Revealing the beginning of Slab Windows at the Chilean Triple Junction

The subduction of active spreading centers is an unusual phenomenon along subduction zones. In southern Chile, the Nazca-Antarctic spreading system (Chile Rise) subducts beneath the South American plate at the Chile Triple Junction (CTJ), forming the Patagonian slab window. The onset of the slab window has been estimated based on plate kinematic reconstructions, but direct observations remain insufficient. To study this tectonic feature in detail, an Ocean Bottom Seismometer (OBS) array was deployed south of the CTJ between 2019 and 2021, and many earthquakes were detected and located around the CTJ.  Using these continuous data and the envelope correlation method, we searched for tectonic tremors to complement the seismic observations and detected more than 500 events in this period. The tremors detected are mainly located beneath the Taitao Ridge, where no fast earthquakes were observed. The tremors exhibit burst and episodic activity, reaching depths less than 20 km. A notable separation between fast seismicity and tremors is observed at the current location of the subducted Chile Rise segment. We interpret this seismic gap as evidence of the Patagonian slab window formation within the last 0.3 Myr. The shallow tremor activity is likely triggered by the migration of fluids, introduced by the subduction of the spreading ridge, into the accretionary prism preserved along the Taitao Ridge.