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The Frontal Thrust Structure seismic line is a northeast to southwest traverse located in the south-central part of Colorado on the eastern flank of the Sangre de Cristo Mountains. The upthrusting of the basement fault block which formed the Sangre de Cristo Mountains displaced the overlying sediments toward the east into the Raton basin syncline. The combination of the block faulting and gravity sliding resulted in a series of east-northeast trending thrusts which affected the Tertiary through Pennsylvanian rocks. Based on the relative thickness of the intervals between the identifiable seismic events on the section, the major detachment zone for these thrusts appears to be between the Pennsylvanian red beds and the Paleozoic carbonate (?) complex. No wells have penetrated beyond the red beds to verify the lithology of the seismic event or the actual detachment zone. Thrusting is evident, however, in several wells drilled in the area. Expanded or repeated beds occur in association with numerous thrusts and are encountered anywhere in the section from Upper Cretaceous to Permo-Pennsylvanian sediments. Overturned strata are occasionally encountered in the thrusted block. In addition, normal faulting is evident behind the main thrust fault zone due to tensional forces exerted on the trailing portion of the thrust block. This resulted in the ramping of the overthrusted plate and the rollover of the overlying materials of the long north to south trending folds.

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