Young Rhyolitic and Alkaline Volcanism of the Ecuadorian Arc - A Result of the Carnegie Ridge Subduction?
Abstract
The subduction of aseismic submarine ridges under continental arcs generally results in the cessation of volcanic activity (Wilson, 1989). The rapidly-subducting (6 cm/yr) Carnegie Ridge under Ecuador, however, is associated with increased and diverse volcanism. The Northern Volcanic Zone in Colombia is a single row of volcanoes, but in Ecuador it is characterized by a high density of Pleistocene and active centers distributed in four parallel bands perpendicular to the convergence axis. Here the NVZ is higher in elevation (3-6 km high) and wider (ca. 140 km) than elsewhere. Ecuador’s volcanic arc has its volcanic front in the western cordillera where it is characterized by on-going dacitic, Si-rich andesitic, and adakitic-like volcanism. Eastwards, in the InterAndean Depression the volcanoes are older and andesitic. In the adjacent eastern cordillera the active volcanoes are normal 2-pyroxene andesites. However, co-existing with this andesite chain is a province of young rhyolitic volcanism (Hall & Mothes, 2001) which ranges in age from 2.7 Ma to historic times. In addition, immediately east of the rhyolitic province in a back-arc setting occurs a small group of alkaline volcanoes with feldspathoid-bearing basanites and tephrites. The rhyolite province extends 130 km southwards from the 50 km-long Chacana caldera complex (800 km3 of flow deposits) to early Holocene pumice flows recently identified at Pisayambo. Between these extremes are found the 20 km-wide Chalupas caldera and associated Late Pleistocene silicic centers and domes. The discovery of young rhyolitic centers at the eastern foot of the Andes (Aliso) extends the province 40 km eastwards. A flare-up of rhyolitic activity began at 200 ka with the 100 km3 Chalupas pumice flow, the coeval rhyolite flows from Chacana caldera, and a 11 km3 andesitic lava flow from nearby Antisana volcano. At 170-180 ka the northern sector of Chacana witnessed rhyolitic lava flows and extensive pumice lapilli falls whose combined volume is near 100 km3. The most recent rhyolitic activity was at Cotopaxi (7 eruptions between 9.6 and 2.3 ka), Conda (11.8 ka), and Aliso (<2 ka); meantime andesitic activity continued normally in this cordillera. The spatial and temporal coincidence of the rhyolitic and alkalic magmatism, the high density of young volcanic centers only in the Ecuadorian arc, the fact that it manifests itself above a -250 mgal negative gravity anomaly, suggesting a 50-60 km-thick crust (Feininger & Seguin, 1983), and the fact that this coincidence is well aligned with the location and direction of Carnegie Ridge subduction under northern Ecuador suggest a causal relationship. Notably the down-going slab is not atypical, but descends with a 32 degree easterly dip to 150 km, where its down-dip projection is aseismic under the eastern cordillera (Guillier et al., 2001). On-going field work and geochemical, petrologic, geodesic, and seismological studies will help to clarify this novel geodynamic situation.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V23B2440H
- Keywords:
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- 8413 VOLCANOLOGY / Subduction zone processes