Basement rocks of the Transantarctic Mountains are believed to record a change in the paleo‐Pacific margin of Gondwana from a passive to a tectonically active margin. Widespread emplacement of calc‐alkaline batholiths (Granite Harbor intrusives) occurred during the active margin phase. We present new concordant zircon and titanite U‐Pb ages for these magmatic rocks in southern Victoria Land and the Scott Glacier area. Most magmatic rocks previously associated with a pre‐late Early Cambrian (>530 Ma) deformational event(s) (Beardmore orogeny) have yielded younger crystallization ages. The lack of definite arc magmatism prior to ∼530 Ma suggests that deformation may have been associated with a strike or oblique‐slip regime, although shallow subduction without significant arc magmatism cannot be ruled out. Local transpressional and transtensional domains may account for compressional deformation and rare alkaline and carbonatite magmatism during this early period. The oldest and most voluminous magmatic rocks were emplaced after ∼530 Ma. This magmatism has been associated with active subduction, and suggests a fundamental change in the plate boundary at ∼530 Ma. Ductile shearing of plutons and contractional deformation of supracrustal rocks after ∼530 Ma (Ross orogeny) may have been due to transpressional tectonics in an oblique subduction setting and/or a collision. Compressional deformation associated with the Ross orogeny may have ceased by ∼500 Ma along the southern Victoria Land‐Scott Glacier segment of the Antarctic margin, as indicated by undeformed magmatic rocks of this age, although magmatic activity continued to at least ∼485 Ma.

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Where and how subduction zones initiate is a fundamental tectonic problem, yet there are few well-constrained geologic tests that address the tectonic settings and dynamics of the process. Numerical modeling has shown that oceanic spreading centers are some of the weakest parts of the plate tectonic system [Gurnis M, Hall C, Lavier L (2004) Geochem Geophys Geosys 5:Q07001], but previous studies have not favored them for subduction initiation because of the positive buoyancy of young lithosphere. Instead, other weak zones, such as fracture zones, have been invoked. Because these models differ in terms of the ages of crust that are juxtaposed at the site of subduction initiation, they can be tested by dating the protoliths of metamorphosed oceanic crust that is formed by underthrusting at the beginning of subduction and comparing that age with the age of the overlying lithosphere and the timing of subduction initiation itself. In the western Philippines, we find that oceanic crust was less than ∼1 My old when it was underthrust and metamorphosed at the onset of subduction in Palawan, Philippines, implying forced subduction initiation at a spreading center. This result shows that young and positively buoyant, but weak, lithosphere was the preferred site for subduction nucleation despite the proximity of other potential weak zones with older, denser lithosphere and that plate motion rapidly changed from divergence to convergence. subduction initiation | tectonics | geochronology | Philippines | ophiolite

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