Tectonic Models

Stratigraphic and geochronologic investigations have led authors to propose a variety of tectonic and accretionary models for the Cheyenne belt. Although there are several interpretations related to this area, in the interest of brevity I have chosen those that are fairly recent and that have held some acceptance. Continued study of this area has allowed authors to further refine these models and each is discussed below:

Figure 5 shows a model proposed by Condie, 1982 that attempts to incorporate a Phanerozoic plate-tectonics framework that results in addition of continental crust. Initially, deposition of quartzite-shale assemblages along the southern-facing continental edge is followed by rifting. Basins produced by rifting collect arkose and conglomerate deposits. Bimodal volcanics accumulate as mixed submarine and subaerial deposits near the arc. Subsequent closing of the marginal basin and collision of an offshore arc system create what he terms the “Wyoming Shear Zone” (WSZ) now referred to as the Cheyenne belt. He then proposes uplift and dissection of the shear zone, deposition of sediments in marginal prisms, and development of a north dipping subduction zone, and finally accretion of southern arc terranes. Condies model attempts to explain the absence or rarity of calc-alkalic volcanics common in arc collisional models and the absence of older crust to be rifted, needed for rifting models.

Duebendorfer and Houston, 1987, proposed a similar model that takes into account the observed geologic relationships present in the Cheyenne belt. Figure 6-I illustrates the tectonic setting of the Cheyenne belt prior to or at the early onset of deformation. The authors propose two island arcs with an intervening marginal basin represented by Barber Lake block sediments (BBLB). Subsequent rifting of this intervening basin is favored by the authors to explain the ocean-floor basalt character of the mafic intrusives of the Bear Lake block (BLB).  The Bear Lake block can be interpreted as either a piece of rifted Archean basement or an exotic microcontinent, but the presence of mafic intrusives in the Bear Lake block and Libby Creek group suggest the former. The development of a south dipping subduction zone is followed by accretion of the arc complex (AC) the Barber Lake block and the Bear Lake block to the Wyoming craton. Figure 6-II is a schematic diagram showing a possible model for the development of the Cheyenne belt due to convergent tectonics. This model relies on the assumption that the pre-D1 relative positions of the blocks with respect to the continental margin is similar to their present-day location. D1 structures are interpreted as related to the inception of subduction and results in recumbent folds, subhorizontal mineral lineations and a subhorizontal transposition foliation. Partial melting of Barber Lake metasedimentary rocks may have produced the source magmas for the late syn-kinematic felsic plutonic rocks. The thickened tectonic pile isostatically rose, strain was localized in discrete mylonite zones, possibly due to decreasing temperature. The development of D1 and D2 in the Barber Lake block is considered part of a single protracted event, whereas in the Libby Creek group only D2 is recorded.