Paleomagnetic and Geochronologic Constraints on the Neoproterozoic Tectonics and Climate of the Western Margin of Laurentia

Abstract

This dissertation focuses on three unique but complementary projects related to Neoproterozoic (1000-542 Ma) tectonics and climate of western North America (Laurentia). I have 1) isolated paleomagnetic evidence for crustal block rotations during rifting along the western margin of Laurentia, 2) refined Neoproterozoic magmatic zircon ages to constrain the age of rifting and onset of global glaciation, and finally 3) created an improved model for the paleogeography of Laurentia in the period leading up to the initial extension of its western margin and the first Cryogenian glaciation. I studied the Mount Harper Volcanics in Canada to understand the rift history of Laurentia and associated crustal rotations. Through mapping and measured sections, I created a robust geological framework for paleomagnetic data. By isolating a primary paleomagnetic pole 50 degrees away from the expected pole, I suggest that the Yukon block rotated counterclockwise relative to Laurentia. My reconstruction facilitates future studies that relate sedimentary and structural patterns to Rodinian rifting and my low latitude pole supports the Snowball Earth interpretation of the related glacial deposits. Next, by studying the strata exposed at Gataga Mountain in Canada, I improved constraints on the nature and timing of Laurentian rifting and the Sturtian Snowball Earth glaciation. I documented three informal sequences: a basal quartzite, the Gataga Volcanics (interpreted to record sub-glacial sedimentation and volcanism), and overlying mixed carbonate-siliciclastics. Using U-Pb chemical abrasion isotope dilution thermal ionization mass spectrometry zircon geochronology, I generated five precise magmatic ages from Gataga Volcanics and a detrital zircon maximum age from underlying non-glacial quartzite that constrains the onset of glaciation. In this research I documented new evidence for the Sturtian glaciation and identified two rift stages: the first indicated by the Gataga Volcanics, and the second by the overlying mixed carbonate-siliciclastics. Finally, my studies took me to the Grand Canyon, where I integrated new paleomagnetic data from the Chuar Group with existing paleomagnetic and geochronological datasets from Laurentia to constrain Neoproterozoic paleogeography and tectonics. In this work I showed that there is paleomagnetic evidence contradicting each of the proposed candidates for the conjugate Laurentian margin. I thus suggest that there may not be a conjugate margin craton for western Laurentia. Instead I posit that the 780-750 Ma southern crustal extension was triggered by strike slip motion along the southwestern Laurentian promontory. Furthermore, post-720 Ma northern crustal extension may have been triggered by the Franklin Large Igneous Province and then propagated down the margin. I speculate that eventual rift event that initiated the Laurentian rift-drift transition may have involved the later rifting of a ribbon continent.