Geosphere

Journal: Geosphere

Publisher

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.
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.
Length scales and types of heterogeneities along the deep subduction interface: Insights from exhumed rocks on Syros Island, Greece
Kotowski, Alissa J.; Behr, Whitney M. (2019)
Geosphere
We use structural and microstructural observations from exhumed subduction-related rocks exposed on Syros Island (Cyclades, Greece) to provide constraints on the length scales and types of heterogeneities that occupy the deep subduction interface, with possible implications for episodic tremor and slow slip. We selected three Syros localities that represent different oceanic protoliths and deformation conditions within a subduction interface shear zone, including: (1) prograde subduction of oceanic crust to eclogite facies; (2) exhumation of oceanic crust from eclogite through blueschist-greenschist facies; and (3) exhumation of mixed mafic crust and sediments from eclogite through blueschist-greenschist facies. All three localities preserve rheological heterogeneities that reflect metamorphism of primary lithological, geochemical, and/or textural variations in the subducted protoliths and that take the form of brittle pods and lenses within a viscous matrix. Microstructural observations indicate that the matrix lithologies (blueschists and quartz-rich metasediments) deformed by distributed power-law viscous flow accommodated by dislocation creep in multiple mineral phases. We estimate bulk shear zone viscosities ranging from ∼1018 to 1020 Pa-s, depending on the relative proportion of sediments to (partially eclogitized) oceanic crust. Eclogite and coarse-grained blueschist heterogeneities within the matrix preserve multiple generations of dilational shear fractures and veins formed under high-pressure conditions. The veins commonly show coeval or overprinting viscous shear, suggesting repeated cycles of frictional and viscous strain. These geologic observations are consistent with a mechanical model of episodic tremor and slow slip (ETS), in which the deep subduction interface is a rheologically heterogeneous distributed shear zone comprising transiently brittle (potentially tremor-genic) sub-patches within a larger, viscously creeping interface patch. Based on our observations of outcrop and map areas of heterogeneous patches and the sizes, distributions, and amounts of brittle offset recorded by heterogeneities, we estimate that simultaneous brittle failure of heterogeneities could produce tremor bursts with equivalent seismic moments of 4.5 × 109–4.7 × 1014 N m, consistent with seismic moments estimated from geophysical data at active subduction zones.
Detrital zircon ages and trace element compositions of Permian–Triassic foreland basin strata of the Gondwanide orogen, Antarctica
Paulsen, Timothy; Deering, Chad; Sliwinski, Jakub; et al. (2017)
Geosphere
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
Incision history of the Verde Valley region and implications for uplift of the Colorado Plateau (central Arizona)
Ott, Richard F.; Whipple, Kelin X.; van Soest, Matthijs (2018)
Geosphere
The record of Tertiary landscape evolution preserved in Arizona’s transition zone presents an independent opportunity to constrain the timing of Colorado Plateau uplift and incision. We study this record of landscape evolution by mapping Tertiary sediments, volcanic deposits, and the erosional unconformity at their base, 40Ar/39Ar dating of basaltic lava flows in key locations, and constructing geological cross sections along canyons to restore the paleorelief on the Tertiary erosional unconformity to test whether canyon incision requires young (<10 Ma) Colorado Plateau uplift. Our cross sections and new 40Ar/39Ar ages document that in the Verde Valley, relief across the ancestral Mogollon Rim that marks the southern edge of the Colorado Plateau was up to 1000 m and averaged ∼700 m in the Early-Middle Miocene, which is close to average modern relief of ∼800–1000 m. Although Middle-Late Miocene volcanics and sediments in places onlap the ancestral Mogollon Rim, suggesting an erosional origin, northeastward erosional retreat of an earlier tectonic escarpment is both plausible and consistent with displacement histories of the Grand Wash fault to the NW and the Diamond Rim fault to the SE. Interestingly, the coincidence of a rugged, sharply defined Mogollon Rim and a wide bench cut into the Hermit shale below the escarpment today suggests that exposure of the Hermit shale may play an important role in cliff retreat and the morphological expression of the Mogollon Rim in the Verde Valley region. Some modern canyons cut into the retreating escarpment reflect re-excavation, deepening, and headward propagation of Miocene paleochannels largely buried by Middle-Late Miocene basalts. Despite evidence for similar total paleorelief, most canyons show significant (∼35%) deepening since 5 Ma with young incision decreasing to the SE. Incision rates during the period of active Miocene volcanism were below 20 m/m.y. but accelerated to 50–80 m/m.y. during the past 5–8 m.y. and were probably 100–160 m/m.y. during the Quaternary. The accelerated incision reflects base-level fall associated with breaching of the Verde Lake basin by ca. 2.5 Ma and integration of the Verde River and thus does not require post–Middle Miocene uplift of the southwestern edge of the Colorado Plateau.
Australia going down under: Quantifying continental subduction during arc-continent accretion in Timor-Leste
Tate, Garrett W.; McQuarrie, Nadine; Hinsbergen, Douwe J.J. van; et al. (2015)
Geosphere
Four-dimensional surface evolution of active rifting from spaceborne SAR data
Casu, Francesco; Manconi, Andrea (2016)
Geosphere
Pleistocene to recent geomorphic and incision history of the northern Rio Grande gorge, New Mexico: Constraints from field mapping and cosmogenic He-3 surface exposure dating
Clow, Travis; Behr, Whitney M.; Helper, Mark A. (2019)
Geosphere
Rates and style of Cenozoic deformation around the Gonghe Basin, northeastern Tibetan Plateau
Craddock, William H.; Kirby, Eric; Zhang, Huiping; et al. (2014)
Geosphere

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