SILICIC TO BIMODAL OLIGOCENE VOLCANISM IN THE ZIMAPÁN AREA, MEXICO: STRATIGRAPHY, AGE AND DEFORMATION

We report new field, petrographic and geochronological data for a voluminous sequence of rhyolitic tuffs and domes associated with basaltic to andesitic lavas exposed southwest of Zimapán, state of Hidalgo. Geologic studies in the area are scarce, but this sequence has been assigned to the initial activity of the Trans-Mexican Volcanic Belt (TMVB) in the late Miocene. Based on this age, normal faults affecting the western part of the study área (Aljibes half-graben) were considered part of the TMVB intra-arc extension (Suter et al., 1995; GSAB).

The oldest volcanic unit of the Zimapan sequence consists of unconsolidated, partly reworked rhyolitic tuffs dated at 31.5 Ma (U-Pb, zircon; with a significant antecryst population from 37 to 30 Ma). This succession is intruded and partly overlain by mafic laccoliths, sills and dikes that in some places reached the surface forming lava flows that cover both the tuffs and the Cretaceous basement. This mafic unit is undated but can be constrained by the overlying silicic unit. This consists of a succession of ignimbrites, up to 250 m in thickness, probably emplaced through fissures as indicated by the presence of WNW- ESE striking pyroclastic dikes. A series of rhyolitic domes cap the succession. The ignimbrites yielded U-Pb ages of 32.9 and 32.8 Ma (with individual ages ranging from 37 to 28 Ma). Four ages obtained for the rhyolitic domes and dikes range between 28.3 Ma and 31.6 Ma.

The Oligocene succession is affected by WNW striking, SSE-dipping normal faults, with displacements up to 150 m and up to 40° tilting. The rhyolitic domes and dikes are also aligned in a WNW direction, suggesting a common extensional regime.Growth faults and decreasing tilting upsection indicate that faulting was synchronous with volcanism and entirely bracketed within the Oligocene. The youngest unit in the area are untilted Ol-basaltic lava flows of Late Miocene age, affected by small extensional faults with less than 50 m of displacement.

Our new data show that the volcanic sequence in the area was emplaced in a short time span during the Oligocene as a part of the Sierra Madre Occidental volcanism. Although small E-W striking faults do affect late Miocene and younger rocks, most of the deformation previously ascribed to the Aljibes half-graben is older and cannot be part of the TMVB intra-arc extension.