History of Subduction Polarity Reversal During Arc‐Continent Collision: Constraints From the Andaman Ophiolite and its Metamorphic Sole

Abstract

Subduction polarity reversal during arc-continent collision has been proposed as a key mechanism to initiate new subduction zones. Despite often interpreted, well-exposed geological record that document the reversal is sparse. The ophiolitic lithounits of the Andaman and Nicobar Islands have been proposed to have formed during the initiation of a new subduction zone following the collision of the Woyla Arc of Sumatra with Sundaland (Eurasia). We here present new field, petrological and geochronological data to evaluate the timing of the initiation of Andaman subduction. We targeted the previously inferred but unstudied metamorphic sole of the Andaman ophiolites that witnessed juvenile subduction. Thermodynamic modeling reveals that the exposed amphibolites of the sole formed at around 0.9 GPa and 675°C. We dated two samples of the metamorphic sole using the Ar/Ar method on amphibole, giving cooling ages of 106.4 ± 2.1 and 105.3 ± 1.6 Ma. This is similar to published ages from plagioclase xenocrysts in recent Barren Island volcanics and in zircons from a gabbro sample from the Andaman ophiolite, which we interpret as the age of the original ophiolite formation. The Ar/Ar ages are considerably older than arc magmatic gabbros and plagiogranites of the overlying ophiolite previously dated at 99-93 Ma and thought to reflect the ophiolite age but recently reinterpreted as a volcanic arc built on the ophiolite. Combined with the ages of Woyla-Sundaland collision, we argue that subduction polarity reversal occurred in a transient period of perhaps some 10 Myr, similar to recent settings.

Differentiating induced versus spontaneous subduction initiation using thermomechanical models and metamorphic soles

“…The SI mode of the Nagaland locality in the low-T region (#15) cannot be differentiated based solely on the temperature conditions; the metamorphic soles may have formed either during SSI up-dip from the slab tip or during ISI, although a petrologic study suggests intra-oceanic ISI 58 . For the three localities along the western margin of North America (localities #1–3) and Andaman (#16), the overriding plate was continental and colder 7 , 59 . The PT conditions at these localities fall in the intermediate temperature range.…”

“…Based on our modeling results, the cold overriding plate and the intermediate temperature range favor neither SSI nor ISI. However, some regional studies on Cascadia, California, and Andaman (#2, 3, 16) suggest ISI due to subduction polarity reversal 7 , 59 . The dynamics of subduction polarity reversal and its impact on the slab surface temperature are not included in our model and may contribute to anomalous conditions.…”

A record of plume-induced plate rotation triggering subduction initiation

“…In the eastern Neotethys, a mid-Cretaceous collision of the intra-oceanic Woyla Arc with the Sundaland continental margin led to a subduction polarity reversal initiating eastward subduction below Sundaland 36 , which is recorded in ophiolites on the Andaman Islands. There, metamorphic sole rocks with 40 Ar/ 39 Ar hornblende cooling ages of 105-106 Ma, and likely coeval SSZ ophiolite spreading ages 37 reveal that this subduction zone may have developed slab pull around the same time as the Indian Ocean-western Neotethys plate boundary formed (Fig 1C). However, eastward slab pull below Sundaland cannot drive E-W convergence in the Neotethys to the west, and Andaman SSZ extension may well be an expression rather than the trigger of Indian plate rotation.…”

“…A key role, however, is possible for the only remaining geodynamic, non-plate-tectonic, plate-motion driver in the region: a mantle plume. India-Madagascar continental breakup is widely viewed 13,27,37 as related to the ~94 Ma and younger formation of the Morondava Large Igneous Province (LIP) on Madagascar 38 and southwest India 39 . This LIP, however, started forming ~10 Ma after initial plate boundary formation.…”

Subduction Polarity Reversal: Induced or Spontaneous?

“…Additionally, the discrepancy of the evolution duration between these two SPR events is also hard to understand. According to geochemical dating results, most SPR events, including Ryukyu, usually last for more than 15 Myrs (Plunder et al., 2020 and the references therein). However, some reconstruction models suggest that the SPR process in New Hebrides is very fast (5–7 Myrs) (Pysklywec et al., 2003; Schellart et al., 2006).…”