TECTONIC SUBSIDENCE OF THE EARLY PALEOZOIC PASSIVE CONTINENTAL MARGIN IN SOUTHWESTERN LAURENTIA RE-EVALUATED IN LIGHT OF CHANGES TO THE GEOLOGICAL TIMESCALE

Early Paleozoic tectonic subsidence in the Nopah Range reference section of eastern California (Levy and Christie-Blick, 1991, GSAB) has been re-evaluated in light of changes to the Cambrian-Ordovician timescale. The estimated time of onset of thermal subsidence (τ₀) based conservatively on implied subsidence rates as a function of the square root of time since τ₀ is updated from the late Precambrian-early Cambrian (590-545 Ma; timescale of Harland et al., 1982) to Terreneuvian–Cambrian Series 2 (538.8-505.5 Ma, and perhaps later still; International Commission on Stratigraphy, June, 2023, updated). This constraint is consistent with U-Pb dating of detrital zircons in the syn-extensional Sixtymile Formation of the Grand Canyon, suggesting that continental rifting continued until <508.6 Ma. An early phase of basin inversion associated with the Butte fault and predating deposition of the Tapeats Sandstone (>505.5 Ma and <506.4 Ma) is taken to indicate a change in regional stress within the upper crust. That is consistent with tentative placement of a comparable regional stratigraphic discontinuity within the Zabriskie Quartzite of California (~506.6 Ma, assuming linear extrapolation of age with stratigraphic thickness below the base of the Miaolingian). The misfit of R1 (tectonic subsidence + eustasy) with respect to a best-fit exponential curve provides a measure of the late Cambrian eustatic maximum. However, a eustatic change as large as 261 m during the Miaolingian (a span of 11.15 Myr) is arguably too rapid for a tectonic mechanism and an interval lacking evidence for continental glaciation. We suggest that some fraction of the misfit may relate to a tectonic phenomenon not accounted for by the best-fit exponential. Given a β factor of ~1.5-2.0, yet only modest evidence for upper crustal extension during deposition of the middle Wood Canyon Formation–lower Zabriskie Quartzite (basal Cambrian; ~538.8-506.6 Ma), we infer that thermal subsidence was driven by extension preferentially in the lower crust and upper mantle. Such deformation may have continued into the Miaolingian, after upper crustal extension had ceased. The Noonday Dolomite–Johnnie Formation (early Ediacaran; 635 Ma to ~580 Ma) is thought to represent an earlier phase of passive margin development following Cryogenian rifting.