The paleovalleys of the northern Sierra Nevada, which contain the ancestral river channels, their sedimentary and volcanic deposits, and hosted flora, are critical to understanding the controversial uplift and climatic history of this mountain range. The paleovalleys and their deposits have been used as tilt indicators and as records of climate. However, the paleovalleys evolved over many tens of millions of years during which tectonics, climate, sediment supply, discharge, topography, and sea level varied tremendously. The stratigraphy of Cenozoic prevolcanic (“auriferous”) gravels, Oligocene−Miocene ignimbrites (Valley Springs Formation), and middle Miocene−Pliocene intermediate lavas and volcaniclastic rocks (Mehrten Formation) has long been recognized but poorly and partly incorrectly dated. New geologic mapping, stratigraphic and mineralogic analyses, new and published detrital zircon U-Pb and K-feldspar 40Ar/39Ar dates from gravels, and abundant, high-precision sanidine 40Ar/39Ar dates of ignimbrites significantly change the interpreted ages of gravel and contained flora, resolve many aspects of paleodrainage evolution and geometry, and provide some constraints but do not resolve the long-standing questions about uplift history. The ancestral rivers and valleys probably formed by 80 Ma during erosion of a Late Cretaceous batholith highland. The rivers and valleys initially were eroding, and deposition only occurred at the western marine boundary at the edge of today’s Great Valley. Erosion changed to major deposition in the paleovalleys, although precisely when and why are not resolved. Prevolcanic deposits consist of lower, thin, kaolinite-rich, coarse gravel and sand and upper, thick sand, fine gravel, and mudstone in which the proportion of smectite increases up section. Detrital zircons in Eocene−Oligocene upper gravel (no lower gravel is dated) throughout the northern Sierra Nevada are mostly Cretaceous and lesser Jurassic grains derived from the batholith. Sparse Cenozoic (43−25 Ma) zircons are present in most samples of the upper gravels. The scarcity of Cenozoic grains reflects great distances to Cenozoic igneous sources, which, at 43−40 Ma, were no closer than northeastern Nevada and east of all proposed paleodivides relative to rivers that drained across the Sierra Nevada. The few Cenozoic grains demonstrate that most upper gravel is younger than or ca. 43 Ma, and some gravel mapped as prevolcanic is the temporally intervolcanic Valley Springs Formation. The famed Chalk Bluff flora, long considered to be ca. 50 Ma and used to assign an age to the prevolcanic gravels and interpret paleoelevation and climate, is restricted to the upper, more smectitic gravels and is mostly, maybe entirely, younger than or ca. 40 Ma. Precise sanidine dating reveals that 18 major ignimbrites erupted from calderas in central Nevada between 31.5 Ma and 23.0 Ma and were channelized into paleovalleys that traversed the northern Sierra Nevada. By 31.5 Ma, rivers drained from the Nevadaplano in the Great Basin across the northern Sierra Nevada; the Sierra Nevada−Basin and Range structural and topographic boundary did not exist until after 23.0 Ma. With some overlap, ignimbrites become younger southward through the Sierra Nevada because magmatism in source areas migrated southwestward. Despite repeated incision between ignimbrites, thick tuffaceous deposits largely filled the paleovalleys and induced two major course changes to intervolcanic channels. Middle Miocene and younger lavas and volcaniclastic rocks filled shallow paleovalley remnants and spread extensively over low-relief interfluves until ca. 4−3 Ma. Sierra Nevada and Great Basin paleovalleys were much wider than deep, ∼6−8 × ≤1 km, which suggests similar evolution. Modern, deep, V-shaped canyons of the northern Sierra Nevada generally postdate the 4 Ma rocks, cut across paleovalleys, and did not reexcavate them. Ignimbrite distributions and detrital dates confirm many of the previously described paleovalleys but also reveal significant inconsistencies. Gravel in a major paleovalley that can be tracked from western Nevada based on correlative ignimbrite sections has the common Cretaceous > Jurassic detrital pattern, but dated sediments in the two closest possible downstream continuations lack Cretaceous zircons. The most likely explanation for these discrepancies is that some generally accepted ancestral river courses are wrong, and we suggest several alternative courses. The distributions of ignimbrites younger than or ca. 31.5 Ma in the northern Sierra Nevada and Great Basin document a north-south paleodivide to east-draining rivers through central Nevada. Ignimbrite distributions and published stable isotope data suggest this divide existed by ca. 45 Ma. In contrast, several studies interpret a Late Cretaceous paleodivide that followed the Late Cretaceous batholith north-northwest through the Sierra Nevada and then northeast through northwestern Nevada. Abundant 100−85 Ma detrital zircons in all samples north of ∼39°N, where batholithic rocks with U-Pb zircon ages younger than 100 Ma are within 10 km of the Nevada border or farther east, require a paleodivide at least as far east as northwestern Nevada during all upper gravel deposition. Although not resolving the relative contributions of Eocene or older and latest Cenozoic uplift, our data indicate uplift of the northern Sierra Nevada between ca. 43−40 Ma and 4 Ma is unlikely. Proposed late, younger than or ca. 5 Ma, uplift of the northern Sierra Nevada would have been coeval with both the latest down-to-the-east faulting/extension that generated the eastern structural/topographic margin of the range and the interpreted ca. 3.5 Ma, delamination-induced uplift in the southern Sierra Nevada. However, the magnitude of extension younger than or ca. 5 Ma probably was insufficient to generate the proposed 1−1.5 km of footwall uplift, and the northern Sierra Nevada lacks the distinctive potassic, basaltic magmatism that accompanied delamination. The enigma of apparent high elevation in the Eocene and late uplift remains.
Polymetamorphosed mafic and pelitic schists of the Worcester Mountain Complex in Stowe, Vermont, northeastern United States, contain a record of subduction zone metamorphism during the Ordovician Taconic orogeny. These rocks occur south of, and along strike of, the Taconic high- and ultrahigh-pressure rocks of Belvidere Mountain and Tillotson Peak. Because the Worcester Mountain Complex preserves minerals that formed during the Taconic orogeny, this key location is ideal to test the hypothesis that additional localities in the northern Appalachian Mountains reached high-pressure metamorphic conditions during the Taconic orogeny. We present trace-element thermobarometric data on inclusions in garnet (Zr-in-rutile; Ti-in-quartz), phengite and biotite 40Ar/39Ar data, and phase equilibria calculations for pelitic schist samples from the Worcester Mountain Complex. Results indicate garnet growth occurred during two distinct episodes that can be correlated with regionally recognized structural fabrics. Garnet cores nucleated at amphibolite-facies conditions (∼0.8 GPa and ∼520 °C), whereas garnet mantles/rims grew at eclogite-facies conditions ranging from 1.35 GPa to 1.75 GPa and 630−695 °C. Phengitic white mica (up to 3.22 Si atoms per formula unit) and biotite preserve Taconic 40Ar/39Ar ages (469−459 Ma) that are concordant with 40Ar/39Ar ages on glaucophane from Tillotson Peak. Our peak pressure estimates are significantly higher than previous constraints and further demonstrate that rocks within the Taconic suture zone in the northern Appalachian Mountains preserve evidence for eclogite-facies metamorphism. Reevaluation of high-pressure metamorphic rocks within the Taconic suture zone in the Appalachian orogen is warranted to document their distribution, and to determine how they formed and how they were exhumed.
The Midcontinent Rift system (ca. 1.1 Ga) is a 2000-km-long series of elongated volcanic and sedimentary troughs and associated intrusive centers exposed chiefly in the Lake Superior region of North America. The rift system represents a long history of intense magmatism and subsequent sedimentation that was arrested by far-field tectonic events before sea-floor spreading was established. The premature cessation preserved a record of processes related to the beginning of continental rifting. The rift system under Lake Superior has been long studied using seismic-reflection data collected as part of the Great Lakes International Multidisciplinary Program on Crustal Evolution (GLIMPCE). We reexamine GLIMPCE Line C by developing a detailed velocity model for time to depth conversion constrained by other legacy data. We corroborate the model and develop a geologic interpretation using gravity and magnetic modeling and ties to geology mapped onshore. We recognize superposed subsiding sedimentary and volcanic basins for the southern half of the Line C depth section. This interpretation differs from previous paradigms that show major crustal faults that bound half-grabens or full grabens. We conclude that high-velocity (6.9 km/s) intrusive zones rather than major crustal faults border the sides of the basins. We speculate that the volcanic basin represents the initiation of seaward dipping reflectors. The syn-magmatic subsidence can be explained by dike injection and volcanic loading. Discrete lava basins throughout the region likely subsided at different times in a disorganized manner along the rift trend, raising questions about the long-term role of lithospheric thinning and melt generation.
The spatial extents of buried Precambrian basement terranes and tectonic features are poorly known below the Allegheny Plateau of New York and Pennsylvania, USA. Geological features around the plateau periphery and geophysical variations within the plateau imply the occurrence of both Mesoproterozoic Grenville province terranes and Neoproterozoic basins associated with Iapetan rifting of the Laurentian margin. Cornell University explored geothermal energy reservoirs below Ithaca, New York, by drilling borehole ESH 1 to 2978 m depth, ending 119 m below the Cambrian-basement unconformity. The substantial dataset for the upper part of the basement enables characterization of the lithologic sequence, which is interpreted to be the fill of a Neoproterozoic basin, metamorphosed to greenschist grade. Petrophysical properties differ among four intervals 18−44 m thick, further divided into meter-scale intervals of repetitively variable properties. Wireline logs and drill cuttings for all four intervals demonstrate high abundance of minerals containing structural water (e.g., phyllosilicates) and potassium (muscovite, biotite, K-feldspar). Cuttings and six sidewall cores reveal fine-grained metamorphic rocks derived from both sedimentary and volcanic protoliths. K-feldspar dominates the upper petrophysical zone while phyllosilicates and quartz dominate the lower three intervals, consistent with alteration of the shallowest basement. Downhole patterns of density and gamma variations are similar to those of sedimentary fills of documented rift basins elsewhere and distinctly different than patterns in high-grade metamorphic and plutonic basement. Applying to other regional wells the criteria developed here may improve mapping of basement boundaries and facilitate identifying Iapetan basins below the Allegheny Plateau.
The 50-km-long Thousand Lake fault (TLF) aligns with a series of west-dipping normal faults forming an ∼200-km-long structural discontinuity along the eastern edge of the Basin and Range in south-central Utah. Quantifying a fault’s slip rate and slip rate variations is important for characterizing earthquake hazards and understanding how deformation within fault networks is accommodated through time. This study uses cross-section analyses to determine that the central TLF has displaced Cenozoic volcanic rocks by 1200−2200 m since fault initiation at 15−10 Ma, implying a long-term slip rate of ∼0.1−0.2 mm/a. In contrast, displacement measurements across a Late Pleistocene fan in Bicknell, Utah, along with luminescence and 3He geochronology, allow us to infer that the fault slip rate is only 0.03−0.08 mm/a over the late Pleistocene. The TLF is an example of a Basin and Range normal fault that has experienced significant slip rate variation over geologic time. Trenching of the fan’s primary fault scarp demonstrates that the last two earthquakes have occurred since 52.8 ± 8.5 ka, and the most recent earthquake occurred after 19.7 ± 4.7 ka. Finally, lidar-based geomorphic and fault scarp mapping along the entire fault shows that the last earthquake clearly post-dates the Last Glacial Maximum, rupturing the ground surface with vertical displacements of ∼0.8−1.4 m for ∼32 km in an ∼Mw 6.8 earthquake.
