structural history of the Basin. It has been controlled by the action and interaction of two dominant structural systems, repeatedly reactivated basement faulting and the "thin skinned" tectonics of the overlying sedimentary sequences, and of several tectonic episodes. These systems and episodes have, in turn, been controlled and driven by major plate tectonics events associated with the spreading of the Pacific Ocean floor during the Paleozoic and, later, with the opening and consequent spreading of the Atlantic and Arctic Ocean floors during the Mesozoic and Cenozoic.

Westward transpressional underthrusting of the Precambrian Shield Plate (PSP) beneath the Cordilleran Semi-Plate (CSP) is the basic mechanism of the proposed structural model for the WCSB . This underthrusting resulted from the westward continental drift and collision(?), and was combined with rotation of the PSP relative to the CSP. Subhorizontal detachment zones have developed within the shale and/or other ductile formations beneath "rigid packages" of thick sedimentary sequences during successive episodes of underthrusting, as a result of interaction of recurrent compression with "thin skinned" tectonics. As a consequence, an apparent reverse sliding (or "sliding back") of the overlying sequences seems to have occurred. These sequences would "lag behind" the underlying sequences due to hindrance from the barrier - The Rocky Mountains Thrust and Fold Belts. The final result is a complex and partly displaced surface regional fault pattern as compared with the basement fault pattern.

Due to the existence of local stress fields compressional and extensional structures and features occurred simultaneously with the rotation and lateral shifting of tectonic megablocks. Such conditions should have been more intensive closer to, and along the edge of the "Disturbed Belt".