Magnetotelluric imaging of a fossil oceanic plate in northwestern Xinjiang, China

Because an oceanic plate colliding with a continental plate will usually be subducted and recycled into the deep mantle, a fossil oceanic plate after the closure of an ancient ocean has rarely been imaged in the subcontinental lithospheric mantle. This has led to a long-standing debate about the fate of subducted ocean plates. The problem can be addressed by imaging the lithosphere in a continental accretion zone with past ocean subduction. We present a study using long-period magnetotelluric data that reveals a large shallow-mantle conductor in a Phanerozoic accretion area in northwestern Xinjiang, China. This conductor extends >300 km laterally at depths from 120 to 220 km and resembles a segment of a fossil oceanic plate. The reduced resistivity is ascribed to the volatile-bearing metasomatic minerals, based on its relatively fertile nature and low temperature. Our results demonstrate that an oceanic plate can be trapped in continental lithosphere, underscoring the significance of oceanic plate subduction to continental accretion, and shedding new light on our understanding of continental formation and evolution.

Tuzo Wilson and the acceptance of pre-Mesozoic continental drift

Tuzo Wilson’s well-known pre-1961 opposition to continental drift stemmed from his early experience as a geologist in the Appalachians and the Canadian Shield, which convinced him that orogenesis did not change drastically over geologic time. Conversely, Taylor (in 1910) and Wegener (in 1912) hypothesized that continental drift began in Cenozoic or Mesozoic time. Between 1949 and 1960, Tuzo Wilson with Adrian Scheidegger developed a quasi-uniformitarian model of progressive continental accretion around fixed Archean nuclei. Tuzo abruptly jettisoned this model in 1961 when, under pressure from paleomagnetic evidence for continental drift and a nascent concept of sea-floor spreading, he finally entertained the possibility of pre-Mesozoic as well as younger continental drift. He immediately found it a superior fit to Appalachian and Shield geology, while his uniformitarian conviction remained intact. Tuzo had blinded himself to the evidence for continental drift so long as he confined it to Taylor or Wegener’s conception. In continental drift operating continuously over geologic time, he found a theory he could eagerly accept.