Geology of the San Pedro – Ceboruco Graben, western Trans-Mexican Volcanic Belt

Abstract

We present a detailed study of the geology of the San Pedro–Ceboruco graben (SPC) supported by extensive field work, subsurface information of eight exploratory wells drilled by the Comisión Federal de Electricidad, as well as the review of 74 published K–Ar and ⁴⁰Ar/³⁹Ar ages and seven new K–Ar unspiked age determinations. The SPC is the northwesternmost tectonic structure of the Tepic–Zacoalco rift, which has developed since the late Miocene in the western Trans-Mexican Volcanic Belt (TMVB). The SPC has a broad WNW–ESE orientation and is located at the boundary between two different basement blocks: the Cretaceous to Paleocene batholith of the Jalisco block to the south, and the Oligocene to early Miocene Sierra Madre Occidental silicic volcanic province to the north. Calcalkaline, transitional and Na-alkaline volcanic products of the TMVB cover the tectonic contact between the two blocks. The oldest volcanism related to the TMVB consists of a thick succession of mafic lava flows only found in a deep drilling beneath Ceboruco volcano. These lava flows have a late Miocene age and are the early fill of a paleo-graben that possibly formed in response to the opening of the Gulf of California. In the early Pliocene, a large amount of rhyolitic lavas and silicic pyroclastic flows shortly followed by mafic lavas were emplaced mainly to the north of the study area. A second phase of faulting occurred between the emplacement of the silicic rocks (~5–4.2 Ma) and the mafic ones (~3.8 Ma). Volcanism resumed at the end of Pliocene along a NW–SE alignment of cinder cones and domes that bounds the southern part of the SPC. Most activity occurred in the last 1 my. It first produced several dacitic to andesitic domes and a small calc-alkaline stratovolcano (Tepetiltic); then, after the formation of a small caldera (San Pedro) a mildly Na-alkaline succession of lava flows made up a small volcano (Amado Nervo). The recentmost volcanism is represented by intracaldera silicic domes, a northern WNW–ESE alignment of monogenetic volcanoes, and the active Ceboruco stratovolcano. Late Miocene to present cumulative extension in the area accounts for a modest 10%. A period of very low volcanic activity in late Pliocene coincides with a low convergence rate between the Rivera and North America plates, confirming a strong relation between subduction regime and upper plate volcanism.