Slab-tearing following ridge-trench collision: Evidence from Miocene volcanism in Baja California, Mexico

Neogene magmatic activity in Central Baja California underwent a major change at ca. 12.5 Ma, when the Pacific-Farallon active oceanic ridge collided with the trench east of Vizcaino Peninsula. The calc-alkaline nagnatism which built the Comondu volcanic arc vanished and was replaced by unusual volcanic associations, which were erupted within six Late Miocene to Quaternary volcanic fields (Jaraguay, San Borja, San Ignacio, Santa Rosalia, Santa Clara, La Purisima), delineating a 600 kin array along the Baja California Peninsula. New fieldwork, K-Ar datings and geochemical analyses on Jaraguay and San Borja lavas, combined with previous data, allow us to show that these associations include: (1) adakites emplaced between 12.5 and 8.2 Ma in the Santa Clara, Santa Rosalia and Jaraguay volcanic fields; (2) mobium-enriched basalts (NEB) in Santa Clara and Santa Rosalia (11.2 to 7.4 Ma); (3) tholeiitic basalts and basaltic andesites (11.3 to 7.2 Ma) displaying a very weak subduction imprint in La Purisima and San Ignacio; (4) alkali trachybasalts in Mesas San Carlos and Santa Catarina, northwest of Jaraguay (9.3 to 7.5 Ma), and finally (5) basalts and associated magnesian basaltic andesites and andesites. Locally referred to as bajaites, the latter lavas display very specific geochemical characteristics, including very high Sr and Ba contents. They were emplaced in all the above-mentioned volcanic fields between 14.6 and 5.3 Ma. The origin of these Late Miocene volcanics has been considered linked either to the opening of an asthenospheric window through which they ascended (tholeiites and alkali traebybasalts), or to the melting of its edges due to thermal erosion (adakites) and the subsequent reaction between adakitic melts and the supraslab mantle (NEB and bajaites). However, the identification of the remnants of the Pacific-Farallon fossil ridge south of Vizcaino Peninsula indicates that active ridge subduction was not responsible for the opening of the asthenospheric window. We propose that the emplacement of slab window-related volcanic rocks between 13 and 7 Ma from ca. 25 degrees N to 30 degrees N, was due to a process of slab-tearing, which started when the active ridge collided with the trench east of Vizcaino Peninsula. The oldest part of the downgoing plate sunk into the mantle, leading to the opening of a tear-in-the-slab, which likely started from the southern limit of the already existing Southern California slab window. It propagated parallel to the trench over about 600 kin and developed into an asthenospheric window. (c) 2006 Elsevier B.V. All rights reserved.