Abstract Laurentia, core of the North American continent, is surrounded by Neoproterozoic to Cambrian rifted margins. This led to early suggestions that it was located within a Neoproterozoic supercontinent, Rodinia. Recent models of Precambrian palaeogeographical development also point to a ‘Laurentia-centric’ Rodinian supercontinent. Before plate tectonics, the geometry of continental margins, comparison of cratonic interiors and sedimentary covers, and orogenic piercing points were employed to postulate the geography of Phanerozoic Pangaea. Marine studies have subsequently demonstrated that the results were remarkably accurate. Absent in situ Precambrian oceanic crust, the same lines of evidence are employed here to reconstruct Rodinia, together with others unavailable at that time. A strong case can be made for the former juxtaposition of the Pacific margins of Laurentia and East Antarctica–Australia approximately as proposed in the 1990s, even though the precise match remains elusive. The Atlantic margin is likely to have rifted from Baltica, Amazonia and other South American cratons along the Grenvillian orogenic suture in the early Paleozoic, although the suture itself makes accurate reconstruction difficult. A piercing point and ‘tectonic tracer’ can be used to position the Kalahari craton and Coats Land crustal block of Antarctica off the present southern margin of Laurentia and contemporaneous large igneous provinces point to Siberia being located off the Arctic margin. Hence Laurentia does appear to be the ‘Key’ to Rodinian palaeogeography even though the exact geometric fit to its surrounding cratons remains to be refined.

Abstract Mesoproterozoic orogenesis is well established on the western and southern flanks of Laurentia in the well-known Racklan–Forward and Mazatzal orogens, but its significance within the previously assembled interior of the supercontinent Nuna has not been established. We examine regional isotopic and structural evidence for Mesoproterozoic deformation in the c. 1.7–1.63 Ga Hornby Bay, Elu, Thelon and Athabasca intracontinental basins, and present evidence for Mesoproterozoic reactivation of Paleoproterozoic structures in the Wopmay and Trans-Hudson orogens. The Racklan–Forward Orogeny in the interior of north Laurentia comprises north–south-trending, high-angle, east-vergent folds and thrusts that occur across a region 1660 km wide and over 1000 km long, stretching from the Yukon to near Hudson Bay and from Banks Island to below the Western Canada Sedimentary Basin. The structures progress from ductile amphibolite and greenschist facies in the Racklan type area to sub-greenschist facies and ultimately brittle or brittle-ductile in the far foreland, showing a predominant thick-skinned style typical of many intracontinental orogens. We present compiled low-temperature thermochronological data, including ages of spatially associated uraninite mineralization, to characterize the scope of reactivation of basement structures in the Archean Rae craton in Nuna's interior. We compare the nature of widespread far-field reactivation in the Racklan–Forward Orogen with other orogens of Nuna's assembly to show it is unusual for Nuna's peripheral margin. We suggest that c. 1.6 Ga continent–continent collision of North Australia with NW Laurentia propagated stresses far into the interior as a result of combined favourable pre-existing structural grain and a weak subcontinental lithospheric mantle in the Rae craton due to repeated episodes of refertilization across 500 Ma of accretion and intrusion. Cratons that experience the complex, two-sided collision and protracted upper plate setting during supercontinent assembly noted herein may be particularly susceptible to extensive foreland propagation of peripheral orogens.

ABSTRACT Supermature siliciclastic sequences were deposited between 1.64 Ga and 1.59 Ga over a broad swath of southern Laurentia in the Archean, Penokean, Yavapai, and Mazatzal Provinces. These siliciclastic sequences are notable for their extreme mineralogical and chemical maturity, being devoid of detrital feldspar and ferromagnesian minerals, containing the clay mineral kaolinite (or its metamorphic equivalent, pyrophyllite), and having a chemical index of alteration >95. Such maturity is the result of a perfect confluence of tectonic and climatic conditions, including a stable continental crust with low topographic relief (the Archean, Penokean, and Yavapai Provinces ca. 1.70 Ga), a warm humid climate, an elevated level of atmospheric CO 2 , and relatively acidic pore fluids in the critical zone. The weathered detritus was transported and deposited by southward-flowing streams across the Archean, Penokean, and Yavapai Provinces, ultimately to be deposited on 1.66 Ga volcanic and volcaniclastic rocks in the Mazatzal continental arc along the southern margin of Laurentia.

ABSTRACT The Mesoproterozoic is a controversial time within the Earth’s history, and is characterized by high temperature/pressure ratios in metamorphic rocks, a large volume of extensional plutons, very few economic mineral deposits, and possibly a slowdown in plate tectonic processes. In Laurentia, ca. 1.48–1.35 Ga is well known as a time of voluminous ferroan magmatism, which led to conflicting tectonic interpretations that range from continental extension to convergent margin settings. Recently, a ca. 1.50–1.35 Ga orogenic belt was proposed that spanned Laurentia from present-day eastern Canada to the southwestern United States. Unlike the preceding Paleoproterozoic Yavapai/Mazatzal orogenies and the subsequent late Mesoproterozoic Grenville orogeny, the early–mid-Mesoproterozoic Picuris orogeny in the southwestern United States was relatively unrecognized until about two decades ago, when geochronology data and depositional age constraints became more abundant. In multiple study areas of Arizona and New Mexico, deposition, metamorphism, and deformation previously ascribed to the Yavapai/Mazatzal orogenies proved to be part of the ca. 1.4 Ga Picuris orogeny. In Colorado, the nature and extent of the Picuris orogeny is poorly understood. On this trip, we discuss new evidence for the Picuris orogeny in the central Colorado Front Range, from Black Hawk in the central Colorado Front Range to the Wet Mountains, Colorado. We will discuss how the Picuris orogeny reactivated or overprinted earlier structures, and perhaps controlled the location of structures associated with Cambrian rifting, the Cretaceous–Paleogene Laramide orogeny, and the Rio Grande rift, and associated mineralization. We will also discuss whether and how the Picuris orogeny, and the Mesoproterozoic in general, were unique within the Earth’s history.

ABSTRACT Analysis of detrital zircon U-Pb ages from the Phanerozoic sedimentary record of central Colorado reveals variability in sediment transport pathways across the middle of the North American continent during the last 500 m.y. that reflects the tectonic and paleogeographic evolution of the region. In total, we present 2222 detrital zircon U-Pb ages from 18 samples collected from a vertical transect in the vicinity of Colorado’s southern Front Range. Of these, 1792 analyses from 13 samples are published herein for the first time. Detrital zircon U-Pb age distributions display a considerable degree of variability that we interpret to reflect derivation from (1) local sediment sources along the southern Front Range or other areas within the Yavapai-Mazatzal Provinces, or (2) distant sediment sources (hundreds to thousands of kilometers), including northern, eastern, or southwestern Laurentia. Local sediment sources dominated during the Cambrian marine transgression onto the North American craton and during local mountain building associated with the formation of the Ancestral and modern Rocky Mountains. Distant sediment sources characterize the remaining ~75% of geologic time and reflect transcontinental sediment transport from the Appalachian or western Cordilleran orogenies. Sediment transport mechanisms to central Colorado are variable and include alluvial, fluvial, marine, and eolian processes, the latter including windblown volcanic ash from the distant mid-Cretaceous Cordilleran arc. Our results highlight the importance of active mountain building and developing topography in controlling sediment dispersal patterns. For example, locally derived sediment is predominantly associated with generation of topography during uplift of the Ancestral and modern Rocky Mountains, whereas sediment derived from distant sources reflects the migrating locus of orogenesis from the Appalachian orogen in the east to western Cordilleran orogenic belts in the west. Alternating episodes of local and distant sediment sources are suggestive of local-to-distant provenance cyclicity, with cycle boundaries occurring at fundamental transitions in sediment transport patterns. Thus, identifying provenance cycles in sedimentary successions can provide insight into variability in drainage networks, which in turn reflects tectonic or other exogenic forcing mechanisms in sediment routing systems.

ABSTRACT The Permian marks a time of substantial climatic and tectonic changes in the late Paleozoic. Gondwanan glaciation collapsed after its earliest Permian acme, aridification affected the equatorial region, and monsoonal conditions commenced and intensified. In western equatorial Pangea, deformation associated with the Ancestral Rocky Mountains continued, while the asynchronous collision between Laurentia and Gondwana produced the Central Pangean Mountains, including the Appalachian-Ouachita-Marathon orogens bordering eastern and southern Laurentia, completing the final stages of Pangean assembly. Permian red beds of the southern midcontinent archive an especially rich record of the Permian of western equatorial Pangea. Depositional patterns and detrital-zircon provenance from Permian strata in Kansas and Oklahoma preserve tectonic and climatic histories in this archive. Although these strata have long been assumed to record marginal-marine (e.g., deltaic, tidal) and fluvial deposition, recent and ongoing detailed facies analyses indicate a predominance of eolian-transported siliciclastic material ultimately trapped in systems that ranged from eolian (loess and eolian sand) to ephemerally wet (e.g., mud flat, wadi) in a vast sink for mud to fine-grained sand. Analyses of U-Pb isotopes of zircons for 22 samples from Lower to Upper Permian strata indicate a significant shift in provenance reflected in a reduction of Yavapai-Mazatzal and Neoproterozoic sources and increases in Grenvillian and Paleozoic sources. Lower Permian (Cisuralian) strata exhibit nearly subequal proportions of Grenvillian, Neoproterozoic, and Yavapai-Mazatzal grains, whereas primarily Grenvillian and secondarily early Paleozoic grains predominate in Guadalupian and Lopingian strata. This shift records diminishment of Ancestral Rocky Mountains (western) sources and growing predominance of sources to the south and southeast. These tectonic changes operated in concert with the growing influence of monsoonal circulation, which strengthened through Permian time. This resulted in a growing predominance of material sourced from uplifts to the south and southeast, but carried to the midcontinent by easterlies, southeasterlies, and westerlies toward the ultimate sink of the southern midcontinent.