Introduction
The Caroline Plate is a minor oceanic tectonic plate in the eastern hemisphere that straddles the Equator and occupies the oceanic region immediately north of New Guinea. Its southern margin forms a plate‑consuming subduction zone with the Bird’s Head Plate and other small plates of the New Guinea region, producing a convergent boundary that links the Caroline Plate to the complex tectonics of New Guinea. To the north, the boundary with the Pacific Plate is dominantly transform, indicating principally lateral (strike‑slip) motion between the two plates. The margin with the Philippine Sea Plate exhibits a convergent regime that grades into extensional rifting, documenting a transition from compression to crustal stretching along that boundary. Collectively, these varied boundary types—subduction to the south, transform to the north, and a convergent‑to‑rift transition with the Philippine Sea Plate—demonstrate that the Caroline Plate accommodates multiple modes of plate interaction (compression, lateral slip, and extension) within a compact oceanic region. The recognition of the Caroline Plate as a distinct tectonic entity was first proposed by Weissel and Anderson (1978).
Geological setting
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The Caroline Plate constitutes a distinct tectonic terrane with an independent evolutionary history, although its low seismicity and relatively small measured boundary velocities have frequently led to its treatment as part of the Pacific Plate. Internally the plate is dominated by two major oceanic basins—the West and East Caroline Basins—separated by the inactive Eauripik Rise; by convention the overlying Caroline Islands and the adjacent Caroline Ridge are excluded from the plate’s defined oceanic domain.
Plate boundaries around the Caroline domain are varied and partly immature. On the southern flank the Caroline lithosphere converges and presently subducts beneath the Bird’s Head region and Australian‑affiliated blocks along the New Guinea Trench, establishing a southward‑directed convergent margin with Australian plate material. The western boundary with the Philippine Sea Plate is bifurcated: a southern segment (the Ayu Trough) records seafloor spreading at roughly 8 mm/yr during ca. 25–2 Ma with diminishing rates thereafter, whereas the northern segment (the Palau and southern Yap trenches) lacks the volcanic and seismic signatures of active subduction. The Caroline–Pacific interface is a complex, evolving boundary system that cannot be described simply as a transform or spreading margin and may be in the process of developing subductional geometry in places.
To the southeast the Manus Trench defines a geometrically convergent margin between the Caroline Plate and the North Bismarck Plate, but it lacks typical hallmarks of active subduction—notably persistent volcanism and high earthquake frequency—and is therefore not considered an active subduction zone at present. The North Bismarck Plate itself is interpreted as a relic plate presently moving coherently with the Pacific Plate. Immediately south of this relic block lies a mosaic of independently moving microplates (including the South Bismarck, Solomon Sea and Woodlark plates), producing a highly fragmented and dynamically active tectonic neighborhood that influences the southern margins of the Caroline domain.
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Tectonic history
The Caroline Plate’s kinematics are complex: contemporary motion vectors closely match those of the Pacific Plate, yet its time of formation and present-day status as an independent plate remain ambiguous. During the Neogene the Caroline block appears to have translated with the Philippine and New Guinea plates despite only very slow seafloor spreading between the Caroline and Philippine margins. A critical test of rigid co-motion with the Pacific—an accompanying record of subduction beneath New Guinea—is largely unmet; the near-absence of corresponding subduction structures beneath New Guinea argues that the Caroline Plate did not move as a simple rigid continuation of the Pacific during key intervals.
Geophysical and geological data record distinct episodes of extension and localized magmatism. Seafloor spreading in the Caroline Sea between about 34 and 27 Ma and the later opening of the Ayu Trough as a slow-spreading zone at ~15 Ma document discrete extensional events that shaped the plate and adjacent basins. North of the plate, the Caroline Ridge—an oceanic crustal feature probably generated by a hotspot—intersects the margin at the Sorol Trough, where evidence points to oblique extensional deformation.
Evidence for past convergence along the northern/northeastern margin is equivocal. Some trench-like bathymetry and geophysical signatures indicate subduction beneath the Pacific Plate, but the lack of associated island-arc systems—normally diagnostic of long-lived, substantial subduction—renders the boundary’s developmental history uncertain. By contrast, the eastern margin preserves a clearer record of plate separation: magnetic anomalies across the Mussau Trench are markedly different, with Oligocene ages on the Caroline side and Cretaceous ages on the Pacific side, demonstrating a long-lived, distinct plate boundary in the geological past.
Regional reorganization since the Miocene further complicates the Caroline Plate’s history. Collision of the Ontong Java Plateau with the Indo-Australian Plate before ~6 Ma initiated back-arc development, forming the Bismarck Sea behind the New Britain arc; subsequent interactions produced a transpressional regime in the western Bismarck Sea associated with subduction of the Caroline and North Bismarck plates at the New Guinea Trench. Collectively, these lines of evidence indicate a history of episodic extension, intermittent convergence, and evolving plate boundaries rather than simple persistent attachment to an adjacent major plate.