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Abstract A long‐standing question is how felsic continental crust is differentiated from its mafic parent mantle magmas. One currently proposed fundamental mechanism is lithospheric foundering and loss of dense material into the mantle. A type locality is the young extinct arc forming the Sierra Nevada, California. Here, we image a distinct anisotropic shear layer below the crust‐mantle boundary using receiver functions. The sense of shear is consistent with west‐ to southwestward removal of lithosphere. The shear signal is strongest in the southern Sierra, where lithospheric foundering was proposed to have concluded several million years ago, and is deeper and less pronounced in the central Sierra, where ongoing lithospheric foundering is corroborated by a band of unusually deep (40+ km) seismicity along the western foothills. Our observations provide progressive snapshots of a lithospheric foundering process spanning several million years and hundreds of kilometers, illuminating a fundamental differentiation process by which continents are built.
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To sink, or not to sink: The thermal and density structure of the modern northern Andean arc constrained by xenolith petrology
Abstract The thermal and compositional structure of arcs influence magmatic differentiation and lower-crustal foundering, two key processes impacting the evolution of the continental crust. Although many studies have proposed time scales of lithospheric recycling based on convective downwelling calculations, these models depend on the composition, density (ρ), and thermal structure of the lower crust and mantle, which are difficult to quantify in active continental arcs. Here, we constrained these properties for the Andean Northern Volcanic Zone using direct petrologic observations from a unique suite of lower-crust and mantle xenoliths from Mercaderes, Colombia. Chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb dates for zircons within the host tuff indicate the xenoliths erupted no earlier than 238 (±19) ka and thus capture a recent snapshot of the arc and subarc mantle. Equilibrium pressure-temperature (P-T) estimates for 81 xenoliths define three distinct thermal domains, interpreted as (1) a steep conductive geothermal gradient in the lower arc crust; (2) a convecting mantle wedge; and (3) cooled mantle in proximity to the subducting slab. Our results indicate the presence of an ~10–14-km-thick, high-density lithospheric root that is ~0.1 g/cm3 denser than the underlying mantle. Unlike records from exhumed paleoarcs, Rayleigh-Taylor instability calculations using our P-T-ρ constraints are unrealistically short for the northern Andes. We suggest the presence of partial melts in this hot arc root as a potential source of buoyancy preventing or significantly slowing down foundering.
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- PAR ID:
- 10450535
- Date Published:
- Journal Name:
- Geology
- Volume:
- 51
- Issue:
- 6
- ISSN:
- 0091-7613
- Page Range / eLocation ID:
- 586 to 590
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/G50973.1
