Orphans of the far-traveled Pulaski thrust sheet; exotic detached duplexes and their implications for original fold-and-thrust geometry

Orphans are exotic, internally complex, detached duplexes within windows of far-traveled thrust sheets. They were amalgamated to the base of the overriding sheet as allochthonous structures. Internal stratigraphy differs significantly from both super- and subjacent rocks of the enveloping thrust, thus indicating that they originated from a distal ramp location where the stratigraphy was intermediate between that of super- and subjacent rocks. They characteristically occur in windows elongated perpendicular to direction of thrust transport. These complex, antiformal stack types of duplexes include a bounding window-roof fault within the overriding thrust sheet that is antiformal and commonly overturned forelandward, whereas the basal fault of orphans is the smoothly curved primary thrust of the overriding sheet in which they are encapsulated. Within an orphan, structurally higher elements are typically part of an inverted sequence of strata that were duplexed after being overturned and may now be antiformal, thus conforming with the antiformal roof fault. Structurally lower elements are typically part of an upright sequence of the same orphan strata and may be anticlinal. The presence of the same strata in the lower upright and higher inverted elements of an orphan suggests derivation from the overturned footwall syncline of a distal ramp that was detached from the footwall and amalgamated to the overriding thrust sheet. Orphans, which individually contain internally different stratigraphic sequences, may be stacked together. Orphans with older strata lie toward the hinterland and those with progressively younger strata lie successively toward the foreland, suggesting their derivation from successive dismembered footwall ramps separated by intervening flats as the thrust climbed upward. Recognition of orphans has serious implications for balancing cross sections and modeling original fold-fault geometry. Orphans result from structural modification of footwall ramps with overturned footwall synclines, the modification of which changes the structure to apparent fault-bend folds through ramp-smoothing processes that involve reducing ramp angle through dismemberment and removal of the footwall syncline.