Out‐of‐Sequence Thrusts

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Abstract
Out‐of‐sequence thrusts (OOST) are those thrusts which do not obey the foreland propagating or in‐sequence deformation style. They include both isolated thrusts which develop hindward of the thrust front and sequences of break‐back thrusts which propagate from the foreland to the hinterland. Two end‐members of a series of OOST types are recognized: (1) older in‐sequence thrusts which are reactivated along their entire length and (2) completely new thrusts which propagate through already deformed thrust sheets. Between the two end‐members are thrusts composed partially of reactivated in‐sequence thrust sequences and partially of new, entirely out‐of‐sequence segments. OOSTs can be initiated for a variety of reasons including: (1) keeping the orogenic wedge at critical taper, (2) break‐back sequences from the suture zone in the overriding plate, (3) ramping to overcome a sticking point which inhibits in‐sequence thrust propagation, and (4) during simultaneous displacement along two stacked thrusts culminations which bow up segment of the upper thrust may be chopped through to permit continued displacement on the upper thrust. Many different types of thrust behavior including gravity sliding, plucking, and derivation of isolated horses from ramps may mimic some of the characteristics of OOSTs. Consequently, it may be difficult to conclusively prove an OOST origin for a complex thrust geometry.