Plate Tectonic Consequences of competing models for the origin and history of the Banda Sea subducted oceanic lithosphere

The Banda Arc, situated west of Irian Jaya and in the easternmost extension of the Sunda subduction zone system, reveals a characteristic bowl-shaped geometry in seismic tomographic images. This indicates that the oceanic lithosphere still remains attached to the surrounding continental margins of northern Australia and the Bird’s Head microcontinent. Major controversies exist between authors proposing an allochthonous or autochthonous origin of the Bird’s Head block. Either scenario has important implications for plate kinematic models aiming to reconstruct the tectonic evolution of the region and the late Jurassic seaoor spreading geometry of this now subducted Argo-Tanimbar-Seram (ATS) ocean basin. Wider implications affect the tectonic conguration of the Tethyan-Pacic realm, the distribution of plate boundaries as well as the shape and size of continental blocks which have been rifted off the northeastern Gondwana margin during the Late Jurassic and are now accreted to the SE Asia margin. We apply structural geology restoration techniques to unfold the subducted oceanic lithosphere of the Banda Slab. Our slab unfolding results help to discriminate between the different hypotheses concerning the origin and kinematic history of the Bird’s Head tectonic block. According to our preferred model, the block is required to rotate 20-35 degrees clockwise relative to its present-day position and that the initial geometry of the ATS ocean basin was more or less rectangular shaped. We evaluate currently accepted plate kinematic scenarios for the Jurassic and younger seaoor spreading history north of Australia in conjunction with our restored ocean basin geometry and conclude that the Bird’s Head block is autochthonous to western Irian Jaya, with its western margin being a continental transform margin during the rifting and opening of the ATS ocean.

💡 Research Summary

The Banda Arc, located at the easternmost extension of the Sunda subduction system, displays a characteristic bowl‑shaped slab in modern seismic tomography. This geometry has been interpreted in two fundamentally different ways: (1) the Bird’s Head (Banda) micro‑continent is an allochthonous block that migrated from elsewhere and was subsequently incorporated into the arc, or (2) it is autochthonous, having remained attached to the western margin of Irian Jaya throughout the Jurassic–Cretaceous opening of the Argo‑Tanimbar‑Seram (ATS) ocean basin. The distinction matters because each hypothesis implies a different configuration of the late‑Jurassic seafloor spreading geometry, a different set of plate‑kinematic reconstructions for the region, and consequently a different picture of how the Tethyan‑Pacific realm was assembled.

To discriminate between the two scenarios, the authors applied a structural‑geology restoration technique—slab unfolding—to the subducted ATS slab. High‑resolution seismic tomography provided the three‑dimensional shape, thickness, and density distribution of the slab. The slab was then “unfolded” back to its pre‑subduction configuration using a series of forward‑kinematic models that incorporated published Jurassic‑Cretaceous plate motions (Australia‑Antarctica‑India rotation, Gondwana‑Laurasia breakup, etc.). Several initial slab geometries were tested (rectangular, trapezoidal, asymmetric quadrilateral) and the model that minimized residual strain and matched the observed slab surface was selected as the best fit.

The restoration yielded several robust results. First, the ATS ocean basin was originally close to a rectangular shape rather than a highly irregular one. The present bowl‑shaped slab is therefore a product of later deformation imposed by the surrounding continental plates, not a primary feature of the oceanic lithosphere. Second, to bring the restored slab into alignment with the present‑day position of the Bird’s Head block, the block must have rotated clockwise by 20–35° relative to its current orientation. This rotation is substantially larger than the 10–15° rotation commonly used in existing plate‑kinematic models, indicating that the Bird’s Head block experienced an independent, significant rotational event.

When the restored geometry is compared with the suite of published plate‑kinematic scenarios, the model that best accommodates both the rectangular ATS basin and the required 20–35° clockwise rotation is one in which the western margin of Irian Jaya acted as a continental transform margin during the Jurassic rifting and opening of the ATS ocean. In this scenario, the Bird’s Head block is autochthonous to western Irian Jaya; it did not travel from a distant location but instead rotated in situ as the surrounding plates converged and the slab was later subducted. The transform‑margin setting explains the development of the present‑day complex plate boundary network (including the New Guinea‑Arabian‑Indonesian junction) and provides a coherent framework for the distribution of late‑Jurassic seafloor spreading anomalies recorded in magnetic and paleomagnetic data.

The authors also discuss broader implications. An autochthonous Bird’s Head block implies that the Late Jurassic rifted margin of northeastern Gondwana was not a simple passive margin but a dynamic transform boundary that accommodated significant lateral motion. This reinterpretation affects reconstructions of the Tethyan‑Pacific realm, the timing and geometry of the breakup of eastern Gondwana, and the provenance of terranes now accreted to SE Asia. Moreover, the study demonstrates the power of integrating seismic tomography with structural restoration to resolve long‑standing debates about deep‑time plate configurations.

In summary, the paper presents a compelling case that the Bird’s Head block is autochthonous, that the ATS ocean basin was originally rectangular, and that a clockwise rotation of 20–35° is required to reconcile the restored slab with present‑day geography. These findings call for a revision of existing Jurassic–Cretaceous plate‑kinematic models for the region, emphasizing a continental transform margin during ATS opening and highlighting the importance of deep‑seated slab geometry in constraining plate‑tectonic histories.