New Articles for Geosphere Posted Online First

Boulder, Colo., USA: GSA’s dynamic online journal, Geosphere, posts articles online regularly. Topics this month include the Mojave Desert; Obsidian Dome, California; and the Appalachian Inner Piedmont, North Carolina. You can find these articles at https://geosphere.geoscienceworld.org/content/early/recent.

Using discordant U-Pb zircon data to re-evaluate the El Paso terrane: Late Paleozoic tectonomagmatic evolution of east-central California (USA) and intense hydrothermal activity in the Jurassic Sierra Nevada arc
Diane Clemens-Knott; Michelle Gevedon
Quantitative modeling of discordant detrital zircon U-Pb isotope data from the northern El Paso terrane reveals metamorphosed Laurentian passive-margin strata within the Kern Plateau (southeastern Sierra Nevada), resolving a 40-year-long debate regarding this terrane’s origin. Previous studies of Kern Plateau pendants identify deep-water metasediments containing detrital zircon populations similar to the Roberts Mountains allochthon; yet structural observations seemingly contradict proposed correlations to the Mississippian Roberts Mountains thrust, which juxtaposes exotic deep-water rocks over shallow-water, passive-margin strata in central Nevada. Here, new samples are combined with published data to identify segments of the thrust within the Kern Plateau, demonstrating that the El Paso terrane was offset ~350 km by late Paleozoic sinistral translation along the braided Kern Plateau shear zone, an abandoned model first proposed more than 20 years ago. New U-Pb-Hf isotope data from plutons intruding the Kern Plateau shear zone are virtually identical to published data from the El Paso Mountains, indicating that the Sierra Nevada–Mojave arc initiated in the late Early Permian (ca. 274 Ma) along the entire length of the El Paso terrane and was active into the Middle Triassic (ca. 240 Ma). Previously implicated Late Triassic arc activity within the Kern Plateau is not corroborated by single-crystal U-Pb data. Published structural evidence indicating reactivation of the late Paleozoic Kern Plateau shear zone is reinterpreted as indicating sinistral-oblique relative plate motion during Permian arc initiation followed by Middle Jurassic extension in the southeastern Sierra Nevada arc, which facilitated intense hydrothermal activity and zircon lead loss.

Thermal history modeling techniques and interpretation strategies: Applications using QTQt
Alyssa L. Abbey; Mark Wildman; Andrea L. Stevens Goddard; Kendra E. Murray
Advances in low-temperature thermochronology have made it applicable to a plethora of geoscience investigations. The development of modeling programs (e.g., QTQt and HeFTy) that extract thermal histories from thermochronologic data has facilitated growth of this field. However, the increasingly wide range of scientists who apply these tools requires an accessible entry point to thermal history modeling and how these models develop our understanding of complex geological processes. This contribution offers a discussion of modeling strategies, using QTQt, including making decisions about model design, data input, kinetic parameters, and other factors that may influence the model output. We present a suite of synthetic data sets derived from known thermal histories with accompanying tutorial exercises in the Supplemental Material ¹. These data sets illustrate the opportunities and limitations of thermal history modeling. Examining these synthetic data helps to develop intuition about which thermochronometric data are most sensitive to different thermal events and to what extent user decisions on data handling and model set-up can control the recovery of the true solution. We also use real data to demonstrate the importance of incorporating sensitivity testing into thermal history modeling and suggest several best practices for exploring model sensitivity to factors including, but not limited to, the model design or inversion algorithm, geologic constraints, data trends, the spatial relationship between samples, or the choice of kinetics model. Finally, we provide a detailed and explicit workflow and an applied example for a method of interrogating vague model results or low observation-prediction fits that we call the “Path Structure Approach.” Our explicit examination of thermal history modeling practices is designed to guide modelers to identify the factors controlling model results and demonstrate reproducible approaches for the interpretation of thermal histories.

Identifying sources of non-unique detrital age distributions through integrated provenance analysis: An example from the Paleozoic Central Colorado Trough
Tyson M. Smith; Joel E. Saylor; Tom J. Lapen; Kendall Hatfield; Kurt E. Sundell
To address the longstanding issue of provenance interpretation of non-unique detrital zircon age populations, we integrated zircon U-Pb, rare earth element (REE), and εHf(t) data from upper Paleozoic strata in the northern Central Colorado Trough and Cambrian intrusions with petrography, paleocurrent data, and structural and stratigraphic observations. This data set indicates that Cambrian sediment was shed by multiple local sources instead of distant sources hundreds of kilometers away, and it reveals a detailed history of tectonic drainage reorganization in the northern Central Colorado Trough during Ancestral Rocky Mountain deformation. During the Early–Middle Pennsylvanian, Cambrian detrital zircons were a minor constituent of northern Central Colorado Trough sediment. However, during the Late Pennsylvanian–early Permian, westward advancement of the adjacent Apishapa Uplift deformation front precipitated drainage reorganization, which resulted in an episode of dominant Cambrian detrital zircon sourcing. Paleocurrent and petrographic data indicate that the source of Cambrian detritus was shed by an igneous rock body that was ≤15 km northeast of the depocenter, which has since been eroded away or mantled by Tertiary volcanic rocks. The addition of zircon petrochronology to the data set applied here was critical in confirming this hidden source of detritus and elucidating the compositional characteristics of that igneous rock. Zircon εHf(t) provided a regional provenance indicator of a western Laurentian affinity, and REE composition aided in discriminating possible local sources of Cambrian zircon. Furthermore, this work serves as a case study of a dominant Cambrian detrital zircon signature sourced from outside of the well-known Amarillo-Wichita Uplift, and it has implications for the interpretation of such detrital spectra in the context of a direct-from-basement source or the recycling of Cambrian zircon-dominated rocks. In summary, we demonstrate the utility of this multi-provenance proxy approach in interpreting a complex structural history of a dynamic hinterland and concomitant drainage reorganization through an in-depth investigation into the basin record.

Making sense of brittle deformation in rhyolitic lavas: Insights from Obsidian Dome, California, USA
Shelby L. Isom; Graham D.M. Andrews; Stuart Kenderes; Alan G. Whittington
The scarcity of observed active extrusive rhyolitic lava flows has skewed research to extensively focus on prehistoric lavas for information about their eruptive and emplacement dynamics. The first ever witnessed silicic lava eruptive events, Chaitén (2008) and Cordón Caulle (2011–2012) in Chile, were illuminating to the volcanology community because they featured a range of emplacement processes (endogenous versus exogenous), movement limiting modes, and eruptive behaviors (explosive versus effusive) that were often regarded as acting independently throughout an eruptive event. In this study, we documented evidence of a continuum of brittle and brittle-ductile deformation and fracture-induced outgassing during the emplacement of the ~600-yr-old silicic lava from Obsidian Dome, California, USA. This study focused on mapping the textural-structural relationships of the upper surface of the lava onto high-resolution (<10 cm² /pixel) orthorectified color base maps. We found that the upper surface is characterized by small (<1 m) mode 1 tensile fractures that grew and initiated new cracks, which linked together to form larger tensile fractures (1–5 m), which in turn penetrated deeper into the lava. We recorded ornamentations on these fracture surfaces that allow snapshot views into the rheological and outgassing conditions during the lava’s effusion. The largest fractures developed during single, large fracture events in the final stages of the lava’s emplacement. Ornamentations preserved on the fractured surfaces record degassing and explosive fragmentation away from the vent throughout the lava’s emplacement, suggesting explosive activity was occurring during the effusive emplacement. Field-based cataloguing of the complexities of fracture surfaces provides qualitative constraints for the future mechanical modeling of effusive lavas.

Magmatic record of changing Cordilleran plate-boundary conditions—Insights from Lu-Hf isotopes in the Mojave Desert
Keith A. Howard; Stirling E. Shaw; Charlotte M. Allen
Belts of Cordilleran arc plutons in the eastern part of the Mojave crustal province, inboard from the southwestern North American plate boundary, record major magmatic pulses at ca. 180–160 and 75 Ma and smaller pulses at ca. 100 and 20 Ma. This cyclic magmatism likely reflects evolving plate-margin processes. Zircon Lu-Hf isotopic characteristics and inherited zircons for different-age plutons may relate magma sources to evolving tectonics. Sources similar in age to the bulk of the exposed Mojave crust (1.6–1.8 Ga) dominated the magmas. Rare zircons having εHf(t) values as low as −52 indicate that Cretaceous melt sources also included more ancient crustal components, such as Archean-derived detritus in supracrustal gneisses of the Vishnu basin. Some rocks signal contributions from mantle lithosphere (in the Miocene) or asthenosphere (middle Cretaceous). Temporal shifts in isotopic pattern in this sample of the Cordillera relate to cyclic pulses of magmatic flux. Hf-isotopic pull-downs suggestive of dominantly crustal sources characterize the Jurassic and Late Cretaceous flare-ups. The Late Cretaceous flare-up, occurring near the onset of flat- slab subduction, produced abundant Proterozoic xenocrystic zircon and Hf isotopes implicating derivation largely from heterogeneous deep Mojave crust. Isotopic pull-ups characterize the lower-flux middle Cretaceous and Miocene magmatic episodes. The middle Cretaceous pulse ca. 105–95 Ma produced Mojave crust signals but also the isotopically most juvenile magmatic zircons, ranging upward to barely positive εHf values and suspected to signal an asthenosphere contribution. This may point toward transtension or slab retreat causing 105–95 Ma backarc extension in the Mojave hinterland of the Cordillera. That possibility of backarc extension raises questions about the tectonic environment of the contemporaneous main Sierra Nevada high-flux arc closer to the continental margin.

Tectonics, geochronology, and petrology of the Walker Top Granite, Appalachian Inner Piedmont, North Carolina (USA): Implications for Acadian and Neoacadian orogenesis
Arthur J. Merschat; Robert D. Hatcher, Jr.; Scott D. Giorgis; Heather E. Byars; Russell W. Mapes ...
The Walker Top Granite (here formally named) is a peraluminous megacrystic granite that occurs in the Cat Square terrane, Inner Piedmont, part of the southern Appalachian Acadian-Neoacadian deformational and metamorphic core. The granite occurs as disconnected concordant to semi-concordant plutons in migmatitic, sillimanite zone rocks of the Brindle Creek thrust sheet. Locally garnet-bearing, the Walker Top Granite contains blocky alkali feldspar megacrysts 1–10 cm long in a groundmass of muscovite-biotite-quartz-plagioclase-alkali feldspar and accessory to trace zircon, titanite, epidote, sillimanite (xenocrysts), and apatite. It varies from granite to granodiorite and contains several xenoliths of biotite gneiss, amphibolite, quartzite, and in one location encloses charnockite (here formally named Vale Charnockite). New sensitive high-resolution ion microprobe U-Pb zircon magmatic crystallization ages obtained from the plutons of the Walker Top Granite are: 407 ± 1 Ma in the Brushy Mountains; 366 ± 2 Ma in the South Mountains; and 358 ± 5 Ma in the Vale–Cat Square area. An age of 366 ± 3 Ma was obtained from the Vale Charnockite at its type locality. Major-, trace-element, and isotopic chemistry indicates that Walker Top is a high-K, peraluminous granite, plotting as volcanic arc or syn-collisional on tectonic discrimination diagrams and suggests that it represents deep-seated anatectic magma with S- to I-type affinity. The alkali calcic, ferroan Vale Charnockite likely formed by deep crustal melting, and similar geochemical and trace-element compositions suggest a similar tectonic origin as Walker Top Granite. The discontinuous nature of the Walker Top Granite plutons precludes it intruded as a volcanic arc. Instead, the peraluminous nature, common xenoliths of surrounding country rock, and geochemical and isotopic signatures suggest it formed by partial melting of Cat Square and Tugaloo terrane rocks. Following emplacement and crystallization, Walker Top plutons were deformed into elliptical to linear shapes—SW-directed sheath folds—enveloped by partially melted, pelitic and quartzofeldspathic rocks. Collectively, Walker Top and other plutons helped weaken the crust and facilitate lateral crustal flow in a SW-directed, tectonically driven orogenic channel during the Acadian-Neoacadian event. A comparison with the northern Appalachians recognizes a similar temporal magmatic and deformational history during the Acadian and Neoacadian orogenies, although while the Walker Top Granite intruded the lower plate during eastward subduction beneath the peri-Gondwanan Carolina superterrane, the northern Appalachian plutons intruded the upper plate during subduction of the Avalon superterrane westward beneath Laurentia. We hypothesize that a transform fault, located near the southern end of the New York promontory, accommodated oppositely directed lateral plate motion and different subduction polarity between the Carolina and Avalon superterranes during the Acadian and Neoacadian orogenies.

Boudinage and the rheology of syntectonic migmatites in the high-strain Taili deformation zone, NE China
Zhiyong Li; Zuoxun Zeng; Yongjiang Liu
This paper presents a detailed field characterization of boudinage in a high-strain zone several kilometers wide in Northern China to establish relationships between boudin types and rheological contrasts between different parts of migmatites during the migmatization process. This zone contains nearly all types of boudins: foliation boudins, block-torn boudins, pinch-and-swell structures, tapering boudins, object boudins, and modified boudins. These boudinage structures record the different stages of melt-involved and solid-state deformation. The boudinage of migmatites is significantly controlled by the evolving rheological contrasts between the leucosome and melanosome. During the melting stage, the deformation and boudinage of rocks are controlled by the melt fraction. Migmatite strength progressively decreases with increasing melt fraction. The occurrence of melt-filled foliation boudins and melanosome block boudins suggests that the residuum and melanosome are more competent than the leucosome. During solid-state deformation after crystallization, the existence of recrystallized solid-state leucosomes and the intrusion of pegmatites cause the migmatite strength to increase. The relationship is reversed: the leucosome is much more competent than the melanosome. The type and geometry of boudins and pinch-and-swell structures are correlated to the fraction of leucosome in the migmatites. The mechanical strength and strain localization of migmatites after crystallization depend on the presence and volume fraction of the different mineral phases, as well as the mineral grain size. The type and geometry of boudins suggest that the effective viscosity of migmatite can be ranked, from high to low, as: quartz veins; coarse-grained, thick pegmatite; coarse-grained, diatexite migmatite; medium-grained leucosome; and fine-grained melanosome. While experiencing partial melting and migmatization, a rheologically homogeneous protolith is turned into two dominant lithologic domains: a competent diatexite migmatite domain and an incompetent melanosome migmatite domain. Spatially, with the increasing leucosome fraction in migmatites, the migmatite rheology of rock changes from homogeneous to heterogeneous and anisotropic, and then back to homogeneous. The strain distribution likewise changes from uniform to partitioned, and then back to uniform. This evolutionary process of strength and rheological properties of rocks during migmatization may promote strain localization at mid crustal conditions.

GEOSPHERE articles are available at https://geosphere.geoscienceworld.org/content/early/recent. Representatives of the media may obtain complimentary copies of GEOSPHERE articles by contacting Kea Giles at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please refer to GEOSPHERE in articles published. Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

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