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

A detailed rock-magnetic, paleomagnetic and paleointensity study was carrie d 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 geomagneti c field strength during Oligocene time, Age of the volcanic units lies betw een 33 and 25 Ma according to available radiometric data. Rock-magnetic exp eriments reveal that remanence is carried in most cases by Ti-poor titanoma gnetite, resulting of oxy-exsolution of original titanomagnetite during the initial flow cooling, Unblocking temperature spectra and relatively high c oercivities point to 'small' pseudo-single domain magnetic grains for these (titano)magnetites. Single-component, linear demagnetization plots were ob served in most cases. Nine sites yield reverse polarity magnetization, thre e are normally magnetized and one unit shows intermediate polarity magnetiz ation. An evidence of strong lightning-produced magnetization overprint was detected for one site, Combining all paleomagnetic data currently availabl e for northern Mexico, we obtained a well-defined Eocene-Oligocene mean pal eomagnetic direction with I = 48.5 degrees, D = 337.1 degrees, k = 20, alph a (95) = 6.8 degrees, N = 24, which deviates counterclockwise from the expe cted direction estimated from the North American apparent polar wander path , This suggests a vertical-axis tectonic counterclockwise rotation of about 16 degrees relative to stable North America, Transition from Laramide comp ression to Basin and Range extension occurred during Oligocene, around 32-3 0 Ma. Tectonic rotation may then reflect east-northeast extension in the mi d- to late-Cenozoic, Twenty-three samples were pre-selected for Thellier palaeointensity experim ents because of their low viscosity index, stable remanent magnetization an d reasonably reversible continuous thermomagnetic curves. Only 12 samples, coming from three individual basaltic lava flows, yielded reliable paleoint ensity estimates with the how-mean virtual dipole moments (VDM) ranging fro m 3.96 to 4.65 x 10(22) Am-2. Combination of Mexican data with the availabl e comparable quality Oligocene paleointensity results yield a mean VDM of 4 x 10(22) Am-2, which is comparable or slightly higher than the mean VDMs c alculated for the Mesozoic low field period, but significantly lower than p resent day value, This low intensity may correlate with the relatively high paleosecular variation rate invoked around 30 Ma by several authors, (C) 2 001 Elsevier Science B.V. All rights reserved.