PALEOMAGNETISM AND TECTONIC INTERPRETATIONS OF THE TAOS-PLATEAU-VOLCANIC-FIELD, RIO-GRANDE-RIFT, NEW-MEXICO

The tectonic response of the Taos Plateau volcanic field in the southe rn San Luis basin to the late stage extensional environment of the Rio Grande rift was investigated using paleomagnetic techniques. Sixty-tw o sites (533 samples) of Pliocene volcanic units were collected coveri ng four major rock types with ages of 4.7 to 1.8 Ma. Twenty-two of the se sites were from stratigraphic sections of the lower, middle and upp er Servilleta Basalt collected in the Rio Grande gorge at two location s 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 uppe rmost flows of the upper member at both sites display normal direction s. Although these sections correlate chemically, they seem to represen t different magnetic time periods during the Gilbert Reversed-Polarity Chron. Alternating field demagnetization, aided by principal componen t analysis, yields 55 sites with stable directions representing both n ormal and reversed polarities, and five sites indicating transitional fields. Mean direction of the normal and inverted reversed sites is I = 49.3-degrees and D = 356.7-degrees (a95 = 3.6-degrees). Angular disp ersion of the virtual geomagnetic poles is 16.3-degrees, which is cons istent with paleosecular variation model G, fit to data from the past 5 m.y. Comparison with the expected direction indicates no azimuthal r otation of the Taos Plateau volcanic field; inclination flattening for the southern part of the plateau is 8.3-degrees +/- 5.3-degrees. Prev ious paleomagnetic data indicate 10-degrees-15-degrees counterclockwis e 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.