WESTWARD GROWTH OF LAURENTIA BY PRE–LATE JURASSIC TERRANE ACCRETION, EASTERN OREGON AND WESTERN IDAHO, UNITED STATES: A REPLY

LaMaskin et al. (2015; J. Geol.), presented evidence to show that deposition of the Coon Hollow Fm. in the Blue Mountains province of Oregon and Idaho spanned ca. 160–150 Ma, entirely during Late Jurassic time. The Coon Hollow Fm. is therefore not a Middle Jurassic overlap assemblage. Instead, new age and provenance data show that the Coon Hollow Fm. is a Late Jurassic overlap assemblage and is associated with a belt of suprasubduction-zone basins that formed ca. 160–150 Ma during a period of trench retreat and slab rollback in the western United States. Thus, the Wallowa terrane was accreted, to and received detritus from, western North America by ca. 160 Ma (early Late Jurassic). To the north in southern British Columbia, outboard terranes (Wrangellia-Stikinia-Cache Creek-Quesnelia) were accreted to the North American plate margin by Middle Jurassic time ca. 175 Ma and to the south, workers have interpreted that any accretion of arc terranes was over by 160 Ma. Thus, any model for Early Cretaceous terrane accretion in the Blue Mountains must explain why accretion in eastern Oregon occurred much later than accretion of other Mesozoic terranes along strike to the north and south. Despite a proliferation in the literature, no documented observations provide evidence for accretion of the Blue Mountains terranes to North America during Cretaceous time. Instead, the concept of Early Cretaceous accretion appears to be based on an assumption that Early Cretaceous peak metamorphism in the SRB must be related to terrane accretion; however, there is no fundamental observation to support, or a reason why, peak metamorphism in the SRB records accretion. As in other terranes of the western US, peak metamorphism was more likely developed during normal subduction along an Andean-type plate margin starting in Early Cretaceous time. We believe it is a mistake to require a unique interpretation for the definition of a geologic province where the affinity and origins of the rocks remain shrouded in uncertainty and debate. Instead, we suggest it will be more productive to identify the rocks on the basis of easily recognized geographical and geological boundaries and continue research to better understand the origins and evolution of this complicated region