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1. Geochronological Evidence of Flat slab Subduction in Colombia

   Ishimwe, Felix; Till, Christy; Cardona, Agustin; Jaramillo, Juan_Sebastián; Harmon, Lydia_Jane; Goltz, Andrea (December 2024, AGU Fall Meeting Abstracts)

   The tectonic setting of Northwestern South America is very complex due to interaction between the Nazca, Caribbean, and South America plates and several oceanic terranes. Of particular interest is the Nazca plate slab geometry change since the Mid-Miocene, and its tectonomagmatic effects on the overlying South American Plate. Several studies indicate that the modern Nazca slab is torn into two segments at 5.5oN in Colombia, the so-called "Caldas tear", where the northern segment dips at a shallow angle while the southern segment dips at a steep angle. This slab geometry difference is manifested in magmatic activity where only the region with steep slab shows typical subduction zone volcanism, while the one with flat slab lacks such activity. However, due to the limited number of geochronological data, the timing of this slab shallowing and tearing remains poorly understood. We conducted an extensive 40Ar/39Ar geochronology study on post mid-Miocene volcanic rocks in Colombia to study the temporal evolution of Nazca plate geometry and its effects on magmatism in Colombia. We find evidence of continuous magmatism north and south of the Caldas tear from 10.5 Ma to 6.4 Ma, with peak activity between 9-8 Ma. This is followed by a ~4 million year magmatic hiatus, until the resurgence of magmatism south of the Caldas tear as monogenetic domes at 2.1 Ma, and continuous volcanic activity in modern composite volcanoes since 1.1 Ma. Results of this study support the presence of a complex subduction system beneath Colombia where the northern segment slab has been flat since ~6.4 Ma ago, and the southern segment has re-steepened at ~2.1 Ma. This study showcases that the Nazca slab geometry was the most important factor driving magmatism in Colombia since the mid-Miocene. 

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2. Miocene Paleogeography of NW Colombia: A review of the sedimentary and magmatic evolution of the Amagá Basin a century after Grosse’s work

   https://doi.org/10.18257/raccefyn.1871  

   Zapata, Sebastian; Jaramillo-Ríos, Juan Sebastián; Eliana_Botello, Gladys; Siachoque, Astrid; Calderon-Día, Laura Cristina; Cardona, Agustín; Till, Christy; Valencia, Victor (May 2023, Revista de la Academia Colombiana de Ciencias Exactas, Físicas y Naturales)

   In 1918, the geologist Emile Grosse was commissioned to conduct geological studies in the Amagá Basin, Antioquia, Colombia. In 1923, Grosse finished a comprehensive cartographic work that became the cornerstone for the geology of the northwest (NW) Colombian Andes. Today, 100 years later, the volcanoclastic strata preserved in the Amagá Basin are crucial for understanding major Oligocene to Pliocene tectonic events that occurred in the NW South-American margin, including the fragmentation of the Nazca Plate, the collision of the Panamá-Chocó Block, and the shallowing of the subducted slab. Our contribution includes new mineral chemistry and zircon petrochronological data from the Combia Volcanic Complex and published data to provide a review of the Oligocene to Pliocene deformation, sedimentation, and magmatic patterns in the Amagá Basin and their implications for the tectonic evolution of NW South America. The Amagá Basin was the result of the Eocene to Oligocene uplift of the Western Cordillera followed by the Middle Miocene to Pliocene uplift of both the Central and Western cordilleras, events that modified the Miocene drainage network in the Northern Andes. Coeval with the final Miocene deformation phases in the Amagá basin, the magmatism of the Combia Complex was the result of subduction magmas emplaced in a continental crust affected by strike-slip tectonics. 

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3. Detrital zircon record of Phanerozoic magmatism in the southern Central Andes

   https://doi.org/10.1130/GES02346.1  

   Capaldi, T.N.; McKenzie, N.R.; Horton, B.K.; Mackaman-Lofland, C.; Colleps, C.L.; Stockli, D.F. (May 2021, Geosphere)

   null (Ed.)

   Abstract The spatial and temporal distribution of arc magmatism and associated isotopic variations provide insights into the Phanerozoic history of the western margin of South America during major shifts in Andean and pre-Andean plate interactions. We integrated detrital zircon U-Th-Pb and Hf isotopic results across continental magmatic arc systems of Chile and western Argentina (28°S–33°S) with igneous bedrock geochronologic and zircon Hf isotope results to define isotopic signatures linked to changes in continental margin processes. Key tectonic phases included: Paleozoic terrane accretion and Carboniferous subduction initiation during Gondwanide orogenesis, Permian–Triassic extensional collapse, Jurassic–Paleogene continental arc magmatism, and Neogene flat slab subduction during Andean shortening. The ~550 m.y. record of magmatic activity records spatial trends in magma composition associated with terrane boundaries. East of 69°W, radiogenic isotopic signatures indicate reworked continental lithosphere with enriched (evolved) εHf values and low (<0.65) zircon Th/U ratios during phases of early Paleozoic and Miocene shortening and lithospheric thickening. In contrast, the magmatic record west of 69°W displays depleted (juvenile) εHf values and high (>0.7) zircon Th/U values consistent with increased asthenospheric contributions during lithospheric thinning. Spatial constraints on Mesozoic to Cenozoic arc width provide a rough approximation of relative subduction angle, such that an increase in arc width reflects shallower slab dip. Comparisons among slab dip calculations with time-averaged εHf and Th/U zircon results exhibit a clear trend of decreasing (enriched) magma compositions with increasing arc width and decreasing slab dip. Collectively, these data sets demonstrate the influence of subduction angle on the position of upper-plate magmatism (including inboard arc advance and outboard arc retreat), changes in isotopic signatures, and overall composition of crustal and mantle material along the western edge of South America. 

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4. Tectonic controls on the origin and segmentation of the Cascade Arc, USA

   https://doi.org/10.1007/s00445-022-01611-2  

   Humphreys, Eugene D.; Grunder, Anita L. (December 2022, Bulletin of Volcanology)

   Abstract The magmatic response above subducting ocean lithosphere can range from weak to vigorous and from a narrow zone to widely distributed. The small and young Cascade Arc, riding on the margin of the tectonically active North American plate, has expressed nearly this entire range of volcanic activity. This allows an unusually good examination of arc initiation and early growth. We review the tectonic controls of Cascade-related magmatism from its inception to the present, with new considerations on the influences of tectonic stress and strain on volcanic activity. The Cascade Arc was created after accretion of the Siletzia oceanic plateau at ~ 50 Ma ended a period of flat-slab subduction. This (1) initiated dipping-slab subduction beneath most of the northern arc (beneath Washington and Oregon) and (2) enabled the more southerly subducting flat slab (beneath Nevada) to roll back toward California. As the abandoned flat slab fragmented and foundered beneath Oregon and Washington, vigorous extension and volcanism ensued throughout the northwest USA; in Nevada the subducting flat slab rolled back toward California. Early signs of the Cascade Arc were evident by ~ 45 Ma and the ancestral Cascade Arc was well established by ~ 35 Ma. Thus, from ~ 55–35 Ma subduction-related magmatism evolved from nearly amagmatic to regional flare-up to a clearly established volcanic arc in two different tectonic settings. The modern Cascades structure initiated ~ 7 Ma when a change in Pacific plate motion caused partial entrainment of the Sierra Nevada/Klamath block. This block pushes north and west on the Oregon Coast Ranges block, breaking the arc into three segments: a southern extensional arc, a central transitional arc, and a northern compressional arc. Extension enhances mafic volcanism in the southern arc, promoting basalt decompression melts from depleted mantle (low-K tholeiites) that are subequal in volume to subduction fluxed calcalkaline basalts. Compression restricts volcanic activity in the north; volcanism is dominantly silicic and intra-plate-like basalts cluster close to the main arc volcanoes. The transitional central arc accommodates dextral shear deformation, resulting in a wide volcanic arc with distributed basaltic vents of diverse affinities and no clear arc axis. 

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5. Geochemical and geochronological records of tectonic changes along a flat-slab arc-transform junction: Circa 30 Ma to ca. 19 Ma Sonya Creek volcanic field, Wrangell Arc, Alaska

   https://doi.org/10.1130/GES02114.1  

   Berkelhammer, Samuel E.; Brueseke, Matthew E.; Benowitz, Jeffrey A.; Trop, Jeffrey M.; Davis, Kailyn; Layer, Paul W.; Weber, Maridee (September 2019, Geosphere)

   The Sonya Creek volcanic field (SCVF) contains the oldest in situ volcanic products in the ca. 30 Ma–modern Wrangell Arc (WA) in south-central Alaska, which commenced due to Yakutat microplate subduction initiation. The WA occurs within a transition zone between Aleutian subduction to the west and dextral strike-slip tectonics along the Queen Charlotte–Fairweather and Denali–Duke River fault systems to the east. New 40Ar/39Ar geochronology of bedrock shows that SCVF magmatism occurred from ca. 30–19 Ma. New field mapping, physical volcanology, and major- and trace-element geochemistry, coupled with the 40Ar/39Ar ages and prior reconnaissance work, allows for the reconstruction of SCVF magmatic evolution. Initial SCVF magmatism that commenced at ca. 30 Ma records hydrous, subduction-related, calc-alkaline magmatism and also an adakite-like component that we interpret to represent slab-edge melting of the Yakutat slab. A minor westward shift of volcanism within the SCVF at ca. 25 Ma was accompanied by continued subduction-related magmatism without the adakite-like component (i.e., mantle-wedge melting), represented by ca. 25–20 Ma basaltic-andesite to dacite domes and associated diorites. These eruptions were coeval with another westward shift to anhydrous, transitional-tholeiitic, basaltic-andesite to rhyolite lavas and tuffs of the ca. 23–19 Ma Sonya Creek shield volcano; we attribute these eruptions to intra-arc extension. SCVF activity was also marked by a small southward shift in volcanism at ca. 21 Ma, characterized by hydrous calc-alkaline lavas. SCVF geochemical compositions closely overlap those from the <13 Ma WA, and no alkaline lavas that characterize the ca. 18–10 Ma eastern Wrangell volcanic belt exposed in Yukon Territory are observed. Calc-alkaline, transitional-tholeiitic, and adakite-like SCVF volcanism from ca. 30–19 Ma reflects subduction of oceanic lithosphere of the Yakutat microplate beneath North America. We suggest that the increase in magmatic flux and adakitic eruptions at ca. 25 Ma, align with a recently documented change in Pacific plate direction and velocity at this time and regional deformation events in southern Alaska. By ca. 18 Ma, SCVF activity ceased, and the locus of WA magmatism shifted to the south and east. The change in relative plate motions would be expected to transfer stress to strike-slip faults above the inboard margin of the subducting Yakutat slab, a scenario consistent with increased transtensional-related melting recorded by the ca. 23–19 Ma transitional-tholeiitic Sonya Creek shield volcano between the Denali and Totschunda faults. Moreover, we infer the Totschunda fault accommodated more than ~85 km of horizontal offset since ca. 18 Ma, based on reconstructing the initial alignment of the early WA (i.e., 30–18 Ma SCVF) and temporally and chemically similar intrusions that crop out to the west on the opposite side of the Totschunda fault. Our results from the SCVF quantify spatial-temporal changes in deformation and magmatism that may typify arc-transform junctions over similar time scales (>10 m.y.). 

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