| Popis: |
New paleomagnetic results for 13 sites in volcanic units from Huaxtla, Paso de Guadalupe and Lazo sections in the Río Grande de Santiago (RGS) canyon and for 5 sites in the silicic center of Sierra de la Primavera (SP) are used to document a preliminary magnetostratigraphy for the Trans-Mexican Volcanic Belt (TMVB) volcanic sequence. The magnetostratigraphic results are correlated with data for 7 nearby sites in the RGS and 6 sites in the SP, and with available K–Ar dates. A geothermal exploratory well drilled by the Federal Comission of Electricity in the SP allowed recovery of samples down to a depth of 1361 m, and 8 core samples were studied for paleomagnetism distributed between depths of 93 m to 1358-1361 m. Normal (N), intermediate (I) and reverse ( R) polarities are documented in the volcanic sequence of RGS at Huaxtla, Paso de Guadalupe and Lazo sections, as well as in the domes and lava sequences of the SP. The three sections studied in the RGS are around 800 m thick, and include a basal rhyolitic ignimbrite (R), the San Cristóbal basalts (mainly R), rhyolites and ash flows of the Guadalajara group (mainly R), the San Gaspar ignimbrite (N), basaltic flows and a rhyolitic dome (N). The SP sequence rests on an andesitic sequence and includes a rhyolite, the San Cristóbal basalts (R), rhyolites and an andesitic flow of the Guadalajara group (R), ash flow tuffs, pre-caldera rhyolites, the Tala tuff (R, I), and intra-caldera lacustrine deposits. The San Cristóbal basalts represent the oldest TMVB units, with dates between 11 and 8 Ma. This interval corresponds to a normal polarity chron that contrasts with the dominant reverse polarity observed in RGS and SP. The normal polarity San Gaspar ignimbrite has been dated at 4.7–4.8 Ma, and provides a marker horizon in the Guadalajara region. This ignimbrite and the units of the Guadalajara group (with dates of 5.2–5.5 Ma) are assigned to the Gilbert R chron, with the San Gaspar ignimbrite possibly corresponding to the Thevera N subchron (4.59–4.79 Ma). The silicic sequence of SP represents the young activity in the region and is assigned to the Brunhes chron. |
