SLAB TEARING FOLLOWING RIDGE-TRENCH COLLISION : EVIDENCE FROM LATE MIOCENE MAGMATISM IN BAJA CALIFORNIA, MEXICO

Neogene magmatism 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 the Vizcaino Peninsula. Calc-alkaline volcanism vanished and was replaced by unusual lavas emplaced within six Late Miocene to Quaternary fields (Jaraguay, San Borja, San Ignacio, Santa Rosalia, Santa Clara, La Purisima), delineating a 600 km long array. They include (1) adakites emplaced in the Santa Clara volcanic field together with niobium-enriched basalts (NEB) between 11.8 and 8.8 Ma, as well as in Santa Rosalia at ca. 12 Ma and in Jaraguay at 9.4 Ma, (2) tholeiites (11.3 to 7.2 Ma) in La Purisima and San Ignacio, (3) alkali basalts (ca. 8 Ma) northwest of Jaraguay, and finally (4) magnesian basaltic andesites (“bajaites”) erupted in the six fields between 13 and 7.5 Ma. The origin of these volcanics is attributed either to the opening of an asthenospheric window through which tholeiites and alkali basalts ascended or to the melting of its lips (adakites) and the subsequent reaction between adakitic melts and the supraslab mantle (NEB and “bajaites”). However, the identification of remnants of the Pacific-Farallon ridge south of Vizcaino Peninsula indicates that ridge subduction was not responsible for the opening of the asthenospheric window. We show that slab window-related lavas erupted between ca. 13 and 7 Ma all along the Peninsula, from ca. 25°N to 30°N. We propose that their emplacement reflects a process of slab tearing, which started at ca. 13 Ma when the active ridge collided with the trench. The old side of the downgoing plate continued to sink into the mantle, leading to the opening of a tear-in-the-slab which propagated parallel to the trench over at least 500 km.