40AR/39AR AGES OF SILICIC ASH FLOW TUFFS AND LAVAS AT THE JALISCO BLOCK (MEXICO) BOUNDARY: CONSTRAINTS ON VOLCANISM, UPLIFT AND RIFTING

Nearly 20 years ago, it was suggested that a small crustal block (Jalisco) in western Mexico would be transferred to the Pacific plate, along with a remnant of the fractured Farallon plate (Luhr et al., 1985). Subsequent kinematic studies (e.g. DeMets and Stein, 1990) disputed this hypothesis and the implications for rifting and uplift. New ⁴⁰Ar/³⁹Ar ages from two basaltic flows and eight silicic ash flow tuffs and lavas in the Tepic-Zacoalco graben establish constraints for the volcanic, uplift and rifting processes in the region. The Tepic-Zacoalco rift zone is the northwest trending segment of a Pliocene triple graben, which, along with the north trending Colima graben and the Middle America trench, defines the boundaries of the Jalisco Block. The Jalisco Block is composed of granitoids and silicic ash flow tuffs (54-114 Ma), devoid of younger basement rocks. In contrast, basement rocks of Oligocene-Miocene age are found to the north and east. To the north, the Sierra Madre Occidental (SMO) is an extensive volcanic province characterized by Oligocene silicic ash flow tuffs. New ⁴⁰Ar/³⁹Ar dates of 25.2 ± 0.2 Ma and 58.3 ± 0.3 Ma from sanidine separates indicate that SMO extended into the Tepic-Zacoalco graben. This raises the question of whether SMO covered the Jalisco Block and has been stripped off due to erosion and post-Oligocene uplift or never covered the Jalisco Block. If the latter is true, a boundary must have existed between the Jalisco Block and Mexico prior to the eruption of SMO. Thus the Tepic-Zacoalco rift may be a reactivation of an older plate boundary. The initiation of rifting in the Tepic-Zacoalco graben was previously based on the eruption of a calc-alkaline basalt at 4.6 Ma in the Colima graben. ⁴⁰Ar/³⁹Ar dates from silicic domes and ashflows in the Tepic-Zacoalco rift yielded ages of 4-5 Ma. The presence of rhyolite lava flows and domes suggest the existence of a caldera. The ash flow tuffs are commonly faulted and at the base of some of the faulted ash flow escarpments, OIB-like basaltic lava flows have erupted. These flows were dated at 3.8 ± 0.1 Ma and thus, post-date the rifting of the ash flows. This bimodal volcanism may be the result of lithospheric extension which produced intraplate basalts that induced crustal melting and the subsequent production of silicic ash flows.