Petroleum exploration and development in areas of deformed sedimentary rocks commonly must be concerned with the interrelations between sedimentary rock deformation and the structure of the basement. Concern with the changes in the nature and attitudes of structures with increasing depth, and increased emphasis on understanding regional structural styles, necessitate an understanding of expected basement behavior during deformation of the overlying sedimentary rocks.

The basement comprises those igneous and metamorphic rocks of the earth's crust which unconformably underlie the unmetamorphosed, dominantly sedimentary rocks of a particular region. As defined here the term bears no connotation of specific age. Although for most of North America the basement is Precambrian, it is Mesozoic in parts of California and Paleozoic in parts of the New England Upland the Appalachian Piedmont.

The widespread occurrence in the basement of severely deformed metasedimentary rocks in association with igneous intrusives typically reveals a long and complex history of deformation under confining pressures ranging up to 5,000-8,000 bars and at temperatures commonly in the order of 300°-800°C. Thus the environment which originally produced the basement rocks was much different from that of the relatively low energy levels in which the overlying sedimentary rocks were deposited and subsequently deformed, and the resulting mechanical properties of basement-type rocks are very different from those of most sedimentary rocks.

It is clear that the interface between the basement and the overlying sedimentary rocks is a mechanical discontinuity as well as a stratigraphic and structural one. Especially in those regions where the basement was closely involved in the deformation of the superjacent sedimentary rocks is a knowledge of the expected mechanical behavior of the basement essential

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to understanding the regional structural framework and the changes in the nature and attitudes of local structures with increasing depth. Triaxial compression tests by several workers on typical basement rocks indicate that the expected mode of failure of the basement in submetamorphic environments is principally fracturing and faulting. This is consonant with numerous field data from many structural provinces in North America and elsewhere.

Present understanding of sedimentary rock deformation related to basement structures is best for regions of relatively thin sedimentary cover, such as in the foreland of the Rocky Mountains. Here the dominant structure of the Laramide orogeny is basement block faulting of diverse trends and great structural relief. Individual blocks are bounded by upthrust faults which commonly die out upward into monoclinal flexures in the overlying sedimentary rocks. Horizontal compression structures on a wide range of scales may be formed even though differential vertical uplift characterizes the fundamental structural style. The deformation of the sedimentary rocks is demonstrably a direct result of differential movement of discrete fault blocks in the underlying basement. This style of deformatio , so well defined in the Rocky Mountains foreland, is at least partly developed in other structural provinces where differential vertical uplift of a rigid basement with a sedimentary rock cover has been recognized.

The roles of lithologic inhomogeneities and fabric anisotropies of the basement in controlling the locus and style of basement-related deformation of sedimentary rocks are complex and not well understood. Pertinent experimental work on the problem is very limited in amount and scope, and field data commonly are conflicting and inconsistent. The much-cited "zone-of-weakness" concept of basement-related structures is deemed not to have real operational significance in the present state of knowledge, but it remains a promising field for continued study.

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