PROVENANCE OF THE MESOPROTEROZOIC UNKAR GROUP AND CORRELATES: A RECORD OF THE TECTONIC EVOLUTION OF THE GRENVILLE OROGENY IN SOUTHERN LAURENTIA

The Late Mesoproterozoic Grenville Orogeny records one of the most significant episodes of orogenesis in Laurentia, producing a Himalayan-scale mountain belt along the eastern and southern margins of the continent. The tectonic evolution of the Grenville Orogeny in eastern Laurentia has been studied in detail and its extension in southern Laurentia is known from isolated exposures in Texas. Although outcrops of the Late Mesoproterozoic core of the Grenville Orogen in southern Laurentia are rare, intracratonic basins of this age are widespread throughout southwest Laurentia and include the Unkar, Apache, and Debaca Groups, Crystal Spring Formation, and sequences in the Franklin Mountains and Van Horn. We examine the provenance of these sequences, through detrital zircon U-Pb-Hf isotopic data and detrital muscovite cooling ages to refine the tectonic setting and re-evaluate correlations between these Late Mesoproterozoic basins. Our goal is to link sedimentation and tectonic events during the Grenville Orogeny in southern Laurentia. The most complete record of Late Mesoproterozoic sedimentation in southwest Laurentia is preserved in the Unkar Group (Grand Canyon, Arizona) where two distinct phases of sedimentation are recognised. The 1.25—1.20 Ga lower Unkar Group (Bass, Hakatai) was deposited in a retroarc basin sourced from a 1.35—1.20 Ga continental arc on the southern margin of Laurentia. An unconformity below the 1.15—1.10 Ga upper Unkar Group (Shinumo, Dox, Cardenas) is attributed to reversal in subduction polarity along the southern margin of Laurentia and coincides with deposition of the upper Unkar Group in a distal foreland basin that contains detrital zircons as young as 1.15 Ga and detrital muscovite cooling ages of 1.16 Ga, which are interpreted to record unroofing of thrust nappes during the early stages of continent-continent collision. Correlation of the Unkar Group with other Late Mesoproterozoic sequences in southwestern Laurentia implies an extensive basin system formed within the continental interior in response to convergence and collision along southern margin of Laurentia. The detrital record of these basins support a new tectonic model for Late Mesoproterozoic sedimentation and further refines the timing of tectonic events during the Grenville Orogeny of southern Laurentia.