Fundamental Advances in Structural Geology Based on Ongoing Studies in Reactivation Tectonics

S. Parker Gay, Jr. Applied Geophysics Inc., Salt Lake City, UT 84111 [email protected]

The author has presented papers in years past showing the pervasive nature of basement fault control on structures and stratigraphic features important to petroleum geology. In this paper I will show new findings resulting from ongoing work in reactivation tectonics and how this obsoletes some existing concepts in the field of structural geology. For example, anticlines are one of the most common geological features found in nature and have been mapped and described for over 100 years. Their formation should be well understood, but it isnÕt. Anticlines are nearly always asymmetrical in cross section because they arise from compression across underlying reverse or thrust faults rooted in basement. This compression created the required transverse basement shortening that results in primary closure parallel to the long axis of the anticline. However, because the reactivated basement fault is seldom at right angles to the compressional direction, there is thus a component of longitudinal compression on the anticline, resulting in Òend-closure,Ó rounding out the necessary Ò4-way closureÓ required for trapping.

Additionally, the author has realized that the size of anticlines (i.e. the length) is also controlled by basement, as it is the basement cross-faults that cut an advancing thrust front or reverse fault into segments that later become individual anticlines. A structural dome (as opposed to salt domes, or compactional domes) apparently results when the angle between the underlying fault and maximum compressive stress varies considerably from 90¼. Another geological situation, also not understood previously, is the side-stepping of a fault, or a side-stepping system of faults, frequently confused with en echelon faults. This results when a series of parallel basement faults are reactivated by compressive stress oblique to the faults. Examples from the midcontinent will be shown of all these features.