Paleomagnetic and paleointensity study of Oligocene volcanic rocks from Chihuahua (northern Mexico)

Abstract

A detailed rock-magnetic, paleomagnetic and paleointensity study was carried out on Oligocene volcanic formations from Chihuahua (northern Mexico) in order to obtain some decisive constraints for the tectonic evolution of the southern Cordilleran Orogenic Belt of North America and for the geomagnetic field strength during Oligocene time. Age of the volcanic units lies between 33 and 25 Ma according to available radiometric data. Rock-magnetic experiments reveal that remanence is carried in most cases by Ti-poor titanomagnetite, resulting of oxy-exsolution of original titanomagnetite during the initial flow cooling. Unblocking temperature spectra and relatively high coercivities point to ‘small’ pseudo-single domain magnetic grains for these (titano)magnetites. Single-component, linear demagnetization plots were observed in most cases. Nine sites yield reverse polarity magnetization, three are normally magnetized and one unit shows intermediate polarity magnetization. An evidence of strong lightning-produced magnetization overprint was detected for one site. Combining all paleomagnetic data currently available for northern Mexico, we obtained a well-defined Eocene–Oligocene mean paleomagnetic direction with I=48.5°, D=337.1°, k=20, α 95=6.8°, N=24, which deviates counterclockwise from the expected direction estimated from the North American apparent polar wander path. This suggests a vertical-axis tectonic counterclockwise rotation of about 16° relative to stable North America. Transition from Laramide compression to Basin and Range extension occurred during Oligocene, around 32–30 Ma. Tectonic rotation may then reflect east–northeast extension in the mid- to late-Cenozoic. Twenty-three samples were pre-selected for Thellier palaeointensity experiments because of their low viscosity index, stable remanent magnetization and reasonably reversible continuous thermomagnetic curves. Only 12 samples, coming from three individual basaltic lava flows, yielded reliable paleointensity estimates with the flow-mean virtual dipole moments (VDM) ranging from 3.96 to 4.65×10 22 Am 2. Combination of Mexican data with the available comparable quality Oligocene paleointensity results yield a mean VDM of 4×10 22 Am 2, which is comparable or slightly higher than the mean VDMs calculated for the Mesozoic low field period, but significantly lower than present day value. This low intensity may correlate with the relatively high paleosecular variation rate invoked around 30 Ma by several authors.