ABSTRACT The highest-grade Barrovian-type metamorphic rocks of the North American Cordillera exposed today are Late Cretaceous in age and found within an orogen-parallel belt of metamorphic core complexes for which the tectonic histories remain controversial. Thermobarometric studies indicate that many of these Late Cretaceous metamorphic assemblages formed at pressures of >8 kbar, conventionally interpreted as >30 km depth by assuming lithostatic conditions. However, in the northern Basin and Range Province, detailed structural reconstructions and a growing body of contradictory geologic data in and around the metamorphic core complexes indicate these metamorphic rocks are unlikely to have ever been buried any deeper than ~15 km depth (~4 kbar, lithostatic). Recent models controversially interpret this discrepancy as the result of “tectonic overpressure,” whereby the high-grade mineral assemblages were formed under superlithostatic conditions without significant tectonic burial. We performed several detailed studies within the Snake Range metamorphic core complex to test the possibility that cryptic structures responsible for additional burial and exhumation might exist, which would refute such a model. Instead, our data highlight the continued discordance between paleodepth and paleopressure and suggest the latter may have reached nearly twice the lithostatic pressure in the Late Cretaceous. First, new detrital zircon U-Pb geochronology combined with finite-strain estimates show that prestrain thicknesses of the lower-plate units that host the high-pressure mineral assemblages correspond closely to the thicknesses of equivalent-age units in adjacent ranges rather than to those of the inferred, structurally overridden (para) autochthon, inconsistent with cross sections and interpretations that assume a lower plate with a deeper origin for these rocks. Second, new Raman spectroscopy of carbonaceous material of upper- and lower-plate units identified an ~200 °C difference in peak metamorphic temperatures across the northern Snake Range detachment but did not identify any intraplate discontinuities, thereby limiting the amount of structural excision to motion on the northern Snake Range detachment itself, and locally, to no more than 7–11 km. Third, mapped geology and field relationships indicate that a pre-Cenozoic fold truncated by the northern Snake Range detachment could have produced ~3–9 km of structural overburden above Precambrian units, on the order of that potentially excised by the northern Snake Range detachment but still far short of expected overburden based on lithostatic assumptions. Fourth, finite-strain measurements indicate a shortening (constrictional) strain regime favorable to superlithostatic conditions. Together, these observations suggest that pressures during peak metamorphism may have locally reached ~150%–200% lithostatic pressure. Such departures from lithostatic conditions are expected to have been most pronounced above regions of high heat flow and partial melting, and/or at the base of regional thrust-bounded allochthons, as is characteristic of the spatial distribution of Cordilleran metamorphic core complexes during the Late Cretaceous Sevier orogeny.

The Caves Branch Cave System is unusual because it developed in highly brecciated, nonbedded Cretaceous limestone, it is one of the largest cave complexes in the tropics, and it has a hydrologic architecture that mimics constant hydrochemical “flushing events.” Its complex growth has assembled a multilevel 31 km network, which is the largest of 45 km of caves in the Caves Branch Valley of Belize that have been surveyed following exploration and cave diving since the late 1960s. The valley is a polje entrenched into a mature cockpit holokarst of 200 m relief. After initiation of high, small, isolated phreatic caves, perhaps 200,000 yr of development progressed vertically downward from massive phreatic chambers (one exceeds 300 m in length) to the present active conduit, which has both deep phreatic loops and low water-table gradients. The primary conduit of the Caves Branch Cave System exceeds 15 km in length and parallels the polje on the east. A series of hydraulically restricted cave channels pirate allogenic river water from the polje into the conduit, to mix with high-solute calcite-saturated discharge from the overlying karst. The discharge of sequential wet season storms overwhelms the river waters in the conduit, producing a rise in solute concentration, which then declines with each karst storm flow recession. The pirate river channels and holokarst inputs join from opposite sides of the same conduit at similar elevations, yet have distinctive morphologies. In the absence of bedding, these may be best explained by differences in clastic sediment load, which are more pronounced than differences in chemistry.

ABSTRACT The Gulf of Mexico formed with the rifting and breakup of Pangaea during Triassic and Early Jurassic time. Its formation and the evolution of its tectonic framework as a passive continental margin can be viewed in terms of the interplay of two factors: (1) stresses operating during the rifting of Pangaea and (2) the inherited tectonic fabric of the rifted passive margin that formed at the divergent plate boundary. Various aspects of continental rifting are examined and evaluated in terms of the stresses operating during the rifting process. A revised model involving double indentation tectonics is proposed to explain the suturing of North and South America and the formation of the Ouachita orogenic belt, which consists of a thrusted allochthonous subduction complex, volcanic arc and forearc basins. It also explains the relationship of this belt to the North America craton, with which it collided. This orogenic belt not only frames the Gulf but forms much of the pre-rift basement of the upper Gulf Coast where segments of the volcanic-magmatic arc terrane, possibly represented in the Sabine and Monroe Uplifts and the Wiggins Arch, are now found in the accreted continental margin and appear to have controlled or at least influenced subsequent Mesozoic rifting. The variable aspects of both these interfacing factors are discussed and their influence on the long and active tectonic and sedimentary history of the Gulf Coast and its basins is evaluated. Also the factors that govern the thermal history of the basin sediments are identified. The Gulf of Mexico is unique among passive margins in that the mechanism which caused uplift, rifting, crustal extension, and drift—with accretion of oceanic crust at a spreading center within the Gulf—aborted and stepped southward into the Caribbean, where it formed that plate. In the Gulf, meanwhile, salt, deposited in basins initially formed and controlled by the two interfacing factors during the early opening, became the dominant tectonic mechanism in controlling subsequent sedimentation and facies distribution. Following deposition of the evaporites, the Gulf began rapid subsidence that resulted in formation of a thick prograding sedimentary wedge of both clastic and carbonate strata around its margin. Its final shaping came with Laramide tectonism that not only rejuvenated hinterland source areas to supply voluminous sediment but transported peninsular Mexico eastward to close the Gulf and form a huge sediment trap in which vast thicknesses of Tertiary to Recent sediment were deposited.