PALEOMAGNETIC STUDY OF OLIGOCENE VOLCANIC ROCKS FROM CHIHUAHUA, NORTHERN MEXICO: TECTONIC IMPLICATIONS

Detailed rock-magnetic and paleomagnetic studies have been carried out on Oligocene volcanic units from central-western Chihuahua, northern Mexico. Age of the volcanic units lies between 33 and 25 Ma according to K-Ar dating. Rock-magnetic experiments reveal that remanence is carried in most cases by magnetite; unblocking temperature spectra and relatively high coercivities point to pseudo-single domain magnetic grains. 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. Evidence of strong lightning-produced magnetization overprint was detected for one site. Combining all paleomagnetic data currently available for northern Mexico, we obtain a well-defined Eocene-Oligocene mean paleomagnetic direction with I=48.5, D=337.1, k=20, a95=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 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 3 basaltic lava flows, yielded reliable paleointensity estimates with the flow-mean virtual dipole moments (VDM) ranging from 3.96 to 4.65 x1022 Am2. Combination of Oligocene data yield a mean VDM of 4 x 1022 Am2, which is significantly lower than present-day value. This intensity low may correlate with the relatively high paleosecular variation rate invoked around 30 Ma.