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1. Modeling the Inception of Supercontinent Breakup: Stress State and the Importance of Orogens

   https://doi.org/10.1029/ 2019GC008538  

   Huang, C. (October 2019, Geochemistry geophysics geosystems)

   The relative significance of various geodynamic mechanisms that drive supercontinent breakup is unclear. A previous analysis of extensional stress during supercontinent breakup demonstrated the importance of the plume‐push force relative to the dragging force of subduction retreat. Here, we extend the analysis to basal traction (shear stress) and cross‐lithosphere integrations of both extensional and shear stresses, aiming to understand more clearly the relevant importance of these mechanisms in supercontinent breakup. More importantly, we evaluate the effect of preexisting orogens (mobile belts) in the lithosphere on supercontinent breakup process. Our analysis suggests that a homogeneous supercontinent has extensional stress of 20–50 MPa in its interior (<40° from the central point). When orogens are introduced, the extensional stress in the continents focuses on the top 80‐km of the lithosphere with an average magnitude of ~160 MPa, whereas at the margin of the supercontinent the extensional stress is 5–50 MPa. In both homogeneous and orogeny‐embedded cases, the subsupercontinent mantle upwellings act as the controlling factor on the normal stress field in the supercontinent interior. Compared with the extensional stress, shear stress at the bottom of the supercontinent is 1–2 order of magnitude smaller (0–5 MPa). In our two end‐member models, the breakup of a supercontinent with orogens can be achieved after the first extensional stress surge, whereas for a hypothetical supercontinent without orogens it starts with more diffused local thinning of the continental lithospheric before the breakup, suggesting that weak orogens play a critical role in the dispersal of supercontinents. 

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2. Paleoproterozoic Plate Tectonics Recorded in the Northern Margin Orogen, North China Craton

   https://doi.org/10.1029/2022GC010662  

   Wu, Chen; Wang, Guosheng; Zhou, Zhiguang; Haproff, Peter J.; Zuza, Andrew V.; Liu, Wenyou (November 2022, Geochemistry, Geophysics, Geosystems)

   Abstract The occurrence of plate tectonic processes on Earth during the Paleoproterozoic is supported by ca. 2.2–1.8 Ga subduction‐collision orogens associated with the assembly of the Columbia‐Nuna supercontinent. Subsequent supercontinent breakup is evidence by global ca. 1.8–1.6 Ga large igneous provinces. The North China craton is notable for containing Paleoproterozoic orogens along its margins, herein named the Northern Margin orogen, yet the nature and timing of orogenic and extensional processes of these orogens and their role in the supercontinent cycle remain unclear. In this contribution, we present new field observations, U‐Pb zircon and baddeleyite geochronology dates, and major/trace‐element and isotope geochemical analyses from the northern margin of the North China craton that detail its Paleoproterozoic tectonic and magmatic history. Specifically, we record the occurrence of ca. 2.2–2.0 Ga magmatic arc rocks, ca. 1.9–1.88 Ga tectonic mélange and mylonitic shear zones, and folded lower Paleoproterozoic strata. These rocks were affected by ca. 1.9–1.8 Ga granulite‐facies metamorphism and ca. 1.87–1.78 Ga post‐collisional, extension‐related magmatism along the cratonal northern margin. We interpret that the generation and emplacement of these rocks, and the coupled metamorphic and magmatic processes, were related to oceanic subduction and subsequent continent‐continent collision during the Paleoproterozoic. The occurrence of ca. 1.77–1.73 Ga mafic dykes and ca. 1.75 Ga mylonitic shear zones along the northern margin of the North China craton may have been related to a regional mantle plume event. Our results are consistent with modern style plate tectonics, including oceanic subduction‐related plate convergence and continent‐continent collision, operating in the Paleoproterozoic. 

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3. Estimation of Absolute Stress in the Hypocentral Region of the 2019 Ridgecrest, California, Earthquakes

   https://doi.org/10.1029/2021JB022000  

   Fialko, Y. (July 2021, Journal of Geophysical Research: Solid Earth)

   Abstract Strength of the upper brittle part of the Earth's lithosphere controls deformation styles in tectonically active regions, surface topography, seismicity, and the occurrence of plate tectonics, yet it remains one of the most debated quantities in geophysics. Direct measurements of stresses acting at seismogenic depths are largely lacking. Seismic data (in particular, earthquake focal mechanisms) have been used to infer orientation of the principal stress axes. I show that the focal mechanism data can be combined with information from precise earthquake locations to place constraints not only on the orientation, but also on the magnitude of absolute stress at depth. The proposed method uses relative attitudes of conjugate faults to evaluate the amplitude and spatial heterogeneity of the deviatoric stress and frictional strength in the seismogenic zone. Relative fault orientations (dihedral angles) and sense of slip are determined using quasi‐planar clusters of seismicity and their composite focal mechanisms. The observed distribution of dihedral angles between active conjugate faults in the area of Ridgecrest (California, USA) that hosted a recent sequence of strong earthquakes suggests in situ coefficient of friction of 0.4–0.6, and depth‐averaged shear stress on the order of 25–40 MPa, intermediate between predictions of the “strong” and “weak” fault theories. 

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4. Sublithospheric diamond ages and the supercontinent cycle

   https://doi.org/10.1038/s41586-023-06662-9  

   Timmerman, Suzette; Stachel, Thomas; Koornneef, Janne M; Smit, Karen V; Harlou, Rikke; Nowell, Geoff M; Thomson, Andrew R; Kohn, Simon C; Davies, Joshua_H_F L; Davies, Gareth R; et al (November 2023, Nature)

   Abstract Subduction related to the ancient supercontinent cycle is poorly constrained by mantle samples. Sublithospheric diamond crystallization records the release of melts from subducting oceanic lithosphere at 300–700 km depths^1,2and is especially suited to tracking the timing and effects of deep mantle processes on supercontinents. Here we show that four isotope systems (Rb–Sr, Sm–Nd, U–Pb and Re–Os) applied to Fe-sulfide and CaSiO₃inclusions within 13 sublithospheric diamonds from Juína (Brazil) and Kankan (Guinea) give broadly overlapping crystallization ages from around 450 to 650 million years ago. The intracratonic location of the diamond deposits on Gondwana and the ages, initial isotopic ratios, and trace element content of the inclusions indicate formation from a peri-Gondwanan subduction system. Preservation of these Neoproterozoic–Palaeozoic sublithospheric diamonds beneath Gondwana until its Cretaceous breakup, coupled with majorite geobarometry^3,4, suggests that they accreted to and were retained in the lithospheric keel for more than 300 Myr during supercontinent migration. We propose that this process of lithosphere growth—with diamonds attached to the supercontinent keel by the diapiric uprise of depleted buoyant material and pieces of slab crust—could have enhanced supercontinent stability. 

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5. Exhumation of the Coyote Mountains metamorphic core complex (Arizona): implications for orogenic collapse of the southern North American Cordillera.

   Gottardi, R. (October 2020, Tectonics)

   A microstructural and thermochronometric analysis of the Coyote Mountains detachment shear zone provides new insight into the collapse of the southern North American Cordillera. The Coyote Mountains is a metamorphic core complex that makes up the northern end of the Baboquivari Mountains in southern Arizona. The Baboquivari Mountains records several episodes of crustal shortening and thickening, and regional metamorphism, including the Late Cretaceous-early Paleogene Laramide orogeny which is locally expressed by the Baboquivari thrust fault. Thrusting and shortening were accompanied by magmatic activity recorded by intrusion of Paleocene muscovite-biotite-garnet peraluminous granites such as the ~58 Ma Pan Tak Granite, interpreted as anatectic melts representing the culmination of the Laramide orogeny. Following Laramide crustal shortening, the northern end of the Baboquivari Mountains was exhumed along a top-to-the-north detachment shear zone, which resulted in the formation of the Coyote Mountains metamorphic core complex. Structural and microstructural analysis show that the detachment shear zone evolved under a strong component of non-coaxial (simple shear) deformation, at deformation conditions of ~450 ± 50°C, under a differential stress of ~60 MPa, and a strain rate of 1.5 ×10-11 s-1 to 5.0 × 10-13 s-1 at depth of ~11–14 km. Detailed 40Ar/39Ar geochronology of biotite and muscovite, in the context of the deformation conditions determined by quartz microstructures, suggests that the mylonitization associated with the formation of the Coyote Mountains metamorphic core complex started at ~29 Ma (early Oligocene). Apatite fission track ages indicate that the footwall of the Coyote Mountains metamorphic core complex experienced rapid exhumation to the upper crust by ~24 Ma. The fact that mylonitization and rapid extensional exhumation post-dates Laramide thickening by ~30 Myr indicates that crustal thickness alone was insufficient to initiate extensional tectonic and required an additional driving force. The timing of mylonitization and rapid exhumation documented here and in other MCCs are consistent with the hypothesis that slab rollback and the effect of a slab window trailing the Mendocino Triple Junction have been critical in driving the development of the MCCs of the southwest. 

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