Megaboudins and lateral extension along the leading edge of a crystalline thrust sheet, Hudson Highlands, New York, U.S.A.

Megascopic lenses of crystalline rock along the leading edge of the Hudson Highland thrust sheet, New York, are boudins that formed as the result of lateral extension during Late Paleozoic compression. These ‘megaboudins’ range from 50 m on each side to 1.2 × 3.2 km and form a NE-trending chain along the northwest side of the Green Pond outlier and Hudson Highlands. The crystalline Hudson Highlands were first thrust northwestward onto shales and siltstones of the Martinsburg Formation during the Middle Ordovician Taconian Orogeny, and later eroded leaving a NE-trending elongate klippe along the leading edge. During subsequent compression, the klippe was laterally extended into a chain of fracture boudins with λ = 3.3. The cleavage and bedding in the surrounding shales and siltstones wraps around the boudins and into the boudin gaps. The cleavage and bedding wrap through 61–114 ° forming shallow to moderately NE-plunging fold axes on the northeast side of the gaps and shallow to moderately SE-plunging fold axes on the southeast side of the gaps. Strike-slip shear indicators at the boudin corners support the mass movement of shale and siltstone. In the wider boudin gaps, conjugate NW-striking normal faults form cross-strike grabens that juxtapose overlying Silurian strata into the gap areas. Fragments of brecciated gneiss from the boudins are also transported into the gap areas along these cross-strike normal faults. The boudins, dextral strike-slip stretching faults along the southeastern margin of the boudins, and asymmetric gap fill geometries are prime criteria for the recognition and characterization of lateral stretching in contractional fold and thrust belts.