Hidden Mantle Plume from Azores Offshoot Rips Open 500km Underwater Canyon System off Portugal – GEOMAR Kiel Study

Discovery of the King's Trough Complex: A Submarine Marvel Larger Than the Grand Canyon

The King's Trough Complex (KTC), often dubbed the 'Grand Canyon of the Atlantic,' is a staggering submarine feature stretching approximately 500 kilometers across the North Atlantic seafloor, located about 1,000 kilometers west of Portugal's coast. This vast system comprises parallel trenches and deep basins, with the Peake Deep marking one of the ocean's deepest points at over 5,000 meters. Unlike terrestrial canyons sculpted by rivers over millennia, the KTC formed through dramatic tectonic forces, ripping open the seafloor like a zipper between 37 and 24 million years ago (Ma).

Researchers from the GEOMAR Helmholtz Centre for Ocean Research Kiel have unveiled how a hidden mantle plume—an upwelling column of hot rock from Earth's deep interior—interacted with shifting plate boundaries to create this colossal structure. This discovery not only rewrites our understanding of Atlantic tectonics but also highlights the pivotal role of marine geophysics in unraveling Earth's dynamic history.

The GEOMAR-Led Expedition: Mapping the Uncharted Depths

In 2020, during expedition M168 aboard the research vessel METEOR, a team led by Dr. Antje Dürkefälden deployed high-resolution multibeam sonar to meticulously map the KTC's rugged terrain. Chain bag dredges recovered volcanic rock samples from depths exceeding 4,000 meters, providing crucial geochemical archives. Bathymetric data was supplemented by surveys from Portugal's Estrutura de Missão para a Extensão da Plataforma Continental (EMEPC), enabling a comprehensive 3D visualization of the canyons' morphology.

Back in Kiel laboratories, rocks underwent inductively coupled plasma mass spectrometry (ICP-MS) for trace element analysis and ⁴⁰Ar/³⁹Ar dating at the University of Wisconsin-Madison, confirming eruption ages aligning with the rifting timeline. Multichannel seismic profiles and gravity modeling revealed anomalies beneath the surface, pointing to crustal irregularities.

Geological Context: Evolution of the North Atlantic Rift System

The North Atlantic's seafloor spreading began around 200 Ma with the breakup of Pangea, but complexities arose as the Eurasian and African plates diverged. Normally, oceanic crust forms at mid-ocean ridges at a uniform ~7 km thickness through mantle melting. However, the KTC region exhibits ~15-20 km thick crust, associated with the 45°N melting anomaly along the Mid-Atlantic Ridge (MAR).

Around 37 Ma, the plate boundary—previously along the Azores-Gibraltar Fracture Zone—jumped northward, exploiting this weakened zone. Oblique extension caused transtension, forming pull-apart basins and strike-slip faults that carved the parallel troughs.

Evidence of Thickened Crust and Mantle Influence

Seismic refraction data showed velocity structures indicative of underplated gabbros and intrusives, elevating crustal thickness. Gravity models confirmed low-density anomalies consistent with hot, buoyant mantle upwelling. Rock geochemistry revealed enriched trace elements (e.g., high Nb/Y ratios) typical of plume-derived magmas, distinct from normal MORB (mid-ocean ridge basalt).

• Crustal thickness: 15-20 km vs. normal 6-8 km.
• Mantle potential temperatures: Elevated by 50-100°C.
• Volcanic ages: 37-24 Ma, bracketing rifting.

This points to an early offshoot of the Azores mantle plume, now feeding the Azores islands and a remnant at 45°N.

Step-by-Step Formation of the Canyon System

The KTC's genesis unfolded in phases:

1. Pre-37 Ma: Azores plume branch impinges, thickening crust via excess volcanism.
2. ~37 Ma: Plate boundary jumps north; E-W extension initiates at Peake Deep.
3. 33-28 Ma: Peak rifting; strike-slip faults propagate westward, forming parallel troughs up to 20 km wide, 2-3 km deep.
4. ~24 Ma: Boundary shifts south; rifting ceases, plume conduit migrates.

Dr. Dürkefälden notes, 'Our results explain why this remarkable structure developed precisely here.'

Advanced Methods: Integrating Geophysics and Geochemistry

The study's rigor stems from multidisciplinary data:

• Bathymetry: Multibeam echo sounders for 3D seafloor models.
• Seismics: 12-channel streamer for P-wave velocities; wide-angle refraction for Moho depth.
• Gravity: Free-air anomalies modeled with 2D forward modeling.
• Petrology: Thin sections, XRF/ICP-MS for major/trace elements.
• Geochronology: ⁴⁰Ar/³⁹Ar incremental heating.