INVERTED NEOPROTEROZOIC GRABENS ON WESTERN NORTH AMERICAN AND SIBERIAN RIFT MARGINS: CONJUGATE PIERCING POINTS IN RODINIA?

A newly recognized Neoproterozoic graben, inverted into the Humbolt anticlinorium in the Pioneer Mountains of SW Montana, closely correlates with the inverted Neoproterozoic Kharaulakh graben in the NE Siberian craton. The grabens lie adjacent to the rift margins of their respective cratons – the western rift margin of Laurentia and the eastern rift margin of the Siberian craton. Their detailed rift-to-drift correlations indicate that they may represent conjugate piercing points between Siberia and Laurentia. They plot adjacent to one another on the tightly-constrained Siberia/Laurentia connection of Sears and Price (2003), which matches correlative Paleoproterozoic basement trends and Mesoproterozoic intracratonic basins and magmatic provinces in the Rodinia supercontinent. Nearby Mesoproterozoic strata contain 1470 Ma mafic sills. Each graben lies on a belt of rift-related Ediacaran to Early Cambrian alkalic igneous rocks. Each contains a thick succession of Cryogenian to earliest Cambrian fossiliferous strata. Transgressive Middle Cambrian marine strata overlapped each graben and buried its fill. Both grabens were inverted by Mesozoic orogenesis.

The grabens record uplift and rifting of their respective cratons beginning by 760-735 Ma, proceeding to thermal subsidence by 635 Ma, followed by a late-rift phase beginning by 570 Ma, breakup from 540 to 510 Ma, and passive margin marine transgression beginning by 500 Ma. Both passive margins were magma-poor, but laced by Cambro-Ordovician mafic igneous intrusions. The grabens may have occupied a transform segment of the Siberia/Laurentia rift system.

The reconstruction implies a quadrupolar geodynamo for the Neoproterozoic era.