Paleomagnetic Sampling in the Cerrillos Hills, New Mexico

Tectonic Map of northern New Mexico showing the location of Tertiary volcanic, volcaniclastic, and intrusive rocks.

Cerrillos Hills, looking north from the Ortiz Mountains.

The Devil’s Throne, a Tertiary intrusion near Cerrillos, New Mexico.

North-striking Tertiary dike cutting Cretaceous shales, southern Cerrillos Hills.

Interbedded volcanic conglomerates and sandstones of the Oligocene Espinaso Formation.

Intermediate composition Oligocene dike cutting Tertiary volcaniclastic rocks of the Espinaso Formation.

Tertiary dike cutting volcaniclastic strata of the Espinaso Formation.

Collecting paleomagnetic samples from a latite dike in the Cerrillos Hills.

Collecting oriented paleomagnetic samples in the volcaniclastic rocks of the Oligocene Espinaso Formation using a Pomeroy orientation tool. This tool uses a magnetic (Brunton) compass, a sun compass, and an inclinometer to orient the samples.

Preliminary paleomagnetic data from 31 sites in breccias of the Espinaso Formation and various Tertiary intrusions in the Cerrillos Hills, Ortiz Mountains, and the La Cienega area. The grand-mean direction from this data set gives a declination of 345.6°, Inclination of 57.6° (k = 28, a95 = 4.4°; 21 sites of normal polarity; 10 of reverse). Comparison of mean direction with the expected direction for this area (e.g., Declination = 355.3°, Inclination = 53.9°) indicates that is discordant (R = -9.6° ± 7.4°) and significant at the 95% confidence level. The apparent rotation is consistent with the results of previous paleomagnetic studies documenting the presence of counterclockwise rotation in this part of the rift, but the overall magnitude of rotation indicated by our data is substantially less than previously reported. Our data are not inconsistent with the hypothesis idea that left slip occurred during development of the Rio Grande rift resulting in significant, although very small magnitude counterclockwise vertical axis rotation during rift development. Alternatively, such a small magnitude of counterclockwise rotation can be readily explained in an extensional setting that does not involve left slip parallel or sub-parallel to the rift.

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