Geology of the boundary between the Sierra Madre Occidental and the Trans-Mexican Volcanic Belt in the Guadalajara region, western Mexico

To investigate the time-space relationship between the Sierra Madre Occidental (SMO) and the Trans-Mexican Volcanic Belt (TMVB), as well as the early volcanic activity of the TMVB, a field study was carried out in the region north of Guadalajara City, along the boundary between the two volcanic provinces. Detailed field mapping at 1:50,000 scale, supported by published and new isotopic ages allow to propose a new stratigraphy (30 lithostratigraphic units) and to clarify the volcanic evolution of the region. The SMO succession is made of about 400 m of regional ash flows tuffs of early Miocene age, capped to the south by a basaltic sequence for which we obtained an 40 Ar/ 39 Ar age of 21.8 ± 0.3 Ma. The SMO units are sub-horizontal north of Garcia de la Cadena but dip up to 25° to the south of this town, where they are covered in unconformity by the TMVB volcanism. A gap of about 10 Ma in the volcanism is observed between the two volcanic provinces. Volcanism pertaining to the TMVB is largely bimodal and can be divided into four stages: 1) a late Miocene mafic episode; it produced the Rio Santiago group that consists of a >800 m thick basaltic succession extending toward the east into the Los Altos de Jalisco; 2) a latest Miocene (7-5 Ma) silicic episode with the emplacement of domes and flows that form the Guadalajara group; 3) an early Pliocene (4.7-3.7 Ma) episode characterized by the emplacement of ignimbrites that show evidence of mingling between a mafic and a silicic magma (San Gaspar and Guadalajara ignimbrites) and alkali-basalts with an intra-plate affinity (Mirador de Ixcatan basalts); 4) a late Pliocene to Pleistocene episode with the eruption of rhyolitic domes and flows (Cerro Chicharron group) and alkalibasalts, also showing an intra-plate affinity (Santa Rosa basalts). New K/Ar ages of 10.2 ± 0.4 and 7.5 ± 0.8 Ma, together with previous dates, confirm that most of the Rio Santiago Group was emplaced between 11 and ~8 Ma. Its aggregate volume in the Guadalajara region is about 1,800 km 3 . The silicic volcanism covers more than 1,000 km 2 and has an aggregate estimated volume of 300 km 3 , which is about nine times the volume of magma extruded by the late Pleistocene La Primavera caldera. The vents of the silicic domes belonging to the Guadalajara group and Cerro Chicharron group follow an average N-S trend, and represent the surficial expression of a long-lived silicic upper crustal chamber with the La Primavera caldera at its southern end. By contrast, the Mirador de Ixcatan and Santa Rosa alkali-basalts are associated with the PlioPleistocene WNW-ESE trending faults of the Plan de Barrancas-Santa Rosa graben and their eruption coincide with the beginning of two extensional pulses along these faults. This agrees with models suggesting that mafic volcanism preferentially erupt along arc-parallel, high strain rate faults, whereas arc-normal, low strain rate faults control the location of more differentiated lavas. Magma mixing found in the San Gaspar and Guadalajara ignimbrites suggest that the arrival of mafic magma in the upper crust during the early Pliocene extensional phase triggered these pyroclastic eruptions.