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1. On the relationship between the uppermost mantle (≤220 km) seismic velocity, crustal thickness, and topography in Tibet

   https://doi.org/10.1130/G52408.1  

   Rowley, David B; Liu, Chujie; Grand, Stephen P; Liang, Xiaofeng; Sandvol, Eric (November 2024, Geology)

   Abstract We propose that the mantle lithospheric density and crustal thickness are correlated in such a way as to produce a flat Tibetan Plateau. We observe that the mantle lithosphere is relatively uniform beneath the Himalaya and southern and central Tibet, despite a near doubling of crustal thickness relative to India. Farther north, cratonic mantle lithosphere disappears over large regions of north-central Tibet, giving rise to large lateral variations in uppermost mantle Vs anomalies (>12%) that are uncorrelated with changes in surface elevation but are closely related to changes in crustal thickness. This decoupling of surface topography from spatial variations in upper mantle seismic velocity, and assumed buoyancy, implies that Tibetan topography is controlled by a crust-mantle interaction that is able to maintain its near constant elevation. This crust-mantle interaction is likely driven by gravitational potential energy with a very weak crust. Magmatism, with ages of ca. 20 Ma to Present, spatially correlated with this region with no sub-Moho mantle lithosphere implies destabilization of mantle lithosphere in northern Tibet. Cratonic Indian underthrusting for the past 25 m.y. has also not led to significant topography in the plateau through time. The magmatism may have helped weaken the crust, allowing it to respond to changes in uppermost mantle buoyancy, resulting in a flat plateau. 

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2. Laramide bulldozing of lithosphere beneath the Arizona transition zone, southwestern United States

   https://doi.org/10.1130/G51194.1  

   Kapp, Paul; Jepson, Gilby; Carrapa, Barbara; Schaen, Allen J.; He, John J.Y.; Wang, Jordan W. (August 2023, Geology)

   Abstract The northwest-trending transition zone (TZ) in Arizona (southwestern United States) is an ~100-km-wide physiographic province that separates the relatively undeformed southwestern margin of the Colorado Plateau from the hyperextended Basin and Range province to the southwest. The TZ is widely depicted to have been a Late Cretaceous–Paleogene northeast-dipping erosional slope along which Proterozoic rocks were denuded but not significantly deformed. Our multi-method thermochronological study (biotite 40Ar/39Ar, zircon and apatite [U-Th-Sm]/He, and apatite fission track) of Proterozoic rocks in the Bradshaw Mountains of the west-central Arizona TZ reveals relatively rapid cooling (~10 °C/m.y.) from temperatures of >180 °C to <60 °C between ca. 70 and ca. 50 Ma. Given minimal ca. 70–50 Ma upper-crustal shortening in the TZ, we attribute cooling to exhumation driven by northeastward bulldozing of continental lower crust and mantle lithosphere beneath it by the Farallon flat slab. Bulldozing is consistent with contemporaneous (ca. 70–50 Ma) underplating and initial exhumation of Orocopia Schist to the southwest in western Arizona and Mesozoic garnet-clinopyroxenite xenoliths of possible Mojave batholith keel affinity in ca. 25 Ma TZ volcanic rocks. 

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3. Expedition 402 Scientific Prospectus: Tyrrhenian Continent–Ocean Transition

   https://doi.org/10.14379/iodp.sp.402.2023  

   Zitellini, N.; Malinverno, A.; Estes, E.R. (March 2023, Scientific prospectus)

   A tenet of plate tectonics is that divergent plates cause the asthenospheric mantle to ascend, decompress, and melt, producing new magmatic crust. However, drilling west of Iberia in the 1980s discovered a continent–ocean transition (COT) made of exposed mantle, revising models of lithospheric thinning and melt generation and defining magma-poor margins. A long-standing argument about mantle in COTs concerns its nature as either subcontinental or being exhumed during ultraslow seafloor spreading. Additionally, two models attribute the apparent lack of melts either to slow extension resulting in low ascent rates with enhanced asthenospheric cooling and reduced melt production or to upwelling mantle originally too depleted to produce a significant melt fraction. The debate on COT models is limited by the scarce evidence obtained in ultra-deepwater drilling, restricted to a few basement highs. Thus, 30 y after its discovery, the nature and genesis of COTs is still controversial. The comparatively shallow water depth and thin sediment cover of the Tyrrhenian Sea provide an optimal location to test COT formation models by drilling. The Tyrrhenian is the only example where extensive modern geophysical data has accurately mapped basement domains of a conjugate pair of COTs. They can be characterized with unprecedented detail in a single drilling expedition to study the time and space evolution of COT processes. Expedition 402 will drill two perpendicular transects. An east–west transect will target the progression from magmatic crust to exhumed mantle; a north–south transect will map the fault zone that exhumed the mantle. Drilling will sample the complete sediment section including Messinian deposits, the sediment/basement interface, the mantle, the associated magmas, and the products of syntectonic, and possibly ongoing, fluid-rock interactions to evaluate the hydrosphere–lithosphere geochemical exchange and potential related ecosystems. 

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4. Protracted mantle heat conduction after lithospheric foundering beneath the Malagasy orogen

   https://doi.org/10.1130/G52316.1  

   Kaare-Rasmussen, Jonas; Horton, Forrest; Holder, Robert; Kylander-Clark, Andrew; Bouvier, Anne-Sophie; Müntener, Othmar; Rakotondrazafy, Michel (October 2024, Geology)

   Abstract Ultrahigh-temperature metamorphism (UHTM) is important for the evolution and long-term stability of continental crust. The Anosyen domain in southeastern Madagascar is a well-preserved UHTM terrane that formed during the amalgamation of Gondwana. The heat source(s) required to reach peak conditions is(are) a matter of debate. One potential cause of extreme crustal heating is the intrusion of mantle-derived melts into the crust. Foundering of the mantle lithosphere can also lead to increased heat flow. To assess the role of these heating mechanisms, we measured zircon δ18O, εHf(t) compositions, and U-Pb dates for plutonic rocks in the midcrustal UHTM domain. Our results indicate that pluton emplacement predated UHTM by as much as 40 m.y. and that all zircons have crustal O and Hf isotopic compositions. We propose that mantle lithosphere foundering caused melting in the lower crust, producing the magmas responsible for plutonism during the early stages of orogenesis. Prolonged conductive heating of the crust—combined with above-average radiogenic heating—may explain why UHTM occurred ∼40 m.y. after foundering. This suggests that foundering of the mantle lithosphere can swiftly lead to partial melting in the lower crust, as well as protracted heating of the middle crust that culminates tens of millions of years later. 

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5. Melt-affected ocean crust and uppermost mantle near Hawaii—clues from ambient-noise phase velocity and seafloor compliance

   https://doi.org/10.1093/gji/ggaa470  

   Doran, A K; Laske, G (November 2020, Geophysical Journal International)

   SUMMARY We present models of crustal and uppermost mantle structure beneath the Hawaiian Swell and surrounding region. The models were derived from ambient-noise intermediate-period Rayleigh-wave phase velocities and from seafloor compliance that were estimated from continuous seismic and pressure recordings collected during the Hawaiian Plume-Lithosphere Undersea Mantle Experiment (PLUME). We jointly inverted these data at the locations of over 50 ocean-bottom instruments, after accounting for variations in local bathymetry and sediment properties. Our results suggest that the crystalline crust is up to 15 km thick beneath the swell and up to 23 km thick closer to the islands. Anomalously thick crust extends towards the older seamounts, downstream of Hawaii. In a second region, anomalies immediately to the south of Hawaii may be associated with the leading edge of the shallow Hawaiian magma conduit. In a third region, thickened crust to the immediate west of Hawaii may be related to Cretaceous seamounts. Low seismic velocities identified in the uppermost mantle to the northeast of Hawaii may be linked to the Molokai fracture zone and may be manifest of complex non-vertical pathways of melt through the upper lithosphere. Velocity anomalies decrease in amplitude towards the surface, suggesting that melt becomes focused into conduits at depths between 20 and 40 km that escape the resolution capabilities of our data set. 

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