M.S. Yancey, D.B. McClellan, 1983. "Drape Fold, Central Wyoming", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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Basement-block uplift and rotation during the Laramide orogeny caused drape folding and faulting in the overlying sediments. The seismic line shown in Figure 1 was acquired across a faulted drape fold in the Wind River basin (see index map). The drape fold was caused by a scissor-like motion of the underlying granitic basement blocks. This scissor-like motion took place along strike (perpendicular to this section) and died out, along with the drape fold, about 5 mi (8 km) away.
On the unmigrated section (Figure 1), the structure resembles a thrust fault. Figure 2, a marked version of Figure 1, suggests that what appears to be thrusting might, in fact, be a series of stacked bow-ties that were truncated because the seismic line did not extend far enough to the right.
Figure 3 shows a series of synclines in depth and their time response. The time response of these synclines is a stack of symmetric bow-ties, caused because the raypaths focus (intersect) before they reach the surface (Figure 4).
Figure 5 shows what happens if the synclines become asymmetric - the time response is a stack of asymmetric bow-ties. This is very similar to what we see on the seismic section in Figure 1.
Thus, assuming the drape-fold model for this line, it is possible to create a depth interpretation with a vertical limb on the right-hand side that synthetically duplicates the real seismic data (Figures 6 and 7). Figure 8, a raypath plot for this model, indicates that if this is the case, this right-hand limb was not illuminated due to the shortness of the line. (The high angle fault on the left was documented by drilling).
A depth migration of the line is shown in Figure 9. The presence of a noise zone (marked A) with arcing could be interpreted as (1) a near-vertical fault, or (2) a no-signal zone due to the lack of seismic illumination of a near-vertical limb or block. The only way to find out for sure whether (1) or (2) is correct is to extend the seismic line.
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Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces
Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface.
Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant.
Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time.
The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the “real world” in their teaching.
The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.