title: Revisiting the Seven Devils-Wrangellia connection: the paleogeography of triassic rocks in western Idaho.
Creators: name: Kalk, Michael Liam
Publisher: name: Western Washington University
Year: 2008

description: <p>The origins of and relationships between allochthonous terranes located west of the <sup>87</sup>Sr/<sup>86</sup>Sr 0.706 line (Armstrong et al., 1977) have profound implications for understanding the Mesozoic paleogeography of western North America. The Wallowa- Seven Devils terrane has long been associated with Wrangellia, whose fragments can now be found in British Columbia, Canada and Alaska. However, stratigraphic, fossil, geochemical, structural, and paleomagnetic evidence linking the Wallowa-Seven Devils terrane to Wrangellia is considered equivocal (Follo, 1992). A new paleomagnetic study of the Seven Devils terrane may yield better results than Hillhouse et al. (1982) and, in conjunction with other evidence, support or refute a linkage of the Wallowa-Seven Devils terrane with Wrangellia. Samples from 22 sites located in the Wild Sheep Creek Formation of the Wallowa-Seven Devils arc were demagnetized and analyzed. Resulting data were divided into groups 1, 2, and 3 based on clustering of paleomagnetic directions calculated from last-removed components. Directions of Group 3 (19 sites) are most common and fail the paleomagnetic fold test. Group 1 (2 sites) and Group 2 (2 sites) directions are significantly different from Group 3 and resemble a subset of sites having similar directions obtained by Hillhouse et al. (1982). Reanalysis of two sets of magnetic directions reported by Hillhouse et al. (1982) that were interpreted to represent primary (Triassic) magnetizations reveals additional complexity. Directions from sites 7 and 18 may be biased northward by an unresolved magnetic component. Two new Group 1 sites (this study) have been compiled with three Group 1 sites from Hillhouse et al. (1982). These revised Group 1 sites do not pass either of two examples of paleomagnetic fold tests. Group 1 preserves a reversed field if Group 1 rocks originated in the northern hemisphere. Hillhouse et al. (1982) Group 2 sites were collected from formations that are now interpreted to reside in two distinct tectonostratigraphic terranes. When parsed into their separate terranes, Group 2 sites no longer pass a McElhinny (1964) or a parametric bootstrap (Tauxe and Watson, 1994) fold test. However, direction clustering for both fold tests is highest at 100% untilting suggesting that the Group 2 magnetizations could be primary. Tests comparing the similarity of the directions from the Wallowa and Olds Ferry subgroups indicate that the components used to define the Group 2 magnetization could have been drawn from a similar distribution (cart_hist test of, Tauxe, 1998); this suggests that the Wallowa-Seven Devils and the Olds Ferry terranes shared a common tectonic framework when the Group 2 magnetization was acquired. Present paleomagnetic data cannot dismiss an association of the Wallowa-Seven Devils terrane with Wrangellia, Stikinia, or Quesnellia. Better structural control and geochronology of the Wild Sheep Creek Formation would greatly benefit any future paleomagnetic studies of the Seven Devils terrane.</p>