Paleomagnetism and tectonic interpretations of the Taos Plateau Volcanic Field, Rio Grande Rift, New Mexico

Abstract

The tectonic response of the Taos Plateau volcanic field in the southern San Luis basin to the late stage extensional environment of the Rio Grande rift was investigated using paleomagnetic techniques. Sixty‐two sites (533 samples) of Pliocene volcanic units were collected covering four major rock types with ages of 4.7 to 1.8 Ma. Twenty‐two of these sites were from stratigraphic sections of the lower, middle and upper Servilleta Basalt collected in the Rio Grande gorge at two locations 19 km apart. Flows from the lower and middle members in the southern gorge record reversed polarities, while those in Garapata Canyon are normal with an excursion event in the middle of the sequence. The uppermost flows of the upper member at both sites display normal directions. Although these sections correlate chemically, they seem to represent different magnetic time periods during the Gilbert Reversed‐Polarity Chron. Alternating field demagnetization, aided by principal component analysis, yields 55 sites with stable directions representing both normal and reversed polarities, and five sites indicating transitional fields. Mean direction of the normal and inverted reversed sites is I = 49.3° and D = 356.7° (a95 = 3.6°). Angular dispersion of the virtual geomagnetic poles is 16.3°, which is consistent with paleosecular variation model G, fit to data from the past 5 m.y. Comparison with the expected direction indicates no azimuthal rotation of the Taos Plateau volcanic field; inclination flattening for the southern part of the plateau is 8.3° ± 5.3°. Previous paleomagnetic data indicate 10°–15° counterclockwise rotation of the Espanola block to the south over the past 5 m.y. The data suggest the Taos Plateau volcanic field, showing no rotation and some flattening in the south and east, has acted as a stable buttress and has been downwarped by overriding of the southeastern end of the plateau by the Picuris Mountains, which make up the northern corner of the counterclockwise rotating Espanola block.