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Blind Imbricate Complexes as Exploration Targets in Fold and Thrust Belts

Nicholas B. Woodward

Field mapping of the Absaroka thrust system of Wyoming documents the existence of thrust and fold complexes in lower Paleozoic rocks that never breach the broadly folded, overlying, upper Paleozoic section. Rather than single blind thrusts, the complexes include from two to five 5- to 10-km long imbricate thrusts that die out upward into brecciated, cleaved, and tightly folded rock. The tight lower folds are damped disharmonically upward into broad folds at the Pennsylvanian level. Stylolitic pressure solution cleavage and brittle fractures break the rock into 50 cm to 5 m pieces, and create extensive fracture porosity.

These complexes show flat or shallow-dipping contraction faults in higher stratigraphic units. They are not duplexes, however, because the core faults never merge with any roof thrust. The effect higher in the section is to create large anticlinal structures above the Mississippian that may reach 5 km wide and 20 km long.

In contrast to the expected ramp anticlines in thrust belts, the blind imbricate complexes will probably not lie along parallel, linear, thrust-ramp trends. They provide a new type of exploration target and a model for the deep structure of folds whose size or stratigraphy indicates their origin is not related to ramping. There is no reason why this type of structure should be unique to any one thrust belt or stratigraphic package.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.