The Grand Canyon encompasses two billion years of Earth history, but more than half this record is absent across a regionally extensive erosional surface known as the Great Unconformity. This enigmatic surface has been linked to multi-phase regional tectonics accompanying supercontinent assembly and breakup and to Cryogenian glacial erosion. Although rift-related uplift during the breakup of Rodinia has long been implicated in erosion associated with the Great Unconformity, we argue that exhumation in the Grand Canyon region was focused along a continent-scale rift-flank escarpment. This “Great Escarpment of Laurentia,” initiated ca. 800−750 Ma, occupied a position analogous to major escarpments generated during Mesozoic Gondwana breakup and experienced similar magnitudes of exhumation. Our landscape evolution models simulating rift-driven topography suggest that rocks within several hundred kilometers of this feature endured substantial erosion, reaching 8 km near the escarpment and decreasing to ≤2 km at lateral distances of ∼200 km from its peak. Such differential erosion, evidenced in prior thermochronology models, necessitates exhumation of mid-crustal basement lithologies including along reactivated crustal fault blocks. This model helps explain the variable magnitude of erosion associated with the Great Unconformity in the southwestern United States.

Previous studies of crustal thickness variation in the cratonic platform and bordering foreland basin of the USA Midcontinent emphasize that Moho relief exceeds topographic relief by an order of magnitude and exceeds structural relief of the Great Unconformity (the Phanerozoic-Precambrian contact) by a factor of 2 to 3. Consequently, the Moho displays significant local slopes. Unfortunately, traditional receiver-function analysis of depth to the Moho can lead to inaccurate results when applied to steeply sloping Moho. Therefore, to decrease measurement uncertainty due to slopes, we applied the recently developed H-kappa-c receiver-function method (Li et al., 2018) to the northern and part of the eastern Midcontinent of the United States to produce a higherresolution map of Moho-depth (i.e., crustal thickness) variation. Results for the central Midcontinent (including the Ozark Plateau and southern Illinois Basin) were reported in Xiao et al. (2022). Here, we extend our coverage eastward and northward, across the Michigan Basin, Grenville front, and western Appalachian Basin. Our results emphasize that crustal thickness varies by almost 13 km in a region of North America where landsurface varies by less than 0.5 km and where relief of the Great Unconformity varies by a maximum of 7.5 km. In contrast to contemporary orogenic belts, crustal thickness does not correlate directly with either land-surface elevation or with sedimentary cover thickness (i.e., depth to the Great Unconformity). For example, the thickest crust of the study area occurs in the southwestern Illinois Basin, where land surface elevation is about 150 m, and the thinnest crust occurs in north central Indiana, where land-surface elevation is 250 m. There is a rough correspondence between crustal thickness and epeirogenic structures. In general, thinner crust underlies domes and arches, whereas thicker crust underlies basins, but there are exceptions. For example, while crust is relatively thin beneath the Kankakee and Cincinnati Arches, it is relatively thick beneath the Findlay Arch and the Wisconsin Arch, crust beneath the Michigan basin, overall, is thinner than that of the Illinois or Appalachian basins, and crust beneath the Canadian Shield is thinner than that beneath the cratonic platform. Notably, the Moho beneath the Canadian Shield and beneath the cratonic platform west of the Grenville front, displays roughly periodic long-wavelength north-south trending undulations. These undulations do not coincide with Precambrian tectonic fabrics or crustal boundaries, hinting that they developed after crustal assembly and could instead reflect variable degrees of thinning, underplating, or crustal delamination during failed Proterozoic rifting, or perhaps of crustal buckling associated with the Grenville collision.

Three-dimensional inversion of regional long-period magnetotelluric (MT) data reveals the presence of two distinct sets of high-conductivity belts in the Precambrian basement of the eastern U.S. Midcontinent. One set, beneath Missouri, Illinois, Indiana, and western Ohio, is defined by northwest- southeast-oriented conductivity structures; the other set, beneath Kentucky, West Virginia, western Virginia, and eastern Ohio, includes structures that are generally oriented northeast-southwest. The northwest-trend-ing belts occur mainly in Paleoproterozoic crust, and we suggest that their high conductivity values are due to graphite precipitated within trans-crustal shear zones from intrusion-related CO2-rich fluids. Our MT inversion results indicate that some of these structures dip steeply through the crust and intersect the Moho, which supports an interpretation that the shear zones originated as "leaky" transcurrent faults or transforms during the late Paleoproterozoic or the early Mesoproterozoic. The northeast-trending belts are associated with Grenvillian orogenesis and also potentially with Iapetan rifting, although further work is needed to verify the latter possibility. We interpret the different geographic positions of these two sets of conductivity belts as reflecting differences in origin and/or crustal rheology, with the northwest-trending belts largely confined to older, stable, pre-Grenville cratonic Laurentia, and the northeast-trending belts largely having formed in younger, weaker marginal crust. Notably, these high-conductivity zones spatially correlate with Midcontinent fault-and-fold zones that affect Phanerozoic strata. Stratigraphic evidence indicates that Midcontinent fault-and-fold zones were particularly active during Phanerozoic orogenic events, and some remain seismically active today, so the associated high-conductivity belts likely represent long-lived weaknesses that transect the crust.

The Baraboo Syncline of Wisconsin is a south-verging, regional-scale fold that formed during the-1.45 Ga Picuris Orogeny. It involves Mesoproterozoic quartzite and phyllite of the Baraboo Quartzite. Phyllite layers preserve two deformational assemblages. D1 includes the regional-scale flexural-slip to flexural-flow syncline and associated mesoscopic parasitic folds. It also includes spaced cleavage in quartzite layers and north-dipping phyllitic cleavage in phyllite layers. On the gently north-dipping south limb of the regional syncline, parasitic folds verge south (out-of-the-hinge). On this limb, D2 includes north-verging monoclinal kink bands as well as south-dipping asymmetric crenulation cleavage; kink bands have not been found on the north limb. Crenulation growth micro-folded phyllitic cleavage domains into asymmetric sigmoids that are shaped like an 'S' on the Baraboo Syncline's south limb and like a 'Z' on the fold's north limb (as viewed looking east). The origin of D2 structures has long been controversial. Some authors attribute them to a post-syncline phase of extensional tectonism on north-dipping shear zones, whereas others consider them to be a consequence of progressive crustal shortening that continued after D1 fabrics had developed. Our structural analysis supports the second hypothesis and therefore implies that the manifestation of the Picuris Orogeny in southern Wisconsin did not include a post-syncline extensional phase. This interpretation is compatible with a tectonic model in which the Baraboo Syncline formed during inversion of a rift or pull-apart basin during north-south crustal shortening. Specifically, we found no north-dipping normal-sense shear zones, and we demonstrate that the kink bands and asymmetric crenulation of the south limb accommodated a component of north-south shortening in the plane of phyllitic cleavage. Published analog models show that the north-verging kink bands of the south limb could have developed in the same deformation regime as did south-verging parasitic folds. The geometry of crenulation in the north limb indicates that it could be an extensional crenulation accommodating stretching of the limb once it was steep. Late-stage flattening at a high angle to bedding on the south limb locally rotated the crenulation cleavage domains to a sub-horizontal dip. Outcrops in the Baraboo Syncline emphasize that asymmetric cren-ulation is a fungible fabric, in that examples with similar appearance can accommodate different strains and displacements depending on local context.

Abstract The central midcontinent of the USA's cratonic platform is a region of low elevation and relief underlain by tectonic basins, domes, faults, and monoclines. To investigate potential correlations among shallow crustal structure and crustal thickness, we produced a high‐resolution Moho‐depth map of the region by applying the recently developed H‐κ‐c receiver‐function method to data from EarthScope Transportable‐Array and Flexible‐Array stations. Results indicate that Moho depth varies from 38 to 57 km. Changes of Moho depth and of Vp/Vs ratios do not correlate with Precambrian tectonic boundaries, suggesting that they reflect post‐accretion tectonics. Deeper Moho underlies sedimentary basins, implying a relationship between crustal thickness and lithospheric subsidence. Thicker crust may be due to rift‐related underplating, and thinner crust may have undergone delamination. Some depth changes underlie known faults and fold zones. Since short‐wavelength undulations were detected only under high‐density seismic arrays, they may occur more widely but have yet to be resolved.

Unlike well‐known plateaus associated with Cenozoic orogens, the Appalachian and Ozark Plateaus of the eastern United States fringe the foreland side of a long inactive and deeply eroded orogen. These foreland intracratonic plateaus (FIPs), which are underlain by sub‐horizontal cratonic‐platform strata and, in places, foreland‐basin strata, now lie 0.5–1.2 km above sea level, notably higher than adjacent fold‐thrust belts. An escarpment lies at or near the boundary between the FIPs and the fold‐thrust belts. Why did the topographic inversion leading to the development of the FIPs take place? To address this question, we built a numerical model, using Landlab, to simulate how topography evolves as foreland lithosphere flexes upward when post‐tectonic erosion causes unloading. In this model, flat‐lying cap‐rock strata (sandstone and limestone) of the foreland have greater resistance to erosion than do the deformed, tilted, cleaved, and fractured strata of the fold‐thrust belt, especially where the fold‐thrust belt contains argillaceous facies. We tested the model by characterizing the development of the Ozark Plateau in the foreland of the Ouachita fold‐thrust belt. Results demonstrate that regional isostatic uplift due to erosion, given reasonable differences in resistance to erosion between the fold‐thrust belt and the foreland, can generate the observed topographic inversion and a distinct escarpment, yielding a plateau. This model may help explain the post‐Paleozoic evolution of the Catskill Mountains, the Deep Valleys Province, and the Cumberland Plateau, highlands which border the Appalachian fold‐thrust belt.

ABSTRACT Phanerozoic continental subduction zones have produced thick continental crust composed almost entirely of granitoid plutons. While ideas about how plutons form have evolved from models that envisioned large, highly molten magma bodies, the exact processes involved remain debated. Geochronology and seismology have led to the view that plutons form by incremental emplacement; stacked sills represent one type of incremental model whereby granitoids grow top-down by sills underplating their predecessor. Still, many questions remain unanswered, including why sill-like contacts are not often seen in more mature plutons, why the mafic residuum is not observed with many granitoid plutons, why some plutons are compositionally zoned (and others are not), and why geochemical characteristics of intrusions systematically change during magmatic cycles. Here, we propose a hypothesis for the construction of batholiths by amalgamation of plutons formed in a two-stage process. During stage 1, intermediate-composition sills underplate previous sills, forming a moving reaction zone mafic complex that produces a thickening granitoid as the process moves downward. The top of this mafic complex also releases a water-rich, low-temperature silicate liquid (LTSL), which begins ascent by reactive porous flow. During stage 2, the upward flux of LTSL further differentiates the overlying granitoids, increasing silica by 5%–10% and resulting in linear “mixing-like” behavior on Harker diagrams. Multiple plutons building downward in a magmatic cycle thicken the crust, leading to garnet gabbro mafic complexes forming at ~20 km depth. Their high density leads to delamination and net silicification of continental crust. The continuous flux of LTSL up through the arc crust can explain many geochemical spatial-temporal trends found in magmatic cycles, including Pb isotope evolution and increasing Fe3+, and provides a mechanism for addition of water to the upper crust, leading to sustained volcanism through time.

Precambrian (1.4 to 1.5 Ga) granite and rhyolite in the St. Francois Mountains at the northeast corner of the Ozark Plateau in Missouri has been altered down to a depth up to 8 meters below the Great Unconformity (the contact between Paleozoic sedimentary rock and underlying Precambrian). Petrographic, geochemical, and mineralogic data indicate that at least two events generated this alteration: 1) surficial weathering due to subaerial exposure of the granite before Cambrian burial-this material is preserved as a paleosol; and 2) alteration due to reaction with basinal fluids channeled along the unconformity from nearby sedimentary basins long after burial by Paleozoic strata. To assess the variation between surficial weathering and basinal fluid alteration, we measured and sampled for petrologic, geochemical, and mineralogic data in the rock at and just below the Great Unconformity at three paleoelevations. Whole-rock geochemical oxide and X-ray diffraction data indicate that K-metasomatism and highly crystalline illite occurred in each profile. The K increase reflects crystallization of authigenic feldspar and illite from basinal fluids that were channeled along the Great Unconformity during younger Paleozoic fluid-flow events. Each profile also exhibits an upward increase in altered feldspar crystals and highly crystalline kaolinite, and an upward decrease in Ca and Na. Such changes reflect soil formation due to reaction with meteoric water before Cambrian burial, indicating that the altered granite was a paleosol before Paleozoic basinal fluid-flow events. Notably, the paleosol at the highest paleoelevation displays the least amount of paleoweathering and the paleosol at the lowest displays the greatest amount of paleoweathering. These results demonstrate that not only can characteristics of the paleosol just below the Great Unconformity be recognized in the St. Francois Mountains, despite subsequent alteration, but also it is possible to detect variations in soil thickness that were controlled by slope steepness and, therefore, water availability and/or soil creep or failure. This spatial relationship is compatible with studies of modern soils which indicate that soil character varies with position on a slope.

The continental interior of the United States encompasses the region between the Rocky Mountain front and the Appalachia-Ouachita front. It is a region of cratonic platform that represents the southern third of the North American craton. Crust of the continental interior consists of Precambrian basement (formed mostly during Proterozoic accretionary orogeny) overlain, at the Great Unconformity (which formed due to late Proterozoic exhumation), by a relatively thin cover of Phanerozoic stratigraphic sequences that were deposited during transgressions and regressions of shallow seas. Nearer the margins of the interior, clastic wedges derived from bordering orogens spread out onto the craton. The continental interior has not undergone orogenic deformation for about 1 billion years, so it does not display the consequences of penetrative deformation or dynamothermal metamorphism, but it does display the consequences of tectonic activity. This Phanerozoic activity produced several epeirogenic domes, basins, and arches, as well as numerous midcontinent fault-and-fold zones. The fault-and-fold zones likely represent transpressional and transtensional reactivation of Proterozoic rift-related normal faults. The timing of Phanerozoic tectonism in the continental interior of the United States corresponds with the timing of orogenic activity in the continent's marginal orogens, suggesting that stress transmitted into the interior of the continent during orogeny caused continental-interior tectonism. Uplifts related to tectonic events also drove groundwater flow that produced distinct pulses of diagenesis. Though the topography of the continental interior is not as dramatic as occurs in orogenic belts, the region nevertheless contains a rich record of lithospheric evolution through the Phanerozoic.