ABSTRACT The Colorado Plateau in the southwestern United States is within the Paleozoic transcontinental arch, an area of thin, cratonic strata. The plateau was broken by latest Mississippian to early Permian Ancestral Rocky Mountain orogenesis, which produced bedrock uplifts that influenced lower Mesozoic sedimentation before Jurassic burial. Clastic sediments shed from uplifts interfinger with eolian Permian strata ultimately derived from eastern Laurentia. Triassic and Jurassic strata of the Colorado Plateau are here divided into five depositional systems, each representing a different sedimentary and tectonic setting and forming stratal associations referred to as “deposystems.” The five deposystems, which largely but not entirely correspond to formation or group names, were deposited during northward continental drift from tropical latitudes (fluvial, tidal, and nearshore marine Moenkopi and fluvial Chinle) through desert latitudes (the erg-dominated Glen Canyon and San Rafael) to temperate latitudes (fluvial Morrison). Paleomagnetically determined paleolatitudes, corrected for inclination shallowing due to postdepositional sediment compaction, place the Glen Canyon and San Rafael eolianites firmly within expected latitudes for desert environmental conditions. Lower Triassic strata of the Moenkopi deposystem form a westward-thickening wedge of fluvial and shallow marine strata and are overlain by entirely fluvial strata of the Chinle deposystem. Both contain 240–280 Ma detrital zircon populations derived from the east Mexico magmatic arc, but more northern Chinle fluvial deposits contain a higher fraction of zircons derived from Paleozoic, Neoproterozoic, and Grenville provinces in eastern Laurentia. Westward thickening of Moenkopi strata is attributed to subsidence in the proforeland basin of the east-vergent Sonoma orogeny in central Nevada, whereas accommodation space for Chinle sedimentation was provided by dynamic subsidence above the upper Triassic subduction zone behind the newly established Cordilleran magmatic arc to the southwest. Overlying, largely Jurassic Glen Canyon and San Rafael deposystems are dominantly eolian. Detrital-zircon geochronologic analysis indicates that eolian sands were derived largely from eastern Laurentia. Interbedded marginal marine, ­lacustrine-sabkha, and fluvial strata have been associated with regional unconformities, but evidence for such unconformities is here regarded as indicating facies transgressions without development of plateau-wide unconformities or disconformities. Upper Jurassic northward continental drift carried the plateau out of the desert belt and into the zone of prevailing westerly winds. This coincided with a flare up of magmatism in the Cordilleran magmatic arc, leading to transgression of Morrison fluvial sediments over erg deposits of the San Rafael deposystem. Eastward dispersal of Morrison sediments marked the initiation of the Cordilleran orogen as the dominant topographic feature of the plateau region.

Abstract Mid-Palaeozoic assembly models for the Arctic Alaska–Chukotka microplate predict the presence of cryptic crustal sutures, the exact locations and deformational histories of which have not been identified in the field. This study presents data on the provenance of polydeformed and metamorphosed strata in the southern Brooks Range Schist Belt and Central Belt of presumed Proterozoic–Devonian depositional age, as well as for the structurally overlying strata, to help elucidate terrane boundaries within the Arctic Alaska–Chukotka microplate and to add new constraints to the palaeogeographical evolution of its constituent parts. The protoliths identified support correlations with metasedimentary strata in the Ruby terrane and Seward Peninsula and suggest a (peri-) Baltican origin in late Neoproterozoic–early Palaeozoic time. Proximity to Laurentia is only evident in what are inferred to be post-early Devonian age strata. By contrast, the North Slope and Apoon terranes originated proximal to Laurentia. The mid-Palaeozoic boundary between these (peri-) Baltican and (peri-) Laurentian terranes once lay between rocks of the Schist/Central belts and those of the Apoon terrane, but is obscured by severe Mesozoic–Cenozoic deformation. Whether this boundary represents a convergent or transform suture, when exactly it formed and how it relates to broader Caledonian convergence in the North Atlantic are still unresolved questions. Supplementary material: Details of the analytical methods together with zircon U-Pb and Lu-Hf isotopic data tables are available at https://doi.org/10.6084/m9.figshare.c.3805696

Cordilleran orogens, such as the central Andes, form above subduction zones, and their evolution depends on both continental shortening and oceanic plate subduction processes, including arc magmatism and granitoid batholith formation. Arc and batholith magma compositions are consistent with partial melting of continental lithosphere and magmatic differentiation, whereby felsic melts rise upward through the crust, leaving a high-density pyroxenite root in the deep lithosphere. We study gravitational removal of this root using two-dimensional thermal-mechanical numerical models of subduction below a continent. The volcanic arc position is determined dynamically based on thermal structure, and formation of a batholith-root complex is simulated by changing the density of the arc lithosphere over time. For the model lithosphere structure, magmatic roots with even a small density increase are readily removed for a wide range of root strengths and subduction rates. The dynamics of removal depend on the relative rates of downward gravitational growth and lateral shearing by subduction-induced mantle flow. Gravitational growth dominates for high root densification rates, high root viscosities, and low subduction rates, resulting in drip-like removal as a single downwelling over 1–2.5 m.y. At lower growth rates, the root is removed over >3 m.y. through shear entrainment as it is carried sideways by mantle flow and then subducted. In all models, >80% of the root is removed, making this an effective way to thin orogenic mantle lithosphere. This can help resolve the mass problem in the central Andes, where observations indicate a thin mantle lithosphere, despite significant crustal shortening and thickening.

Abstract The Greenland and Svalbard Caledonides make up an important part of the Palaeozoic Caledonian orogen, and preserve a complex history of Palaeoproterozoic arc accretion, Proterozoic to Palaeozoic sedimentation within various basins and extensive magmatism, metamorphism and deformation during the Caledonian orogeny. In this summary, the current understanding of the structure and lithological content of the Greenland and Svalbard Caledonides is first reviewed, and open questions are highlighted. The Greenland Caledonides are divided into three different segments, and the term terrane is abandoned for the Svalbard Caledonides. Then, other Caledonian fragments in the Arctic region are discussed, including Bjørnøya, Pearya and Cordilleran terranes and parts of the Barents Shelf. Finally, a regional synthesis covering the geological evolution of the Greenland and Svalbard Caledonides from the Palaeoproterozoic to the end of the Caledonian orogeny is presented and controversial issues and open questions are discussed.

The Mesozoic Peninsular Ranges batholith, part of a long-lived Cordilleran subduction orogen, is located at a critical juncture at the southwest corner of cratonal North America. The batholith is divided into northern and southern segments that differ in their evolution. In this paper, we focus on the more poorly understood southern Peninsular Ranges batholith, south of the Agua Blanca fault at ~31.5°N latitude, and we compare its evolution with the better-known northern Peninsular Ranges batholith. Adding our new insights to previous work, our present understanding of the geologic history of the Peninsular Ranges consists of the following: (1) stronger connections between the Paleozoic passive-margin rocks in the eastern Peninsular Ranges batholith and similar assemblages in Sonora, Mexico, to the east and the Sierra Nevada batholith to the north that were originally proposed by earlier workers; (2) continuity of the Triassic–Jurassic accretionary prism and forearc basin assemblage from the northern Peninsular Ranges batholith through the southern Peninsular Ranges batholith; (3) possible synchronous subduction of an ocean ridge or ridge transform along the Peninsular Ranges batholith in late Middle Jurassic time; (4) continuity of the Early Cretaceous Santiago Peak continental arc from the northern Peninsular Ranges batholith along the entire margin, including the southern Peninsular Ranges batholith; (5) development of the Alisitos oceanic arc in Jurassic and possibly Triassic time, much earlier than originally thought; and (6) removal of part of the Santiago Peak assemblage in the southern Peninsular Ranges batholith during collision of the Alisitos terrane in latest Early Cretaceous time.

The North American portion of the Cordilleran orogen extends continuously from Alaska to southern Mexico, and from east to west over much of its length the orogen comprises an easterly vergent fold-thrust belt, a complexly deformed metamorphic hinterland that collapsed gravitationally, and an interlaced mosaic of exotic terranes. Although most models for the development of the Cordilleran orogen invoke Late Jurassic–Cretaceous intraplate, backarc shortening above an eastwardly dipping subduction zone, a simple collisional model in which the leading edge of North America was subducted to the west, beneath a segmented, arc-bearing microcontinent, better fits the data. During the early Mesozoic, Panthalassic Ocean crust was subducted westward beneath a ribbon continent named Rubia, where it created a generally low-standing continental arc. At about 124 Ma, the widespread deposition of intraformational gravels and conglomerates atop the passive margin marked the passage of the North American shelf over the outer bulge of the trench and its entry into the subduction zone. Loading by the “bulldozed” and thickened accretionary wedge—as well as the overlying eastern edge of the Rubian ribbon continent—depressed the lithosphere to create the Cretaceous foredeep, which migrated eastward during progressive convergence. As the westernmost edge of North America was subducted, the dewatering of slope-rise and rift deposits abruptly created voluminous melts that rose to thicken and assimilate the overlying exotic crust, where they formed Cordilleran-type batholiths. Owing to the difficulty of subducting an old craton, convergence slowed to a halt by 80–75 Ma, causing the shutdown of Cordilleran-type magmatism, and finally, during the Maastrichtian, break-off of the North American plate. The first segment to fail was likely the Great Basin segment, located south of the Lewis and Clark lineament and north of the Sonoran segment. There, slab failure rates were apparently slow enough that there was considerable lithospheric necking, and so slab-failure magmas were prevented from rising into the overriding plate. The diachronous break-off caused a catastrophic stress inversion in both upper and lower plates. Released from its oceanic anchor, the partially subducted edge of the North American craton rose rapidly, causing its stress regime to change from extensional to compressional, which, along with continued convergence, generated the thick-skinned Laramide deformation. Uplift and gravitational collapse of the overlying Rubian plate formed the linear belt of Paleocene-Eocene metamorphic core complexes within the orogenic hinterland. In the Canadian segment, located north of the Lewis and Clark line, the Coast plutonic complex was uplifted rapidly as asthenosphere rose through the torn lower-plate lithosphere to invade Rubia with a 1500-km linear belt of break-off–generated magmas. Within the Sonora segment to the south, break-off magmatism was also prevalent. Both the Canadian and Sonoran segments have abundant porphyry copper mineralization temporally and spatially associated with the break-off magmas, which suggests a genetic link between slab failure and porphyry copper mineralization. By 53 Ma, eastwardly dipping subduction of Pacific Ocean crust was generating arc magmatism on the amalgamated Cordilleran collision zone in both the Canadian and Sonoran segments. Oceanic schists, such as the Orocopia-Pelona-Rand, were formed in the ocean basin west of Rubia and accreted during initiation of the new easterly dipping subduction zone. A major transform fault, called the Phoenix fault, connects the Sevier fold-thrust belt at the California-Nevada border with that in eastern Mexico and separates the Great Basin and Sonoran segments. It juxtaposes the Sierra-Mojave-Sonora block alongside the Transition Zone of the Colorado Plateau. Cordilleran events affected the subsequent development of western North America. For example, the structural Basin and Range Province appears to coincide with the region where exotic allochthons sit atop North American crust in both the Great Basin and Sonoran segments. Also, within the triangular Columbia embayment, large segments of Rubia appear to have escaped laterally during the Cordilleran orogeny to create a lithospheric “hole” that was later filled by basalt of the Columbia River and Modoc plateaux.