Palaeointensity determination and K?Ar dating of the Tertiary north-east Jalisco volcanics (Mexico)

Abstract

Sixteen samples representing eight Tertiary volcanic units from north-east Jalisco, Mexico were studied in an attempt to estimate the palaeointensity of the Earth's magnetic field. The experimental technique used was similar to that proposed by Shaw (1974) and an attempt was made to incorporate further criteria using the directional behaviour of the NRM, TRM and two ARMS during the af treatment and measuring the rate of ARM acquisition before and after heating. The directional information was used for checking the stability of all remanent magnetizations involved and for correcting the ARM test values. In addition the TRM directional behaviour could be a valuable yet simple test to detect the problem of insufficient heating. The use of the ARM acquisition test in the palaeointensity determination permits the full investigation of the coercive force spectrum and could lead to a more reliable palaeointensity method. This combined ARM method was applied to one sample. Further work is needed to understand the NRM-ARM1 and TRM-ARM2 relationships if the ARM tests are to be used for correcting TRM alteration effects. Whole rock K-Ar age determinations were carried out on samples from four selected units. Ten samples, representing six of the units, are considered to yield reliable palaeointensity values. Mean values were computed for each unit and reduced to the palaeoequator. The mean palaeoequatorial values and K-Ar ages determined are: I (13 f 2 Myr), 0.504 Oe; 11, 0.453 Oe; 111, 0.439 Oe; VI (52 f 10 Myr), 0.074 Oe; VII (14 ? 2 Myr), 0.187 Oe and VIII (12 f 2 Myr), 0.251 Oe.These results agree reasonably well with those from previous studies. A large number of palaeointensity estimations, many more than currently available, are required to obtain an average estimate of the behaviour of the Earth's magnetic field during the Tertiary.

Palaeointensity results from Ethiopian basalts: implications for the Oligocene geomagnetic field strength

“…Results from six other studies, obtained from different rocks types, are available for the Oligocene (Table 2): a lateritic horizon baked by overlying Tertiary lava in Northern Ireland (Wilson 1961), Tertiary volcanic units from northeastern Jalisco in New Mexico (Urrutia Fucugauchi 1980), a Scottish baked contact with an age of 33.9 ± 1.2 Ma (Smith 1967), Oligocene rhyolites and welded tuffs of the San Juan volcanic field in Colorado (Kono & Ueno 1977), a new result obtained mainly on Oligocene baked contacts from Armenia (Solodovnikov 1997), and recently published palaeointensity estimates obtained on submarine basaltic glasses (Juárez et al 1998). Including the Ethiopian results, 45 cooling units (Fig.…”

“…However, two studies (Wilson 1961; Urrutia Fucugauchi 1980) have to be considered with caution because of the imprecision on the age of the cooling units, with an error comparable to the assigned age. To avoid any bias on the mean VDM, these two data sets, and the three corresponding cooling units, were rejected from the selected data set.…”

Tectonic Control of the Volcano-Sedimentary Sequence of the Chapala Graben, Western Mexico

“…To the north, the late Miocene basaltic flows form extensive plateaus (~1900 m above sea level) (Fig. 2), with coeval ignimbritic and rhyolitic flows (Urrutia-Fucugauchi, 1981;Verma et al, 1985;Ferrari et al, 1994b). The basaltic plateau, called the Basal Mafic Sequence of the TMVB by Rosas-Elguera (1998), possesses a calc-alkaline affinity (Verma et al, 1985) and could represent the first widespread, uniform volcanic event along this volcanic arc (Ferrari, Garduño et al, 1994).…”

“…2). The Chapala graben extends eastward from the triple-junction zone and narrows within some 30 km (Delgado-Granados and Urrutia-Fucugauchi, 1986). It is characterized by recent faulting and may represent the most active structure in the region (Urrutia-Fucugauchi, 1986;Urrutia-Fucugauchi and Rosas-Elguera, 1994).…”

Propiedades magnéticas y petrofábrica de unidades volcánicas terciarias de Ia región de Arandas-Atotonilco, Jalisco, México

“…Entre los trabajos subsecuentes, se tiene un estudio de paleointensidad del campo geomagnetico y de fechamientos radiometricos de K-Ar (Urrutia, 1980a). En 16 muestras no desmagnetizadas del estudio anterior se aplico Ia tecnica experimental descrita por Shaw (1974) para la determinacion de paleointensidades, que incorpora una magnetizaci6n remanente anhisteretica (ARM) en cada paso y estudia su espectro de coercitividad, para identificar alteraciones en la muestra durante el calentamiento.…”