Abstract

In southeastern Glacier National Park the Lewis thrust is discordant to structures in its hanging wall, strongly suggesting that it formed out of sequence. Faults that are truncated downward against the Lewis thrust are inferred to have originally soled into a structurally lower thrust fault, in which case displacement must have shifted out of sequence from this original fault (here named the Lewis I thrust) to the present-day, structurally higher surface (the Lewis II thrust) at some time during the evolution of the Lewis thrust system. Because of variations in the geometry of the Lewis thrust system along its 450 km strike length, it is likely that the formation of such out-of-sequence thrusts varied in time and in space. Consequently the present-day Lewis thrust may be a composite surface, composed of numerous segments that were not all active at the same time.Many of the fold and fault structures in the study area that are truncated downward agains the Lewis thrust lie within the Spot Mountain duplex, in the basal part of the Lewis allochthon. The Spot Mountain duplex is noteworthy in that both its roof thrust (the Spot Mountain fault) and its floor thrust (the Lewis thrust) formed out of sequence, after most of the structures within the duplex. The term "duplex" in this case has no genetic connotations but refers only to a geometry in which the highly deformed rocks constituting the base of the Lewis thrust plate are separated from less deformed rocks by an enveloping pair of thrust faults.

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