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1. Receiver Function Derived Structural Constraints on Dynamic Processes Associated with the Young Nazca Lithosphere Subducting beneath Colombia

   Bishop, Brandon; Cho, Sung-Won; Warren, Linda M; Pedraza, Patricia; Prieto, German A; Dionicio, Viviana (January 2024, AGU Fall Meeting 2023)

   Subduction of the very young (<15 Myr old) oceanic lithosphere of the Nazca plate in central to southern Colombia is observationally related to an unusually high and unusually variable amount of intermediate (>50 km) depth seismicity. From 2010 through 2019 89% of central and southern Colombia’s 11,466 intermediate depth events occurred between 3.5°N and 5.5°N, highlighting these unusual characteristics of the young slab. In addition, morphologic complexity and possible tears characterize the Nazca slab in Colombia and complicate mantle flow in the region. Prior SKS-phase shear-wave splitting results indicate sub-slab anisotropy is dominated by plate motion parallel-to-subparallel orientations in the region, suggesting the young slab has entrained a relatively thick portion of the sub-slab mantle. These observations suggest the subduction of young lithosphere has significant effects on both the overlying and underlying asthenosphere in the Colombia subduction zone. Here we use more than 10 years of data to calculate receiver functions for the Red Sismológica Nacional de Colombia’s network of broadband seismometers. These receiver functions allow us to tie these prior observations of the Colombia subduction zone to distinct, structural features of the slab. We find that the region of high seismicity corresponds to a low seismic velocity feature along the top of the subducting plate between 3.5°N and 5.5°N that is not present to the south. Moderately elevated P-wave velocity to S-wave velocity ratios are also observed within the slab in the north. This feature likely represents hydrated slab mantle and/or uneclogitized oceanic crust extending to a deeper depth in the north of the region which may provide fluids to drive slab seismicity. We further find evidence for a thick layer of material along the slab’s lithosphere-asthenosphere boundary characterized by spatially variable anisotropy. This feature likely represents entrained asthenosphere at the base of the plate sheared by both the overlying plate and complex flow related to proposed slab tears just north and south of the study region. These observations highlight how structural observations provide key contextual constraints on short-term (seismogenic) and long-term (anisotropic fabric) dynamic processes in the Colombia subduction zone. Plain-language Summary The Nazca oceanic plate is very young (<15 million years old) where it is pulled or subducted beneath the South America plate in central and southern Colombia. Earthquakes occurring in the subducted Nazca plate at depths greater than 50 km are nearly 9x more common in central Colombia than in southern Colombia. The subducted Nazca plate also has a complex shape in this region and may have been torn both in northern Colombia and to the south near the Colombia-Ecuador border. The slow flow of mantle rock beneath the subducted plate is believed to be affected by this and earlier studies have inferred this flow is mostly in the same direction as the subducting plate's motion. We have used 10+ years of data to calculate receiver functions, which can detect changes in the velocity of seismic waves at the top and bottom of the subducted plate to investigate these features. We found that the Nazca plate is either hydrated or has rocks with lower seismic velocities at its top in the central part of Colombia where earthquakes are common. We also find that a thick layer of mantle rock at the base of the subducted plate has been sheared. 

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2. Linking Pacific Plate Motions to Metamorphism and Magmatism in Japan During Cretaceous to Paleogene Times

   https://doi.org/10.2138/gselements.20.2.103  

   Wu, Jonny; Wu, Tsung-Jui; Yamaoka, Ken (April 2024, Elements)

   Plate reconstructions of oceanic domains are generally based on paleo-magnetic and seafloor spreading records. However, uncertainties associated with such reconstructions grow rapidly with increasing geological age because the original oceanic plates have been subducted. Here we synthesize advances in seismic tomographic mapping of subducted plates now lying within the mantle that assist plate reconstructions. Our proposed Japan–NW Pacific subduction histories incorporate tomography results and show three distinct stages comparable to those revealed by geochronology, petrology, and geochemistry. We propose major revisions to previously accepted ideas about the age, kinematics, and identity of the plates outboard of Japan during the Cretaceous–Paleogene Sanbagawa-Ryoke paired metamorphism. These revisions require updates to relevant plate convergence boundary conditions and thermo-dynamic models. 

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3. Slab-derived sulfate generates oxidized basaltic magmas in the southern Cascade arc (California, USA)

   https://doi.org/10.1130/G48759.1  

   Muth, Michelle J.; Wallace, Paul J. (June 2021, Geology)

   Abstract Whether and how subduction increases the oxidation state of Earth's mantle are two of the most important unresolved questions in solid Earth geochemistry. Using data from the southern Cascade arc (California, USA), we show quantitatively for the first time that increases in arc magma oxidation state are fundamentally linked to mass transfer of isotopically heavy sulfate from the subducted plate into the mantle wedge. We investigate multiple hypotheses related to plate dehydration and melting and the rise and reaction of slab melts with mantle peridotite in the wedge, focusing on electron balance between redox-sensitive iron and sulfur during these processes. These results show that unless slab-derived silicic melts contain much higher dissolved sulfur than is indicated by currently available experimental data, arc magma generation by mantle wedge melting must involve multiple stages of mantle metasomatism by slab-derived oxidized and sulfur-bearing hydrous components. 

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4. Miocene-Pliocene Volcanism in the Sierra San Francisco, Central Baja California Peninsula, Mexico

   https://doi.org/10.1130/abs/2022CD-374066  

   Grandy, Sam; Hausback, Brian; Bennett, Scott; Dorsey, Rebecca; Darin, Michael (April 2022, GSA Cordilleran 2022 Meeting)

   Volcanic rocks of the Sierra San Francisco (SSF), in northern Baja California Sur, Mexico, record post-subduction magmatism related to slab melting and slab window opening. The range is composed of andesitic and dacitic domes, mafic lavas, and volcaniclastic deposits (debris and block-and-ash-flow, lahar, and fluvial) that constitute the proximal to distal facies of a volcanic field with local eruptive ages that postdate the regional transition from subduction to transtension. Lowest observed volcanic units consist of interbedded and hydrothermally altered mafic lavas, tuff breccias, and andesite/dacite domes. These are overlain by volcaniclastic units and dacite domes that erupted between ~11-10 Ma. Volcaniclastic deposits comprise a section up to 800 m thick, locally flank and dip radially away from domes, and are likely associated with dome collapse. These deposits are unconformably overlain by a series of ~5.5-4.5 Ma Mg-enriched basaltic andesites (bajaites) that typically erupted along NNW-trending normal faults. Low interbedded mafic lavas are chemically similar to syn-subduction lavas (>15 Ma) SE of the SSF, suggesting a typical subduction supraslab mantle source during waning, late Miocene Farallon plate subduction. ~11-10 Ma dacite domes and debris flow blocks display an adakitic geochemical signature, implying an origin involving late Miocene foundering and melting of the edges of the subducted Farallon plate during the opening of a slab window after the 12.3 Ma transition from subduction to transtension. Adakitic rocks of the SSF and the Santa Clara volcanic field 60 km to the SW may constrain the E-W extent of the slab window. The ~5.5-4.5 Ma bajaites display enriched REE and trace element patterns, potentially resulting from the rise of enriched subslab mantle through the slab window and interaction with supraslab mantle, previously metasomatized by slab melts. Thermal pulses associated with Gulf of California rifting may have provided the heat to generate Mg-rich magmas which ascended along rift-related faults, precluding significant crustal contamination or fractionation, and allowing magmas to retain their primitive character. Further analysis will elucidate the timing of slab window development and the post-subduction mantle processes that drove the chemical evolution of SSF magmas. 

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5. On the Evolution and Fate of Sediment Diapirs in Subduction Zones

   https://doi.org/10.1029/2021GC009873  

   Klein, Benjamin Z.; Behn, Mark D. (November 2021, Geochemistry, Geophysics, Geosystems)

   Abstract At subduction zones, significant volumes of sediments and other crustal material are carried on top of the downgoing plate past the trench and into the mantle. This represents the dominant process by which material from the Earth's surface is recycled to the interior. However, the fate of these recycled materials is uncertain. Subducted material may be carried with the slab into the deep mantle, or it may form diapirs that ascend into the hotter portions of the mantle wedge, where they can melt and/or be relaminated to the base of the arc crust. While this material can be a mixture (or “mélange”) of sediments, oceanic crust and mantle rocks, here we focus on the dynamics of the uppermost layer of sediments on the downgoing slab. We modified a thermodynamic model to accurately predict the equilibrium mineral assemblage, melting behavior, and density of a range of subducted sediment compositions at pressure and temperature conditions relevant to subduction zones. Using this thermodynamic model, we constructed a coupled dynamic model of sediment diapirs and identified the primary parameters that control diapir behavior: sediment thickness and composition, and the thermal state of the subduction zone. Relamination of ascending diapirs is favored by greater sediment thicknesses, more felsic compositions, and warmer thermal conditions. By contrast, diapirism is suppressed in colder arcs, or where subducted sediment layers are thin or more mafic. Applying this model to modern subduction zones suggests that multiple processes are active today, with relamination occurring in a significant subset of modern arcs. 

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