UNRESOLVED PROBLEMS IN THE PRECAMBRIAN BASEMENT OF SOUTH-CENTRAL SOUTH DAKOTA

The Precambrian basement of south-central South Dakota is a poorly understood confluence of several major Paleoproterozoic crustal domains and structures. The principal domains involved are the Trans-Hudson orogen, extending southward from Canada between the Superior Province (SP) and Wyoming Province (WP), and the geon 17 northern part of the Central Plains orogen (CPO) in Nebraska, a continuation of the Yavapai Province (YP) in Colorado and southern Wyoming, and which continues eastward to the Great Lakes region. In Wyoming the structural boundary between the YP and the WP is the Cheyenne Belt (CB); in the Great Lakes area the structural boundary between the SP-Penokean orogen and the geon 17 basement is the Spirit Lake tectonic zone (SLtz). Although both boundaries could join in central SD, truncating the THO, there are few data to constrain such an interpretation. Rather, general relationships for the CB and the SLtz suggest that they have opposite polarities, complicating potential merger. A more likely scenario is that the structure in SD is more complex, possibly involving a transform offset formed during post-THO rifting, followed by differential convergence of geon 17 YP and CPO terranes toward the resulting, irregular cratonic margin. This implies that the CB and the SLtz are separate, though related, structural boundaries, but this model will have to be tested as better data become available.