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Abstract

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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