Abstract

Basement shear zones, as observed on surface geological maps and airphoto, Landsat, and radar images of outcropping basement on all the world’s Precambrian shields, occur pervasively in parallel sets on the cratons and cut the earth’s crust into a series of separate blocks. These bounding shear zones/weakness zones are reactivated under sedimentary basins in subsequent tectonic events or by later sedimentary or tectonic loading, affecting all younger rocks. This process is termed “reactivation tectonics”, and its reality requires reconsideration of many geological phenomena. For example, one-on-one correlations obtained in the Paradox Basin of the 4-Corners region in the western U.S. between basement shear zones mapped with aeromagnetics and 1) Kelley and Clinton’s map (1960) of the Comb Ridge monocline and 2) Hodgson’s classic study (1961) of jointing showed that basement shear zones controlled the Laramide-age monocline and were also responsible for the joint pattern (Gay, 1972, 1973). In the 35 years since 1973, basement faults have been mapped in sedimentary basins throughout the U.S. with the same rigorous aeromagnetic techniques and compared to the locations of hundreds of known, reliably-mapped faults and stratigraphic features in the sedimentary section. From this work it can be stated definitively that most faults in the sedimentary section (excluding thin-skinned thrusts and “growth faults”) are reactivated basement faults, and that a majority of stratigraphic features also arise from lesser movements of basement faults.

The work on reactivation tectonics has also explained some very common geological features that geologists thought were well understood but weren’t, such as basement involved anticlines and domes. Anticlines are nearly always asymmetrical in cross-section and arise from compression across underlying reverse or thrust faults. This compression created the required transverse basement shortening under the anticlines that results in the primary closure parallel to the long axis of the anticline. However, since the causative reactivated basement fault is seldom at right angles to the compressional direction, there is thus always a component of longitudinal compression on the anticline, resulting in “end-closure”, rounding out the necessary “4-way closure”, of petroleum geologists. Additionally, the author has realized recently 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, with more movement, become individual anticlines.

A structural dome, as opposed to salt domes or compactional domes over underlying basement hills, apparently results when the angle between the underlying fault and maximum compressive stress varies considerably from 90°, for in this situation the longitudinal compression becomes a sizeable percentage of the transverse compression. Another geological situation, also not fully understood previously, is the sidestepping 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 maximum compressive stress oblique to the strike of the faults.

Finally, reactivation tectonics explains the origin of regional jointing in sedimentary rocks and the connection among jointing, fracturing, lineaments and linears, of which the latter two to this day, in spite of their ubiquity, are still considered “controversial”. The clear connection of these features to basement faults, as demonstrated herein, should dispense with that uncertainty.