Geosphere

Journal: Geosphere

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Geological Society of America

ISSN

1553-040X

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Publications 1 - 10 of 22

The rise and demise of deep accretionary wedges: A long-term field and numerical modeling perspective
Angiboust, Samuel; Menant, Armel; Gerya, Taras; et al. (2022)
Geosphere
Several decades of field, geophysical, analogue, and numerical modeling investigations have enabled documentation of the wide range of tectonic transport processes in accretionary wedges, which constitute some of the most dynamic plate boundary environments on Earth. Active convergent margins can exhibit basal accretion (via underplating) leading to the formation of variably thick duplex structures or tectonic erosion, the latter known to lead to the consumption of the previously accreted material and eventually the forearc continental crust. We herein review natural examples of actively underplating systems (with a focus on circum-Pacific settings) as well as field examples highlighting internal wedge dynamics recorded by fossil accretionary systems. Duplex formation in deep paleo-accretionary systems is known to leave in the rock record (1) diagnostic macro- and microscopic deformation patterns as well as (2) large-scale geochronological characteristics such as the downstepping of deformation and metamorphic ages. Zircon detrital ages have also proved to be a powerful approach to deciphering tectonic transport in ancient active margins. Yet, fundamental questions remain in order to understand the interplay of forces at the origin of mass transfer and crustal recycling in deep accretionary systems. We address these questions by presenting a suite of two-dimensional thermo-mechanical experiments that enable unravelling the mass-flow pathways and the long-term distribution of stresses along and above the subduction interface as well as investigating the importance of parameters such as fluids and slab roughness. These results suggest the dynamical instability of fluid-bearing accretionary systems causes either an episodic or a periodic character of subduction erosion and accretion processes as well as their topographic expression. The instability can be partly deciphered through metamorphic and strain records, thus explaining the relative scarcity of paleo-accretionary systems worldwide despite the tremendous amounts of material buried by the subduction process over time scales of tens or hundreds of millions of years. We finally stress that the understanding of the physical processes at the origin of underplating processes as well as the forearc topographic response paves the way for refining our vision of long-term plate-interface coupling as well as the rheological behavior of the seismogenic zone in active subduction settings.
Multifluid geo-energy systems: Using geologic CO2 storage for geothermal energy production and grid-scale energy storage in sedimentary basins
Buscheck, Thomas A.; Bielicki, Jeffrey M.; Edmunds, Thomas A.; et al. (2016)
Geosphere
Continental arc-island arc fluctuations, growth of crustal carbonates, and long-term climate change
Lee, Cin-Ty A.; Shen, Bing; Slotnick, Benjamin S.; et al. (2013)
Geosphere
Differentiation of an upper crustal magma reservoir via crystal- melt separation recorded in the San Gabriel pluton, central Chile
Payacán, Italo; Gutierrez, Francisco; Bachmann, Olivier; et al. (2023)
Geosphere
Crystal- melt separation has been invoked as a mechanism that generates compositional variabil-ities in magma reservoirs hosted within the Earth's crust. However, the way phase separation occurs within such reservoirs is still debated. The San Gabriel pluton of central Chile is a composite pluton (12.82 +/- 0.19 Ma) with wide textural/compositional variation (52- 67 wt% SiO2) and presents a great natural laboratory for studying processes that occur in upper crustal magma reservoirs. Geochemical and geochronological data supported by numerical models reveals that shallow magma differentiation via crystal- melt separation occurred in magma with intermediate composition and generated high-silica magmas and cumulate residues that were redistributed within the reservoir.The pluton is composed of three units: (1) quartz-monzonites representing the main hosting unit, (2) a porphyritic monzogranite located at the lowest exposed levels, and (3) coarse-grained quartz-monzodiorites with cumulate textures at the middle level of the intrusive. Calculations of mass balance and thermodynamic modeling of major and trace elements indicate that <40 vol% of haplogranitic residual melt was extracted from the parental magma to generate quartz-monzonites, and 50- 80 vol% was extracted to generate quartz-monzodiorites, which implies that both units represent crystal- rich residues. By contrast, the monzogranites are interpreted as a concentration of remobilized residual melts that followed 30-70 vol% fractionation from a mush with 0.4-0.55 of crystal fraction. The monzogranites represent the upper level of a pulse that stopped under a crystal- rich mush zone, probably leaving a mafic cumulate zone beneath the exposed pluton. This case study illustrates the role of the redistribution of residual silicic melts within shallow magma reservoirs.
Rates and style of Cenozoic deformation around the Gonghe Basin, northeastern Tibetan Plateau
Craddock, William H.; Kirby, Eric; Zhang, Huiping; et al. (2014)
Geosphere
Assessing the effect of melt extraction from mushy reservoirs on compositions of granitoids: From a global database to a single batholith
Cornet, Julien; Bachmann, Olivier; Ganne, Jérôme; et al. (2022)
Geosphere
Mafic and ultramafic plutonic rocks are often considered to be crystal cumulates (i.e., they are melt-depleted), but such a classification is much more contentious for intermediate to silicic gran-itoids (e.g., tonalite, granodiorite, granite, and syenite). Whether or not a given plutonic rock has lost melt to feed shallower subvolcanic intrusive bodies or volcanic edifices has key implications for understanding igneous processes occurring within the crust throughout the evolution of the Earth. We use statistical analyses of a global vol-canic and plutonic rock database to show that most mafic to felsic plutonic rock compositions can be interpreted as melt-depleted (i.e., most of the minerals analyzed are more evolved than their bulk-rock compositions would allow). To illustrate the application of the method to natural samples (from the Tertiary Adamello Batholith in the south-ern Alps), we estimate the degree of melt depletion using a combination of magmatic textures, bulk-rock chemistry, modal mineralogy, distributions of plagioclase composition (using scanning electron microscope phase mapping/electron microprobe analyses), and thermodynamic modeling. We find that melt depletion correlates with the magmatic foliation and is accompanied by bulk depletion in incompatible elements, low amounts of near-sol-idus minerals, and mineral compositions that are too evolved (i.e., depleted in Ca or Mg, depend-ing on the mineral) to be in equilibrium with their bulk-rock chemistry. The analytical and modeling workflow proposed in this study provides a path to quantifying melt depletion in any plutonic samples.
Zircon age spectra to quantify magma evolution
Schmitt, Axel Karl; Sliwinski, Jakub; Caricchi, Luca; et al. (2023)
Geosphere
The past decades have seen tremendous advances in analytical capabilities regarding the sensitivity, spatial selectivity, and instrumental precision of U-Th-Pb zircon geochronology. Along with improved zircon pretreatment to mitigate the effects of Pb-loss, these advancements have resulted in the emergence of U-Th-Pb dating as the most widely used geochronometer. In paral-lel, it became increasingly obvious that modern analytical techniques can resolve zircon age dis-persal beyond instrumental uncertainties and that this dispersion cannot be attributed to Pb-loss or inheritance. Hence, there is a pressing need to refine statistical procedures for displaying and interpreting dispersed age data from volcanic and plutonic rocks, where zircon ages were traditionally assigned to the quasi-instantaneous events of erup-tion and magma emplacement, respectively. The ability to resolve zircon age spectra, which often range over timescales of 103-106 years, also offers new opportunities to monitor magmatic processes, because zircon crystallization directly relates to the temperature and composition of its host melt. This relation is, at least for typical subalkaline melt compositions, well calibrated by multiple zircon saturation experiments, although absolute satu-ration temperatures derived from them can vary by tens of degrees. Moreover, zircon saturation thermometry is supported by the trace element and isotopic inventory of zircon, which records the thermochemical and compositional evolution of melts at high fidelity. Here, we first review the prop-erties of true zircon age spectra that are defined by a statistically robust overdispersion relative to analytical uncertainties. Secondly, we evaluate existing models and present new models that aim to quantitatively translate the properties of zircon age spectra into parameters controlling the longev-ity and thermal evolution of crustal magma bodies such as magma recharge flux and duration. These developing approaches, which aspire to capture all processes that affect the formation and dispersal of zircon in dynamic crustal magma systems, have the potential to foster an improved understanding of magmatism with implications for volcanic haz-ard assessment, geothermal energy uses, and the origins of ore deposits.
The relative roles of inheritance and long-term passive margin lithospheric evolution on the modern structure and tectonic activity in the southeastern United States
Wagner, Lara S.; Fischer, Karen M.; Hawman, Robert; et al. (2018)
Geosphere
We perform inversions for the shear-wave velocity structure of the southeastern United States (SEUS) using Rayleigh-wave phase and amplitude data from the broadband stations of the South Eastern Suture of the Appalachian Margin Experiment (SESAME) and EarthScope Transportable Array (TA). Our tomographic images of shear-wave velocities in the upper mantle beneath the SEUS provide new constraints on the evolution of mantle lithosphere, both from the inheritance of structures from repeated Wilson cycles and from processes that have occurred while at a passive margin setting. Our images also allow us to correlate these structures to evidence of Eocene to recent tectonism observed at the surface. We find evidence for both inherited structures and more recently evolved structures, both of which bear some correlation to observations of ongoing tectonism. Our results suggest that lithospheric mantle continues to evolve while in a passive margin setting and that even relatively “stable” continental mantle lithosphere is subject to episodes of delamination, foundering, and erosion due to processes that are still not well understood. Our results provide structural constraints on the types of processes that may be ongoing and on possible explanations for the numerous observations of comparatively recent tectonic activity occurring along this passive margin setting.
rmacrostrat: An R package for accessing and retrieving data from the Macrostrat geological database
Jones, Lewis A.; Dean, Christopher D.; Gearty, William; et al. (2024)
Geosphere
The geological record is a vast archive of information that provides the only empirical data about the evolution of the Earth. In recent years, concentrated efforts have been made to compile macrostratigraphic data into the online centralized database Macrostrat. Macrostrat is a global stratigraphic database containing information regarding surface and subsurface rock units and their respective ages, lithologies, geographic extents, and various other associated metadata. However, these raw data are currently directly accessible only through the Macrostrat application programming interface, which is a barrier to potential users that are less familiar with such services. This data accessibility hurdle currently prevents full capitalization of the value offered by Macrostrat, particularly its potential to improve understanding of the geological and biological evolution of the Earth. Here, we introduce rmacrostrat, an R package that interfaces with the Macrostrat database to access and retrieve a variety of geological, paleontological, and economic data directly into the R programming environment. In this article, we provide details about how the package can be installed, its implementation, and potential use cases. For the latter, we showcase how rmacrostrat can be used to visualize regional stratigraphic columns, produce regional geologic outcrop maps, and investigate temporal trends in macrostratigraphic units. We hope that this package will make geological data more readily accessible and in turn will facilitate new research utilizing Earth system data.
Numerical modeling of tectonic underplating in accretionary wedge systems
Ruh, Jonas B. (2020)
Geosphere
Many fossil and active accretionary wedge systems show signs of tectonic underplating, which denotes accretion of underthrust material to the base of the wedge. Underplating is a viable process for thickening of the rear part of accretionary wedges, for example as a response to horizontal growth perpendicular to strike. Here, numerical experiments with a visco-elasto-plastic rheology are applied to test the importance of backstop geometry, flexural rigidity, décollement strength, and surface erosion on the structural evolution of accretionary wedges undergoing different modes of sediment accretion, where underplating is introduced by the implementation of two, a basal and an intermediate, décollement levels. Results demonstrate that intense erosion and a strong lower plate hamper thickening of a wedge at the rear, enhancing localized underplating, antiformal stacking, and subsequent exhumation to sustain its critical taper. Furthermore, large strength contrasts between basal and intermediate décollements have an important morphological impact on wedge growth due to different resulting critical taper angles. Presented numerical experiments are compared to natural examples of accretionary wedges and are able to recreate first-order structural observations related to underplating.

Publications 1 - 10 of 22

Journal: Geosphere

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