COUNTERCLOCKWISE DECLINATION ANOMALY IN PALEOMAGNETISM OF THE PLIOCENE CERROS DEL RIO VOLCANIC FIELD, NEW MEXICO - AN EXPRESSION OF LATE RIO GRANDE RIFT DISTRIBUTED SHEAR?

Distributed strike-slip shear is difficult to detect, particularly where shear is accommodated by vertical-axis rotation of fault blocks. Declination anomalies in paleomagnetic data have become the principal evidence used to detect such rotations.

The >700 sq km Cerros del Rio volcanic field (CdRVF) in northern New Mexico lies within the Rio Grande rift and overlaps the boundary between Española and Santa Domingo basins. The CdRVF comprises mostly flat-lying lava and pyroclastic deposits erupted from multiple basaltic to dacitic volcanic centers between 2.7 Ma and 1.1 Ma. We have sampled CdRVF units for paleomagnetic analysis at 51 sites distributed over the field. Extensive alternating-field demagnetization was required to isolate primary magnetization in many of these sites due to overprints from past lightning strikes. A mean direction of 49 sites is D=346.3°, I=51.5°, a95=4.5°, k=22. These data should average geomagnetic secular variation; they are distributed subequally between Gauss-chron normal-polarity sites and younger mostly reversed-polarity sites of the Matuyama chron and they sample units erupted from at least 12 volcanic centers. Compared to an expected direction calculated from a 0-Ma pole of Besse and Courtillot (2003), the CdRVF mean direction has a significantly counterclockwise (CCW) declination (-9.0° ± 6.3°), whereas the inclination is slightly shallow (4.2° ± 3.6°). The CCW declination anomaly confirms earlier findings by Brown and Golombek (1986) based on fewer paleomagnetic data; they attributed the declination anomaly to CCW rotation of the entire Española basin. Alternatively, CCW rotation may be linked to local shear developed as interaction increased between faults of the Española and Santo Domingo basins during known, late-stage narrowing of Rio Grande rifting.

Besse and Courtillot, 2003, JGR, 108, 2469, doi:10.1029/2003JB002684.

Brown and Golombek, 1986, Tectonics, v. 5, p. 423-438.