NEOPROTEROZOIC TO CAMBRIAN RIFTING IN THE NORTHERN ROCKY MOUNTAINS OF IDAHO AND MONTANA: A TECTONIC MODEL

While workers have established a tectonic framework for much of the Neoproterozoic and early Paleozoic margin of western Laurentia, the central ID/southwestern MT segment is less understood. The earliest Neoproterozoic record in this region is emplacement of the ~780 Ma Gunbarrel dikes in southwestern MT. In northern UT and northeastern WA, localized ca. <760 to 720 Ma clastic sedimentation followed the Gunbarrel event. However, in central ID, the earliest record of sedimentation is much later, after ca. 680 Ma, and is recorded by fine-grained siliciclastic strata, (probably Sturtian) diamictite and reworked tuffaceous rocks. These rocks can be traced from southeastern ID into north-central ID, signaling the development of a regionally continuous rift margin. In central ID, this early episode of rifting was concurrent with extensive isotopically juvenile (εHf_t>0) magmatism recorded by ca. 670-640 Ma syn-depositional zircons, and emplacement of the northwest-trending Big Creek alkalic plutons. After ca. 635 Ma—perhaps coeval with Marinoan glacial drawdown—development of lateral stratigraphic facies and thickness variations suggest localized lithospheric thinning outboard of the preserved sequences as the N-S oriented rift margin was deflected westward around the Mesoproterozoic Belt basin. During this interval, central ID records ca. 600 Ma mafic volcanism and east-central ID experienced a lower magnitude of subsidence, including development of a less-extended block called the Lemhi arch. Low-T thermochronometric data from southwestern MT record heterogeneous ca. 600-510 Ma exhumation, indicating widespread denudation along this segment of the margin. This was followed by late Early Cambrian (ca. 510 Ma) deposition of Flathead Sandstone as the Sauk II trangression flooded central ID and around the Lemhi arch into southwestern MT. Elsewhere along the margin, this was followed by the onset of passive margin carbonate deposition. Along the ID-MT margin, however, limited subsidence above the subaqueous Lemhi arch was flanked on the northeast by a more rapidly subsiding intrashelf basin in southwestern MT. At ca. 500 Ma, emplacement and rapid exhumation of the alkalic Beaverhead plutons within the Lemhi arch signals tectonism associated within the central ID segment of the otherwise “passive” margin.