--> Intra-Formation, “Sub-Seismic Resolution” Deformation in the Lewis Allochthon, Montana, and Arbuckle Anticline, Oklahoma: Preliminary Results and Implications

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Intra-Formation, “Sub-Seismic Resolution” Deformation in the Lewis Allochthon, Montana, and Arbuckle Anticline, Oklahoma: Preliminary Results and Implications

Abstract

The spatial scale over which thrust sheets remain rigid during transport remains an intriguing question. New structural observations from the Lewis allochthon, MT, and the Arbuckle uplift, OK, illustrate the complexity of internal deformation within a thrust sheet. On-going outcrop and photomosaic mapping of exposures of the hangingwall of the Lewis thrust, at Scenic Point, Glacier National Park, reveal the following: (1) km-scale imbricates and duplexes within the basal 600 m of the allochthon, are composite duplexes-in-duplexes (DID's), some of which contain recumbent, tight folds and inverted bedding, analogous to Alpine nappe structures. (2) Disharmonic, overturned, asymmetric folds (amplitudes = cm- to 10's-of-m's) occur within the lower portion of the allochthon at Scenic Point and commonly are rooted by structurally deeper thrusts with tips in the anticlinal core and structurally higher thrusts with tips rooted in the adjacent synclines. (3) Apparent offsets along the same faults observed in different cliff-face perspectives coupled with the non-systematic orientation of drag folds within DID's is consistent with a demonstrable but unknown magnitude of slip out of a given cross section plane. Similar localized structural styles are prominent at diverse scales in the Arbuckle anticline (OK), where late Paleozoic transpressional tectonics produced a series of en echelon W-NW-trending anticlinal uplifts (kms wavelength) and adjacent fault zones. Previous workers have infered 10's of kms of left-lateral shearing associated with the Washita Valley fault (WVF) that bounds the anticline. However, many published reports describe NE-directed shortening and include balanced sections displaying significant shortening perpendicular to the regional W-NW-trending fault. New fault striae data from the WVF do not show a consistent dip-slip or strike-slip vector distribution. Rather, available observations indicate that the WVF is a zone dominated by braided fault networks separating phacoids of variably internally deformed rock - viz., composite duplexes. We hypothesize that such sub-km scale structures may be more prominent in the host rocks surrounding large-slip faults such as the Lewis thrust or the WVF than previously recognized. We speculate that these intra-formation structures may be difficult to detect in conventional seismic imaging and that out-of-plane slip documented here may compromise the accuracy of 2D balanced cross sections.